• If precipitate of CuS, BaSO4 or Prussian blue are washed continuously with water, after sometime the
precipitate are converted into colloidal state which thus pass through the fitter paper and thus can be
detected in wash water.
Purification of colloidal solution.
The colloidal solutions prepared by the above methods usually contain impurities especially electrolytes which
can destabilize the sols. These impurities must be eliminated to make the colloidal
solutions stable. The following methods are commonly used for the purification of
colloidal solutions.
(1) Dialysis Water Sol
Parchment membrane
(i) The process of separating the particles of colloid from those of crystalloid,
by means of diffusion through a suitable membrane is called dialysis.
(ii) It’s principle is based upon the fact that colloidal particles can not pass Dialysis
through a parchment or cellophane membrane while the ions of the electrolyte can pass through it.
(iii) The colloidal solution is taken in a bag (parchment paper).
(iv) The bag is suspended in fresh water.
(v) The impurities slowly diffused out of the bag leaving behind pure colloidal solution
(vi) The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start
diffusing back into the bag.
(vii) Dialysis can be used for removing HCl from the ferric hydroxide sol.
(2) Electrodialysis
(i) The ordinary process of dialysis is slow.
(ii) To increase the process of purification, the dialysis is carried out by applying electric field. This process is
called electrodialysis.
(iii) Kidneys in the human body act as dialysers to purify blood which is +–
colloidal in nature.
Sol
(iv) The important application of dialysis process in the artificial kidney
machine used for the purification of blood of the patients whose kidneys have Parchment membrane
failed to work. The artificial kidney machine works on the principle of dialysis. Electrodialysis
(3) Ultra – filtration
(i) Sol particles directly pass through ordinary filter paper because their pores
are larger (more than 1µ or 1000mµ ) than the size of sol particles (less than 200mµ ).
(ii) If the pores of the ordinary filter paper are made smaller by soaking the filter paper in a solution of gelatin
of colloidion and subsequently hardened by soaking in formaldehyde, the treated filter paper may retain colloidal
particles and allow the true solution particles to escape. Such filter paper is known as ultra - filter and the process
of separating colloids by using ultra – filters is known as ultra – filtration.
(4) Ultra – centrifugation
(i) The sol particles are prevented from setting out under the action of gravity by kinetic impacts of the
molecules of the medium.
(ii) The setting force can be enhanced by using high speed centrifugal machines having 15,000 or more
revolutions per minute. Such machines are known as ultra–centrifuges.
Properties of colloidal solutions.
The main characteristic properties of colloidal solutions are as follows.
(1) Physical properties
(i) Heterogeneous nature : Colloidal sols are heterogeneous in nature. They consists of two phases; the
dispersed phase and the dispersion medium.
(ii) Stable nature : The colloidal solutions are quite stable. Their particles are in a state of motion and do not
settle down at the bottom of the container.
(iii) Filterability : Colloidal particles are readily passed through the ordinary filter papers. However they can
be retained by special filters known as ultrafilters (parchment paper).
(2) Colligative properties
(i) Due to formation of associated molecules, observed values of colligative properties
like relative decrease in vapour pressure, elevation in boiling point, depression in freezing
point, osmotic pressure are smaller than expected.
(ii) For a given colloidal sol the number of particles will be very small as compared to
the true solution.
(3) Mechanical properties Brownian movement of
(i) Brownian movement colloidal particles
(a) Robert Brown, a botanist discovered in 1827 that the pollen grains suspended
in water do not remain at rest but move about continuously and randomly in all directions.
(b) Later on, it was observed that the colloidal particles are moving at random in a zig – zag motion. This
type of motion is called Brownian movement.
(c) The molecules of the dispersion medium are constantly colloiding with the particles of the dispersed phase.
It was stated by Wiener in 1863 that the impacts of the dispersion medium particles are unequal, thus
causing a zig-zag motion of the dispersed phase particles.
(d) When a molecule of dispersion medium colloids with a colloidal particle, it is then displaced in one
direction. Then another molecules strikes it, displacing it to another direction and so on. This process give rise to zig-
zag motion.
(e) This can be confirmed by the fact that the suspensions do not show any such movement due to large
molecular size.
(f) Brownian movement provides a direct demonstration of the ceaseless motion of molecules as postulated by
kinetic energy.
(g) The Brownian movement explains the force of gravity acting on colloidal particles. This helps in providing
stability to colloidal sols by not allowing them to settle down.
(ii) Diffusion : The sol particles diffuse from higher concentration to lower concentration region. However,
due to bigger size, they diffuse at a lesser speed.
(iii) Sedimentation : The colloidal particles settle down under Light beam Light beam
the influence of gravity at a very slow rate. This phenomenon is used not visible visible
for determining the molecular mass of the macromolecules.
Source of light Scattering
(4) Optical properties : Tyandall effect of light
(i) When light passes through a sol, its path becomes visible Lens True solution Colloidal solution
because of scattering of light by particles. It is called Tyndall effect. Tyndall effect
This phenomenon was studied for the first time by Tyndall. The illuminated path of the beam is called Tyndall
cone.
(ii) In a true solution, there are no particles of sufficiently large diameter to scatter light and hence no Tyndall
effect is observed.
(iii) The intensity of the scattered light depends on the difference between the refractive indices of the
dispersed phase and the dispersion medium.
(iv) In lyophobic colloids, the difference is appreciable and, therefore, the Tyndall effect is well - defined. But
in lyophilic sols, the difference is very small and the Tyndall effect is very weak.
(v) The Tyndall effect confirms the heterogeneous nature of the colloidal solution.
(vi) The Tyndall effect has also been observed by an instrument called ultra – microscope.
Note : The smoke is colloidal, so when it is viewed at an angle to the source of light, it appears blue
due to Tydnall effect.
Dust in the atmosphere is often colloidal. When the sun is low down on the horizon, light from it has to
pass through a great deal of dust to reach your eyes. The blue part of the light is scattered away from
your eyes and you observe red part of the spectrum. Thus red sunsets are Tyndall effect on a large scale.
Tail of comets is seen as a Tyndall cone due to the scattering of light by the tiny solid particles left by
the comet in its path.
Due to scattering the sky looks blue.
The blue colour of water in the sea is due to scattering of blue light by water molecules.
Visibility of projector path and circus light.
Visibility of sharp ray of sunlight passing through a slit in dark room.
(5) Electrical properties : Colloidal particles carry an electric charge and the dispersion medium has an
opposite and equal charge, the system as a whole being electrically neutral. The presence of equal and similar
charges on colloidal particles is largely responsible in giving stability to the system because the mutual forces of
repulsion between similarly charged particles prevent them from coalescing and coagulating when they come closer
to one another.
(i) Electrophoresis
(a) The phenomenon of movement of colloidal particles under an applied electric field is called
electrophoresis.
(b) If the particles accumulate near the negative electrode, the charge on the particles is positive.
(c) On the other hand, if the sol particles accumulate – + – +
near the positive electrode, the charge on the particles is
negative. Battery Battery As2S3 particles
get
(d) The apparatus consists of a U-tube with two Pt- Pt electrode
electrodes in each limb. Sol of As2S3 accumulated
(e) Take a sol of As2S3 in the U-tube. (a) Before electrophoresis (b) After electrophoresis
(f) The intensity of the colour of the sol in both the Process of electrophoresis
arms is same. Now pass the current through the sol.
(g) After some time it is observed that the colour of sol near the positive electrode become dark. This indicates
that the As 2S3 particles are negatively charged and they move towards oppositely charged electrodes.
(h) Similarly, when an electric current is passed through positively charged Fe(OH)3 sol, it is observed that
they move towards negatively charged electrode and get accumulated there.
(i) Thus, by observing the direction of movement of the colloidal particles, the sign of the charge carried by
the particles can be determined.
(j) When electrophoresis of a sol is carried out with out stirring, the bottom layer gradually becomes more
concentrated while the top layer which contain pure and concentrated colloidal solution may be decanted. This is
called electro decanation and is used for the purification as well as for concentrating the sol.
(k) The reverse of electrophoresis is called Sedimentation potential or Dorn effect. The sedimentation
potential is setup when a particle is forced to move in a resting liquid. This phenomenon was discovered by Dorn
and is also called Dorn effect.
(ii) Electrical double layer theory
(a) The electrical properties of colloids can also be explained by electrical double layer theory. According to
this theory a double layer of ions appear at the surface of solid.
(b) The ion preferentially adsorbed is held in fixed part and imparts charge to colloidal particles.
(c) The second part consists of a diffuse mobile layer of ions. This second layer consists of both the type of
charges. The net charge on the second layer is exactly equal to that on the fixed part.
(d) The existence of opposite sign on fixed and diffuse parts of double layer leads to appearance of a
difference of potential, known as zeta potential or electrokinetic potential. Now when electric field is
employed the particles move (electrophoresis)
(iii) Electro-osmosis
(a) In it the movement of the dispersed particles are prevented from moving by semipermeable membrane.
(b) Electro-osmosis is a phenomenon in which dispersion medium is allowed to move under the influence of
an electrical field, whereas colloidal particles are not allowed to move.
(c) The existence of electro-osmosis has suggested that when liquid forced through a porous material or a
capillary tube, a potential difference is setup between the two sides called as streaming potential. So the reverse of
electro-osmosis is called streaming potential.
Note :Distance traveled by colloidal particles in one second under a potential gradient of one volt per cm is
called electrophoretic mobility of the colloidal particles.
The principle of electrophoresis is employed for the separation of proteins from nucleic acids,
removing sludge from sewage waste etc.
Origin of the charge on colloidal particles.
The origin of the charge on the sol particles in most cases is due to the preferential adsorption of either
positive or negative ions on their surface. The sol particles acquire electrical charge in any one or more of the
following ways.
(1) Due to the dissociation of the surface molecules : Some colloidal particles develope electrical
charge due to the dissociation / ionisation of the surface molecules. The charge on the colloidal particles is balanced
by the oppositely charged ions in the sol. For example, an aqueous solution of soap (sodium palmitate) which
dissociates into ions as, C15 H 31COONa C15 H 31COO− + Na + . The cations (Na+) pass into the solution while the
Sodium palmitate
anions (C15 H 31COO− ) have a tendency to form aggregates due to weak attractive forces present in the hydrocarbon
chains.
(2) Due to frictional electrification
(i) It is believed that the frictional electrification due to the rubbing of the dispersed phase particles with that of
dispersion medium results in some charge on the colloidal particles.
(ii) The dispersion medium must also get some charge, because of the friction. Since it does not carry any
charge, the theory does not seem to be correct.
(3) Due to selective adsorption of ions
(i) The particles constituting the dispersed phase adsorb only those ions preferentially which are common with
their own lattice ions.
(ii) For example, when a small quantity of silver nitrate (AgNO3 ) solution is added to a large quantity of
potassium iodide (KI) solution, the colloidal particles of silver iodide adsorb I − from the solution to become
negatively charged, (at this stage KI is in excess, and I − being common to AgI )
AgI + I − → (AgI)I −
(Colloidal (in excess (Colloidal particle
particle) in the becomes positively
medium) charged)
But, when a small quantity of potassium iodide (KI) solution is added to a large quantity of silver nitrate
solution (AgNO3 ) ; the colloidal silver iodide particles adsorb Ag + from the solution to become positively
charged, (at this stage AgNO3 is in excess and Ag + is common to AgI ),
AgI + Ag + → (AgI)Ag + Ag+ I–
(Colloidal (in excess in (Colloidal particle Ag+ – – – Ag+ I– + + + I–
particle) the medium) becomes positively
charged) – AgI – + AgI +
Ag+ – – – I– + + + I–
– Ag+ +
Ag+ I–
I– ions remain in dispersion
Ag+ ions remain in dispersed
medium (Agl, I– Sol) medium (Agl, Ag+ Sol)
(iii) Similarly, the ferric hydroxide colloidal particles develop positive charge due to the adsorption of Fe 3+
ions from the solution.
Fe(OH)3 + Fe 3+ → Fe(OH)3 .Fe 3+
(Colloidal (in exess in (Colloidal particle of ferric hydroxide
particle) the medium ) get positively charged due to the
adsorption of Fe3+ ions on it)
Ferric hydroxide colloidal particles develop negative charge due to adsorption of either OH − or Cl −
(CollFoied(aOl H()in3 + OH − → Fe(OH)3 .OH − Fe(OH)3 + Cl − → Fe(OH)3 .Cl −
excess) (Negatively charged ferric (Colloidal (in excess) (Negatively charged ferric
particle) hydroxide colloidal particle) particle) hydroxide colloidal particle)
(iv) Depending upon the nature of charge on the particles of the dispersed phase, the colloidal solutions are
classified into positively charged and negatively charged colloids. Some typical examples are as follows
(a) Negatively charged colloids (b) Positively charged colloids
• Metal sulphides : As 2S3 , CdS
• Metal hydroxides : Al(OH)3 , Fe(OH)3
• Metal dispersions : Ag, Au, Pt
• Metal oxide : TiO2
• Acid dyes : Eosin, congo red • Basic dyes : Methylene blue
• Sols of starch, gums, gold, gelatin etc. • Haemoglobin
• Sulphur sol
Note : SnO2 forms positively charged colloidal sol in acidic medium and negative charged colloidal
in basic medium this is due to SnO2 is amphoteric reacting with acid and base both. In acidic medium (say
HCl)Sn4+ ion is formed which is preferentially adsorbed on SnO2 giving positively charged colloidal sol.
SnO2 + 4HCl → SnCl4 + 2H 2O ; SnO2 + SnCl4 →[SnO2 ]Sn4+ : 4Cl −
Positively charged
In basic medium, SnO32− is formed which is preferentially adsorbed on SnO2 giving negatively charged
colloidal sol.
2NaOH + SnO2 → Na2 SnO3 + H 2O ; SnO2 + Na2 SnO3 →[SnO2 ]SnO32− : 2Na +
(Negatively charged)
Stability of sols.
Sols are thermodynamically unstable and the dispersed phase (colloidal particles) tend to separate out on long
standing due to the Vander Waal's attractive forces. However sols tend to exhibit some stability due to
(1) Stronger repulsive forces between the similarly charged particles : All colloidal particles in any sol
possess similar charge. Therefore, due to the electrostatic repulsion these are not able to come closer and form
aggregates. Thus stronger repulsive forces between the similarly charged particles in a sol promote its stability.
(2) Particle-solvent interactions
(i) Due to strong particle-solvent (dispersion medium) interactions, the colloidal particles get strongly solvated.
(ii) Due to solvation, the effective distance between the colloidal particles increases, and therefore, the Vander
Waal’s force of attraction decreases. As a result, the particles are not able to form aggregates.
(iii) Lyophilic sols are mainly stabilized by solvation effects due to strong interactions between the sol particles
and the dispersion medium.
Coagulation or Flocculation or Precipitation.
“The phenomenon of the precipitation of a colloidal solution by the addition of the excess of an electrolyte is
called coagulation or flocculation.” or
“The stability of the lyophobic sol is due to the presence of charge on colloidal particles. If the charge is
removed, the particles will come nearer to each other and thus, aggregate or flocculate and settle down under the
force of gravity. This phenomena is known as coagulation or flocculation.”
The coagulation of the lyophobic sols can be carried out by following methods.
(1) By electrophoresis : In electrophoresis the colloidal particles move towards oppositely charged
electrode. When these come in contact with the electrode for long these are discharged and precipitated.
(2) By mixing two oppositely charged sols : When oppositely charged sols are mixed in almost equal
proportions, their charges are neutralised. Both sols may be partially or completely precipitated as the mixing of
ferric hydroxide (+ve sol) and arsenious sulphide (–ve sol) bring them in precipitated form. This type of coagulation
is called mutual coagulation or meteral coagulation.
(3) By boiling : When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with the
molecules of dispersion medium. This reduces the charge on the particles and ultimately they settle down to form a
precipitate.
(4) By persistent dialysis : On prolonged dialysis, the traces of the electrolyte present in the sol are
removed almost completely and the colloids become unstable.
(5) By addition of electrolytes : The particles of the dispersed phase i.e., colloids bear some charge. When
an electrolyte is added to sol, the colloidal particles take up ions carrying opposite charge from the electrolyte. As a
result, their charge gets neutralised and this causes the uncharged, particles to come closer and to get coagulated or
precipitated. For example, if BaCl2 solution is added to As2S3 sol the Ba 2+ ions are attracted by the negatively
charged sol particles and their charge gets neutralised. This lead to coagulation.
(6) Hardy schulze rule : The coagulation capacity of different electrolytes is different. It depends upon the
valency of the active ion are called flocculating ion, which is the ion carrying charge opposite to the charge on the
colloidal particles. “According to Hardy Schulze rule, greater the valency of the active ion or flocculating ion, greater
will be its coagulating power” thus, Hardy Schulze law state:
(i) The ions carrying the charge opposite to that of sol particles are effective in causing coagulation of the sol.
(ii) Coagulating power of an electrolyte is directly proportional to the valency of the active ions (ions causing
coagulation).
For example to coagulate negative sol of As2S3 , the coagulation power of different cations has been
found to decrease in the order as, Al 3+ > Mg 2+ > Na +
Similarly, to coagulate a positive sol such as Fe(OH)3 , the coagulating power of different anions has been
found to decrease in the order : [Fe(CN)6 ]4− > PO 3− > SO42− > Cl −
4
(7) Coagulation or flocculation value
“The minimum concentration of an electrolyte which is required to cause the coagulation or flocculation of a
sol is known as flocculation value.”
or
“The number of millimoles of an electrolyte required to bring about the coagulation of one litre of a colloidal
solution is called its flocculation value.”
Thus , a more efficient flocculating agent shall have lower flocculating value.
Flocculation values of some electrolytes
Sol Electrolyte Flocculation value (mM) Sol Electrolyte Flocculation value (mM)
NaCl 51.0 KCl 9.5
As2S3 KCl 49.5 Fe(OH)3 BaCl 2 9.3
(-vely charged CaCl2 0.65 (+ vely K 2SO4 0.20
) MgCl2 0.72 charged) MgSO4 0.22
MgSO4 0.81
AlCl3 0.093
Al2(SO4 )3 0.096
Al(NO3 )3 0.095
Note : Coagulating value or flocculating value ∝ 1 power .
coagulating
(8) Coagulation of lyophilic sols
(i) There are two factors which are responsible for the stability of lyophilic sols.
(ii) These factors are the charge and solvation of the colloidal particles.
(iii) When these two factors are removed, a lyophilic sol can be coagulated.
(iv) This is done (i) by adding electrolyte (ii) and by adding suitable solvent.
(v) When solvent such as alcohol and acetone are added to hydrophilic sols the dehydration of dispersed
phase occurs. Under this condition a small quantity of electrolyte can bring about coagulation.
Note: Hydrophilic sols show greater stability than hydrophobic sols.
Protection of colloids and Gold number.
(1) Lyophilic sols are more stable than lyophobic sols.
(2) Lyophobic sols can be easily coagulated by the addition of small quantity of an electrolyte.
(3) When a lyophilic sol is added to any lyophobic sol, it becomes less sensitive towards electrolytes. Thus,
lyophilic colloids can prevent the coagulation of any lyophobic sol.
“The phenomenon of preventing the coagulation of a lyophobic sol due to the addition of some lyophilic
colloid is called sol protection or protection of colloids.”
(4) The protecting power of different protective (lyophilic) colloids is different. The efficiency of any protective
colloid is expressed in terms of gold number.
Gold number : Zsigmondy introduced a term called gold number to describe the protective power of
different colloids. This is defined as, “weight of the dried protective agent in milligrams, which when added to 10 ml
of a standard gold sol (0.0053 to 0.0058%) is just sufficient to prevent a colour change from red to blue on the
addition of 1 ml of 10 % sodium chloride solution, is equal to the gold number of that protective colloid.”
Thus, smaller is the gold number, higher is the protective action of the protective agent.
Protective power ∝ 1
Gold number
Gold numbers of some hydrophilic substances
Hydrophilic substance Gold number Hydrophilic substance Gold number
Gelatin 0.005 - 0.01 Sodium oleate 0.4 – 1.0
Sodium caseinate Gum tragacanth 2
Hamoglobin 0.01 Potato starch 25
Gum arabic 0.03 – 0.07
0.15 – 0.25
The protective colloids play very significant role in stabilisation of the non–aqueous dispersions, such as
paints, printing inks etc.
(5) Congo rubin number : Ostwald introduced congo rubin number to account for protective nature of
colloids. It is defined as “the amount of protective colloid in milligrams which prevents colour change in 100 ml of
0.01 % congo rubin dye to which 0.16 g equivalent of KCl is added.”
(6) Mechanism of sol protection Lyophobic Lyophilic
(i) The actual mechanism of sol protection is very complex. However it may particles
be due to the adsorption of the protective colloid on the lyophobic sol particles, protecting
followed by its solvation. Thus it stabilises the sol via solvation effects.
(ii) Solvation effects contribute much towards the stability of lyophilic particles
systems. For example, gelatin has a sufficiently strong affinity for water. It is only Protection of colloids
because of the solvation effects that even the addition of electrolytes in small
amounts does not cause any flocculation of hydrophilic sols. However at higher concentration, precipitation occurs.
This phenomenon is called salting out.
(iii) The salting out efficiency of an electrolyte depends upon the tendency of its constituents ions to get
hydrated i.e, the tendency to squeeze out water initially fied up with the colloidal particle.
(iv) The cations and the anions can be arranged in the decreasing order of the salting out power, such an
arrangement is called lyotropic series.
Cations : Mg 2+ > Ca 2+ > Sr 2+ > Ba 2+ > Li + > Na + > K + > NH 4 + > Rb + > Cs +
Anions : Citrate 3− > SO4 2− > Cl − > NH 3 − > I − > CNS −
Ammonium sulphate, due to its very high solubility in water, is oftenly used for precipitating proteins from
aqueous solutions.
(v) The precipitation of lyophilic colloids can also be affected by the addition of organic solvents of non-
electrolytes. For example, the addition of acetone or alcohol to aqueous gelatin solution causes precipitation of
gelatin. Addition of petroleum ether to a solution of rubber in benzene causes the precipitation of rubber.
Emulsion.
“The colloidal systems in which fine droplets of one liquid are dispersed in another liquid are called emulsions
the two liquids otherwise being mutually immiscible.” or
“Emulsion are the colloidal solutions in which both the dispersed phase and Oil droplets
the dispersion medium are liquids.”
A good example of an emulsion is milk in which fat globules are dispersed in Water Oil Oil Oil
Oil Oil
water. The size of the emulsified globules is generally of the order of 10−6 m.
Emulsion resemble lyophobic sols in some properties. Oil Oil
Oil in water
(1) Types of Emulsion : Depending upon the nature of the dispersed
phase, the emulsions are classified as;
(i) Oil-in-water emulsions (O/W) : The emulsion in which oil is present as the dispersed phase and water as
the dispersion medium (continuous phase) is called an oil-in-water emulsion. Milk is an example of the oil-in-
water type of emulsion. In milk liquid fat globules are dispersed in water. Other examples are, vanishing cream etc.
(ii) Water-in-oil emulsion (W/O) : The emulsion in which water forms the dispersed phase, and the oil acts
as the dispersion medium is called a water-in-oil emulsion. These emulsion are also Water droplets
termed oil emulsions. Butter and cold cream are typical examples of this types
of emulsions. Other examples are cod liver oil etc.
Note : The emulsion can be inter converted by simply changing Oil
the ratio of the dispersed phase and dispersion medium. For example, an oil-in-
water emulsion can be converted to water in oil emulsion by simply adding excess Water in oil
of oil in the first case.
(2) Preparation of Emulsions
(i) Emulsions are generally prepared by vigorously agitating a mixture of the relevant oil and water by using
either a high speed mixer or by using ultrasonic vibrators.
(ii) The emulsions obtained by simple mechanical stirring are unstable. The two components (oil and water)
tend to separate out.
(iii) To obtain a stable emulsion, a suitable stabilizing substance is generally added.
(iv) The stabilizing substance is called emulsifier of emulsifying agent. The emulsifier is added along with
the oil and water in the beginning. For Examples : substances which can act as emulsifiers are soaps, detergents,
long chain sulphonic acid, lyophilic colloids like gelatin, albumin, casein etc.
(3) Nature of emulsifier : Different emulsifiers may act differently in the case of a particular emulsion.
For example,
(a) Sodium oleate is used to prepare oil-in-water (O/W) emulsions.
(b) Magnesium and calcium oleates are used to prepare water-in-oil (W/O) emulsions. When calcium oleate is
added to an emulsion stabilized by sodium oleate, the stability of the system decreases. At a certain ratio of
Na + : Ca 2+ , the oil-in-water emulsion becomes unstable. If the Ca 2+ ions concentration is increased further very
quickly, then the reversal of the emulsion type occurs, that is the oil-in-water emulsion gets converted into a water-
in-oil type.
(4) Identification of emulsions : Several methods are available to find out whether an emulsion is of the
oil-in-water type or of the water-in-oil type emulsion. An emulsion can be identified as follows.
(i) Dilute test : Add water to the emulsion. If the emulsion can be diluted with water this means that water
acts as the dispersion medium and it is an example of oil-in-water emulsion. In case it is not diluted, then oil acts as
dispersion medium and it is an example of water-in-oil emulsion.
(ii) Dye test : An oil soluble suitable dye is shaken with the emulsion. If colour is noticed on looking at a drop
of the emulsion, it is oil-in-water type emulsion. In case the entire background is coloured, it is an example of water-
in-oil type.
(iii) Conductivity test : Add small amount of an electrolyte (e.g. KCl) to the emulsion. If this makes the
emulsion electrically conducting , then water is the dispersion medium. If water is not the dispersed phase.
(5) Properties of emulsion
(i) Emulsions show all the characteristic properties of colloidal solution such as Brownian movement, Tyndall
effect, electrophoresis etc.
(ii) These are coagulated by the addition of electrolytes containing polyvalent metal ions indicating the
negative charge on the globules.
(iii) The size of the dispersed particles in emulsions in larger than those in the sols. It ranges from 1000 Å to
10,000 Å. However, the size is smaller than the particles in suspensioins.
(iv) Emulsions can be converted into two separate liquids by heating, centrifuging, freezing etc. This process is
also known as demulsification.
(6) Applications of emulsions
(i) Concentration of ores in metallurgy
(ii) In medicine (Emulsion water-in-oil type)
(iii) Cleansing action of soaps.
(iv) Milk, which is an important constituent of our diet an emulsion of fat in water.
(v) Digestion of fats in intestine is through emulsification.
Gels.
(1) “A gel is a colloidal system in which a liquid is dispersed in a solid.”
(2) The lyophilic sols may be coagulated to give a semisolid jelly like mass, which encloses all the liquid
present in the sol. The process of gel formation is called gelation and the colloidal system formed called gel.
(3) Some gels are known to liquify on shaking and reset on being allowed to stand. This reversible sol-gel
transformation is called thixotropy.
(4) The common examples of gel are gum arabic, gelatin, processed cheese, silicic acid, ferric hydroxide etc.
(5) Gels may shrink by loosing some liquid help them. This is known as synereises or weeping.
(6) Gels may be classified into two types
(i) Elastic gels : These are the gels which possess the property of elasticity. They readily change their shape
on applying force and return to original shape when the applied force is removed. Common examples are gelatin,
agar-agar, starch etc.
(ii) Non-elastic gels : These are the gels which are rigid and do not have the property of elasticity. For
example, silica gel.
Application of colloids.
(1) Purification of water by alum (coagulation) : Alum which yield Al 3+ ions, is added to water to
coagulate the negatively charged clay particles.
(2) In rubber and tanning industry (coagulation and mutual coagulation) : Several industrial processes
such as rubber plating, chrome tanning, dyeing, lubrication etc are of colloidal nature
(i) In rubber platting, the negatively charged particles of rubber (latex) are made to deposit on the wires or
handle of various tools by means of electrophoresis. The article on which rubber is to be deposited is made anode.
(ii) In tanning the positively charged colloidal particles of hides and leather are coagulated by impregnating,
them in negatively charged tanning materials (present in the barks of trees). Among the tanning agent chromium
salts are most commonly used for the coagulation of the hide material and the process is called chrome tanning.
(3) Artificial rains : It is possible to cause artificial rain by throwing the electrified sand or silver iodide from
an aeroplane and thus coagulating the mist hanging in air.
(4) Smoke precipitation (Coagulation) : Smoke is a negative sol consisting of carbon particles dispersed in
air. Thus, these particles are removed by passing through a chamber provided with highly positively charged
metallic knob.
(5) Formation of deltas (coagulation) : River water consists of negatively charged clay particles of colloidal
dimension. When the river falls into the sea, the clay particles are coagulated by the positive Na + , K + , Mg 2+ ions
etc. present in sea water and new lands called deltas are formed.
(6) Blood consists of negatively charged colloidal particles (albuminoid substance). The colloidal nature of
blood explains why bleeding stops by applying a ferric chloride solution to the wound. Actually ferric chloride
solution causes coagulation of blood to form a clot which stops further bleeding.
(7) Colloidal medicine : Argyrol and protargyrol are colloidal solution of silver and are used as eye lotions
colloidal sulphur is used as disinfectant colloidal gold, calcium and iron are used as tonics.
(8) Photographic plates : These are thin glass plates coated with gelatin containing a fine suspension of
silver bromide. The particles of silver bromide are colloidal in nature.
Note : Isoelectric point of the colloid : The hydrogen ion concentration at which the colloidal
particles are neither positively charged nor negatively charged (neutral) is known as isoelectric point
of the colloids. At this point, the lyophilic colloids are expected to have minimum stability because at
this point particles have no charge or equal quantum of positively and negatively charge. For
example,
isoelectric point of gelatin is 4.7 (at pH 4.7 gelatin has no electrophoretic motion; at pH<4.7, gelatin
moves towards anode)
Colloidal solution of graphite is called Aqua dug.
Ultrasonic dispersion : Various substances such as mercury, oils, sulphur, sulphides and oxide of
metals can be dispersed into colloidal state very easily with the help of ultrasonic waves.
Stem technology: Colloidal particles in a sol are very small and most of them are not visible through
an ultramicroscope or light microscope. Recently, new techniques have been developed to determine
the size and shape of the colloidal particles. These are
(i) Scanning Electron Microscope (SEM), (ii) Transmission Electron Microscope (TEM)
A modified form of the above methods has also been developed. It is called Scanning
Transmission Electron Microscope (STEM). All these techniques are superior to the light microscope
because they have greater resolving power.
Bencroft rule : The phase in which the emulsifier is more soluble becomes outer phase of the emulsion.
***
General principals of extraction of metals
All the materials found in the earth are composed of elements. Oxygen 49.5 Al 7.5
There are about 112 elements known which constitute the entire Silicon 27.7 Fe 4.7
matter on the earth. Therefore, the elements are regarded as the
building blocks of the universe. These are distributed in all the three Calcium 3.4
main parts of the earth; atmosphere and lithosphere. Among these, Sodium 2.6
lithosphere constitutes the main source of most of the elements. The Potassium 2.4
elements have been broadly divided into metals and non-metals on the Magnesium 0.6
basis of their physical and chemical properties. Entire distribution of Titanium 0.6
sixteen most abundant elements in the earth's crust and their Chlorine 0.2
percentages by weight are shown in figure. Phosphorus 0.1
Manganese 0.1
Others < 0.1
Distribution of elements in the earth's crust
Occurance of Metals .
Element which have low chemical reactivity generally occur native or free or metallic state. e.g.
Au, Pt, noble gas etc. Element which are chemically reactive, generally occur in the combined state. e.g.
halogens, chalcogens etc. The natural materials in which the metals occur in the earth are called minerals. The
mineral from which the metal is conveniently and economically extracted is called an ore. All the ores are minerals
but all minerals cannot be ores. Ores may be divided into four groups,
(1) Native ores : These ores contain metals in free state, e.g., silver, gold, platinum, mercury, copper, etc.
Sometimes lumpes of pure metals are also found. These are termed nuggets. Iron is found in free state as
meteroites which also have 20 to 30% nickel.
(2) Sulphurised and arsenical ores : These ores consist of sulphides and arsenides in simple and complex
forms of metals.
(3) Oxidised ores : In these ores, metals are present as their oxides or oxysalts such as cabonates, nitrates,
sulphates, phosphates, silicates, etc.
(4) Halide ores : Metallic halides are very few in nature. Chlorides are most common e.g., Common salt,
Horn silver, Fluorospar, Cryolite.
Metals in Biology .
Metals are also found in living organisms, e.g., (2) Potassium is present in plant roots.
(1) Magnesium is found in chlorophyll. (4) Zinc is present in eyes of cats and cows.
(3) Manganese, Iron and copper are present in chloroplast. (6) Calcium is present in bones.
(5) Iron is present in haemoglobin. (8) Chromium is present in prown.
(7) Vanadium is present in cucumbers.
Mineral Wealth of India .
Minerals are the gifts of nature. They occur at definite spots. They are needed both for plants and animal life.
The development of Saudi Arabia is a dramatic story of how minerals transform a barren waste into a thriving land.
Some of the minerals which occur in India are given below.
(1) Gold : India is a minor producer of gold and her production comes from Hutte and Kolar gold field in
Karnataka. It also occurs in the alluvial sands of Ganges, Irrawaty and Swarnrekha. India stands the eight position
among the gold producing countries of the world. India produces 2824 kg of gold in 1975.
(2) Copper : It is formed in Singhbhum and Hazaribagh district in Bihar, Agnigundala in Andra Pradesh,
Malanjkh in Madhya Pradesh and Khetri in Rajasthan. Copper production in India was 25,000 tons in 1976.
(3) Iron : It occurs as haematite, Fe2O3. The major iron producing states in India are Orissa, Bihar, Madhya
Pradesh, Mysore and Goa. The other iron producing states of minor importance are Maharashtra, Andhra Pradesh,
Rajasthan, Tamil Nadu, Utter Pradesh, Punjab and Jammu and Kashmir. Annual product of iron ore in the country
is about 22 million metric tons.
(4) Aluminium : It is, one of the most important metals in the modern industrial era. Its chief minerals bauxite
and corundum are found in Rewa, Bhopal, Orissa, Bihar, Tamil Nadu, Saurashtra and Kashmir. Its production in
the country in 1975 was 175,000 metric tons.
(5) Manganese : India is a important producer of the best quality of manganese in the world. Our production
of its ore in 1975 was 134–136 metric tons. It occurs in Madhya Pradesh, Maharastra, Madras, Mysore, Andhra
Pradesh, Bihar, Rajasthan etc.
(6) Tin : It is an important non ferrous metal. It is found in Bihar, Orissa and Rajasthan.
(7) Chromium : The metal occurs as chromate or chrome iron ore in Mysore, Bihar, Orissa and Maharashtra.
(8) Lead : Its sulphide ore galena occurs in Bihar, Madhya Pradesh and Rajasthan. India produced 5200 tons
of lead in 1975.
(9) Lime Stone : It occurs in Satna, Katni and Rohtasgarh. Marble is formed in Jaipur and Madhya Pradesh.
(10) Mica : India produces about 80% of world’s mica production. The major mica producing states in India
are Bihar, Andhra Pradesh and Rajasthan. Minor producing states are Tamil Nadu, West Bengal and Madhya
Pradesh. The Bihar mica belts extends for a distance of about 160 km. passing through Hazaribagh district.
(11) Thorium : It occurs in monazite sands in Travancore. India enjoys a sole monopoly in this mineral.
Extraction of Metals: Metallurgy.
The extraction of a pure metal from its ore is called metallurgy. In order to extract the metal from ores, several
physical and chemical methods are used. The method used depending upon chemical properties and nature of the
ore from which it is to be extracted. It involves four main steps,
(1) Crushing and grinding of the ore.
(2) Concentration or dressing of the ore.
(3) Reduction to free metal.
(4) Purification or refining of the metal.
(1) Crushing and grinding of the ore : Those ores occur in nature as huge lumps. They are broken to small
pieces with the help of crushers or grinders. These pieces are then reduced to fine powder with the help of a ball mill
or stamp mill. This process is called pulverisation.
(2) Concentration or dressing of the ore : The ore are usually obtained from the ground and therefore
contained large amount of unwanted impurities, e.g., earthing particles, rocky matter, sand, limestone etc. These
impurities are known collectively as gangue or matrix. It is essential to separate the large bulk of these impurities
from the ore to avoid bulk handling and in subsequent fuel costs. The removal of these impurities from the ores is
known as concentration. The concentration is done by physical as well as chemical methods.
Physical Methods
(i) Gravity Separation or levigation : This process of concentration is based on the difference in the specific
gravity of the ore and gangue. The sieved ore is either subjected to dry Water Crushed ore
centrifugal separation or is placed in big shallow tanks in which a strong Riffles
current of water blows. Heavy ore particles settle down to the bottom of the
tanks while lighter gangue particles are carried away by the current of water. Ore particles Gangue
The process removes most of the soluble and insoluble impurities. For this mixed with
purpose wilfley table and hydraulic classifier are widely used. The method is
water
particularly suitable for heavy oxide and carbonate ores like Cassiterite The Wilfley table
(SnO2) and haematite.
(ii) Froth floatation process : In some cases for example, sulphides ores of copper, zinc and lead
concentration is brought by this method. In this method advantage is taken of the preferential wetting of the ore by
an oil. The finely ground ore is taken in a tank containing water and 1% of pine oil or terpentine oil. A strong
current of air is blown through the suspension, producing a heavy froth or foam on the surface. The metal sulphide
is wetted by the oil but the gangues is not and the sulphide-oil mixture is carried to the surface by films of oil The
froth is skimmed off, the gangue settles down on the bottom or remains underneath the froth. By this floation
method it is possible to concentrate over 90% of a sulphite ore to 1/10 of its original bulk.
Powdered Compressed air
ore + oil + water
Light ore particles
in froth
Gangue Concentrated
ore
Froth floatation process
Note : Pine oil acts as froth since it forms a stable froth with the ore particles.
In the floatation process, ethyl xanthate and potassium ethyl xanthate are also used which
act as Collectors. These attach themselves to mineral particles by polar group which become water
repellent and passed on into the froth.
Activators and Depressants : During the floatation process of some ores, these substances are added
which activate or depress the floatation property of the minerals and thus help in the separation of minerals present
in the ore. For instance, galena (PbS) is usually associated with sphalerite (ZnS) and pyrites (FeS2). Concentration
of galena is carried out by passing potassium ethyl xanthate (Collector) alongwith sodium cynamide and alkali
(depressants) where by the floatation property of ZnS and FeS2 is depressed. Mainly PbS passes into the froth when
air current in flown in, which is collected. After PbS is removed with the froth, same CuSO4 (activator) is added and
air is blown. The floatation property of ZnS is increased which is now removed with the froth. The slurry is acidified
and process is repeated when FeS2 passed into the froth and is collected.
(iii) Electromagnetic separation : If the mineral and not gangue is attracted by a magnet, it can be
concentrated by magnetic separation. For example chromite ore, FeCr2O4 being Powdered ore
magnetic can be separated from non–magnetic silicons impurities by this Electromagnets
method. Sometimes two minerals occur together, in which one happens to be
magnetic. By magnetic separation method the nonmagnetic minerals is
separated from the magnetic mineral. For example tin–stone or cassiterite, SnO2 Moving belt Gangue
(non-magnetic) containing wolfram, FeWO4 (magnetic) is separated by this Magnetic ore
method. In this method a thin layer of finely ground ore is spread over a rubber
belt carried over a pulley in a magnetic field. The gangue particles or the Electromagnetic separation
particles of non–magnetic mineral fall off as the belt becomes vertical, and the
magnetic particles collect.
Chemical methods
(i) Calcination : In this process the concentrated ore is heated in a suitable furnace generally in reveratory
furnace much below its melting point in absence of air. As a result of which the ore dries up and moisture and
volatile impurities are driven off and carbonates are converted into oxides and the ore becomes porous. For example,
Al2O3 .2H 2O → Al2O3 + 2H 2O ; 2Fe 2O3 . 3H 2O → 2Fe 2O3 + 6H 2O
ZnCO3 → ZnO + CO2 ; CaCO3 → CaO + CO2 ; CuCO3 .Cu(OH)2 → 2CuO + CO2 + H 2O
(ii) Roasting : The process of heating the ores strongly in presence of air with or without certain substances,
below its melting point is termed as roasting. It differs from calcination in the respect that heating is done in
presence of air and at a higher temperature. In this process the impurities of sulphur and arsenic etc. are volatilized
away as oxides and the ore is converted into oxide. For example zinc oxide is formed by the oxidation of zinc
blende, 2ZnS + 3O2 → 2ZnO + 2SO2
(iii) Leaching : It involves the treatment of the ore with a suitable reagent as to make it soluble while impurities
remain insoluble. The ore is recovered from the solution by suitable chemical method. For example, the chief ore of
aluminium, bauxite (Al2O3.2H2O) contains varying amounts of ferric oxides, titanium oxide and silica. Since alumina is
amphoteric, it can be separated from the other two oxides. Finely powdered bauxite is digested with caustic soda
solution at 150–170oC under pressure for some hours. Alumina dissolves forming soluble sodium aluminate.
Al2O3. 2H2O + 2NaOH → 2NaAlO2 + 3H2O
The impurities remain unaffected and separated as insoluble red mud which is filtered off. The filtrate is
diluted and some freshly precipitated aluminium hydroxide is added when Al(OH)3 is precipitated as follows,
NaAlO2 + 2H2O → NaOH + Al(OH)3
The precipitated hydroxide is filtered off and calcified to get highly pure aluminium oxide (alumina).
2Al(OH)3 calcination → Al 2O3 + 3H 2O
Gold and silver are also extracted from their native ores by Leaching (Mac-Arthur forest cyanide process).
(3) Reduction to free metal : Some of the methods commonly used to get free metal from the concentrated
ore are given below,
(i) Smelting : The process of extracting a metal in the state of fusion is called smelting. In this process the ore is
mixed with carbon, obtained after the above reactions and heated in suitable furnace. A suitable flux is added during the
operation to convert the non–fusible gangue to fusible slag. The metallic oxide is reduced by carbon and the metal may be
obtained in the molten state or as vapours which are condensed. Metals like tin, zinc or lead are obtained by this process.
SnO2 + 2C → Sn + 2CO ; ZnO + C → Zn + CO ; Fe 2O3 + 3CO → 2Fe + 3CO2
Flux and slag : Flux is a substance that is added during smelting to convert infusible silicons or earthy
impurities into fusible material known as slag. Impurities + Flux = Slag. The slag is immiscible with the metal and
has a low melting point and density. The slag floats on the metal and protects it from oxidation. It is removed from
the furnance through the slag hole. If the impurities in the ore are acidic (SiO2) in nature, a basic flux e.g., CaO,
MgO, FeO etc. are added ; and if the impurities are basic (CaO, FeO, etc.) then on acidic flux (SiO2) is used. The
gangue or matrix present in the ore is refractory or non–fusible in nature but it reacts with the flux forming fusible
slag which does not mix with the molten metal and forms the upper layer. Slag are usually silicates.
CaO + SiO2 → CaSiO3 ; SiO2 + FeO → FeSiO3
Flux Impurities Slag
Impurities Slag
(ii) Reduction by Aluminium (Gold-schmidt alumino thermic process) : The process of reduction is used in
the case of those oxides which can not be easily reduced by carbon. In this process, metallic oxides ore are mixed
with aluminium powder commonly called as thermite and placed in a steel crucible lined inside with a refractory
material and ignited by magnesium ribon. By the use of this process a number of metals such as chromium and
manganese are obtained on a commercial scale in highly pure state.
Cr2O3 + 2Al → Al2O3 + 2Cr ; 3 MnO2 + 4Al → 2Al2O3 + 3Mn
Large amount of heat energy is released during reduction, which fuses both the alumina and the metal.
(iii) Self reduction process : This process is also called autoreduction process or air reduction process. The
sulphide ores of less electropositive metals like Hg, Pb, Cu, etc., are heated in air as to convert part of the ore into
oxide or sulphate which then reacts with the remaining sulphide ore to give the metal and sulphur dioxide. No
external reducing agent is used in this process.
(iv) Electrolytic reduction process : This process is used in the extraction of the alkali and alkaline earth
metals, zinc and aluminium. The material from which a metal to be obtained is first smelted by heating and then
electrolysed. Sometimes, some other salt is mixed to lower the melting point of the substance taken. For example,
(electrolysis)
NaCl Na+ + Cl−
At anode (Oxidation) : Cl − −e− → Cl ; Cl + Cl → Cl2 , At cathode (Reduction) : Na + + e − → Na
(v) Precipitation or metal displacement method (Hydrometallurgy) : This method is used for extraction
of metals such as cadmium, copper, gold and silver. A metal having higher electrode potential is added into the
solution of a metal of lower electrode potential with the result that the latter is displaced or precipitated.
CuSO4 + Fe → Cu + FeSO4 ; CdSO4 + Zn → Cd + ZnSO4
2K[Ag(CN)2 ] + Zn → 2Ag + K2[Zn(CN)4 ] ; 2K[Au(CN)2 ] + Zn → 2Au + K 2[Zn(CN)4 ]
(vi) Amalgamation process : This method is used for the extraction of noble metals like gold, silver, etc.,
from the native ores. the finely powdered ore is brought in contact with mercury which combines with the particles
of the metal present in the ore and form amalgam. The metal recovered from the amalgam by subjecting it to
distillation, where the mercury distills over leaving behind the metal.
(4) Purification or refining of metals : Metals obtained as above are usually impure and need purification.
Some of the methods used in the refining of metals are given below,
(i) By poling : The molten metals is stirred with green wood poles. Wood at the high temperature of the molten
metals form hydrocarbons like methane which being about the reduction of any oxide present in the metal e.g., copper
oxide present in the blister copper. In the case of the tin the impurities are oxidised and float on the molten metal as scum
which is removed.
(ii) By cupellation : In this method the impure metal is heated in a blast of air when impurities are oxidised
and blown away. For example, when impure silver is heated in air, lead present in it is oxidised to litharge (PbO)
and blown away leaving a shining of silver.
(iii) By liquation : This process is used for refining easily fusible metals like lead and tin. The impure metal is
heated on the slopy hearth of a reverberatory furnace. The metal melts and flows down leaving the impurities.
(iv) By distillation : Some metals have very low melting point and soon vaporize on behind heating, while
the associated impurities remains in the solid state. Zinc, mercury and arsenic are purified by this method. Vacum
distillation gives very pure product and is used in the refining of the metals of IA and IIA Groups.
(v) By fractional distillation : This process is applied for the separation of cadmium from zinc. In the
metallurgy of zinc, the metal is invariably associated with cadmium. The impure zinc is mixed with powdered coke
and heated when the first portion of the condensate contain cadmium while zinc is obtained in the subsequent
portions.
(vi) By thermal dissociation : In this process the metal is first converted into some compound which is then
decomposed into pure metal by heating. For example, impure nickel is heated with carbon monoxide at 60oC to
form nickel carbonyl Ni(CO)4 which is then decomposed at higher temperature, 150-180oC to give very pure nickel.
Sometimes iron is also purified by this method.
(vii) By Electrolytic refining : Most of the metals such as copper, silver, gold, zinc, nickel, and chromium
are refined electrolytically. The impure metal is made the anode and a thin sheet of the pure metal the cathode in a
suitable electrolytic bath. On passing current the metal from the anode passes in the solution and pure metal from
the electrolyte is deposited on the cathode. The electrolyte used in the bath is usually a complex salt of the metal to
enable the smooth deposition of pure metal on the cathode.
(viii) Special methods
(a) Mond's process : Nickel is purified by this method. Impure nickel is treated with carbon monoxide at
60-80°C when volatile compound, nickel carbonyl, is formed. Nickel carbonyl decomposes at 180°C to form pure
nickel and carbon monoxide which can again be used. Molten zone
(b) Van Arkel process : This methods is generally applied for containing impurity
obtaining ultrapure metals. The impure metal is converted into a volatile
compound while the impurities are not affected. The volatile compound is
then decomposed electrically to get the pure metal. Ti, Zr, Hf, Si, etc.,
have been refined by this method. Pure metal Moving circular Impure metal
heater
(c) Zone refining or fractional crystallisation : Elements such as
Si, Ge, Ga, etc., which are used as semiconductors are refined by this Zone refining of metals
method. Highly pure metals are obtained. The method is based on the difference in solubility of impurities in
molten and solid state of the metals. A movable heater is fitted around a rod of the metal. The heater is slowly
moved across the rod. The metal melts at the point of heating and as the heater moves on from one end of the rod
to the other end, the pure metal crystallises while the impurities pass on the adjacent melted zone.
Furnaces.
In the extraction of metal different types of furnaces are used. Each Charge
furnace has its own characteristics. Some principal furnaces have been
described below,
(1) Blast furnace : It is a special type of tall cylindrical furnace, 200-250° C Waste gases
about 100 feet high with a diameter of 15-28 feet. It is made of steel sheets 400° C
lined inside with fire-proof bricks. The charge is added through a cup and 600° C Refractory
cone arrangement at the top. At the upper part of the furnace there is a hole 700° C bricks
for the escape of the waste gases of the furnace. There are two outlets in the Hot air
hearth of the furnace, one for tapping the molten metal and the other 800-1000° C
above it for the slag. The waste gases are heated and a hot air blast under 1300° C
pressure is blown into the furnace by means of bellows or fans through 1600° C
water cooled nozzles ortuyers. The temperature of the furnace varies from
250oC. to 1500oC. Thus the charge descends slowly into zone of increasing Molten Tuyeres
temperatures. The blast furnace is used for the extraction of metal like slag
copper and iron. Molten
metal
Blast Furnace
(2) Reverberatory Furnace : In this furnace fuel burns in a separate
part and does not mix with the charge. The furnace may be divided into 3 Tie-rod Charge
hopper
parts,
(i) Fire Grate : It is on one side where the fuel burns. Hangers
(ii) Flue or Chimney : It is on the other side of the fire grate. The
waste gases escape through it.
(iii) Hearth : It is the middle part of the furnace where the charge is Air and oil
heated with the flames and hot gases. Furnace charge
The material to be heated is placed on the hearth or bed of the Silica Magnesite
furnace and is heated by the hot gases or flames produced by the burning Modern reverberatory furnace
of fuel. The waste gases escape out of the chimney. Since the fuel does not
come in contact with the charge, the furnace is very suitable for calcination and roasting and is employed for both
oxidising and reducing purposes. For oxidation, the material is heated by the current of hot air while for reduction
the material is mixed with coke and heated. The furnace find wide application in the extractive metallurgy.
(3) Electric Furnace : The fuel burnt furnaces described in this chapter produce temperature in the range of
1000-1500oC. Although these furnaces have the great utility in the extraction of metals yet these are unsuitable
where higher temperatures are needed. One commonly used electric furnace is Heroult’s furnace shown in fig. It
consists of a steel shell lined inside with dolomite or magnesite. It is provided with movable water jacketed
electrodes suspended from the roof or from the sides. Heat is generated by striking an arc between the electrodes,
thereby, a temperature of over 3000oC may be reached. The charge melts and the impurities e.g., Si, Mn, P and S
etc. present in the ore combine with the basic lining to form slag, which is free from sulphur or gas bubbles. Steel of
very fine quality is prepared by this method. Electric furnaces are largely used where,
(i) Cheap power supply is available. (ii) High temperature are required. (iii) Pure product are required.
As such they find wide applicability in a number of industries such as metallurgy, ceramices plastics chemical
and also in the research laboratories. These furnaces are easily operated and involve the problem of the storage of
fuel and disposal of fuel waste.
(4) Muffle Furnace : In this furnace the material to be heated does not come in the contact with the fuel or
flames. A muffle is a chamber made of refractory material and is surrounded by flames
and hot gases on all sides. The products of combustion are removed through a door Flames and hot gases
provided in the furnace. Muffle furnace is used for the extraction of zinc, preparation of Muffle
red lead, Pb3O4 and for testing the purity of precious metals like silver and gold. In an
electric muffle furnace the chamber is surrounded by resistance coils.
(5) Bessemer Converter : A Bassemer converter is a pear–shaped 10 or more Fire
feet high, open at the top, lined with a refractory material such as silica or magnesia place
Muffle furnace
which also acts as a flux. The converter is mounted on trunnions, so that it can be
tilted to collect the products formed. There is an arrangement of introducing a hot blast of air from a number of small
openings in the bottom of the furnace. The converter is used mostly for manufacturing of copper of steel from pig iron.
Passing a current of hot air into the molten metal taken in the converter, the impurities are oxidised and escaped as
gases or from slag. The Bessemer process is rapid one and does not take more than 15 minutes in the production of
one bath.
(6) Regenerative Furnace : These are furnaces in which the heat of the gases escaping out from the chimney is
utilized. Most of the furnaces particularly blast furnaces are fitted up with regenerated system which means an economy
of the fuel. A flowing column of air is heated by the hot flue gases, it is then brought back to the fire and returned to the
furnace. This furnace is largely used in the production of steel.
Refractory materials.
The materials which can withstand very high temperatures without melting or becoming soft are known as
refractory materials. These are not affected by slags formed during the extraction of metals. These are used in the form of
bricks for the internal linings of furnaces. Refractory materials used are of three types,
(1) Acid refractories : Silica, quartz, silicious sand stones, etc., are the examples.
(2) Basic refractories : Lime, dolomite, magnesite, etc., are the examples.
(3) Neutral refractories : Graphite, chromite, bone ash, etc., are the examples.
Silica (92% SiO2, 2.7% Al2O3) and quartz, can tolerate temperatures upto about 1750°C, bauxite upto 1800°C,
alumina upto 2000°C and magnesite, chromite, etc., upto 2200°C. Some carbides such as silicon carbide is used as
refractory for special purposes.
Alloys.
A metallic product containing two or more metals or sometimes one of the ingredients a non–metal provided
that the mixture is homogenous and possesses metallic properties, is known as an alloy. Alloys are usually prepared
by melting two or more metals together in the proportions and then allowing the melting to solidify. If one of the
metals is mercury the alloy is known as amalgam.
Alloys are prepared with a view to impart some desirable properties which the individual metals do not
possess. These are,
(1) Change in the chemical reactivity : Sodium acts vigorously with water, but Na–Hg amalgam reacts
slowly to suit the requirement of a number of chemical reactions.
(2) Hardness : Silver, gold and soft metals but become hard when alloyed with copper.
(3) Melting Points : Melting points of an alloy may be higher or lower than any of its components. Wood-
metal, which is an alloy of Bi, Pb, Sn and Cd fuses at 60.5oC., while none of these metals fuses at this low
temperature.
(4) Change of colour : Aluminium bronze is an alloy of aluminium and copper. It is of golden, yellow colour
and is used in making decoration articles, jewellery and coins while the colour of aluminium is white and that of
copper is red.
(5) Corrosion resistance : Iron gets corroded soon whereas stainless Steel, an alloy of iron and chromium,
resists corrosion.
(6) Casting : An alloy of lead and antimony is known as type metal is used for casting type required in
printing works.
Alloys of Al
Alloy Percentage Important Properties Uses
Aluminium Light, strong alloy with golden Coins, utensils, jewellary picture frames
bronze Al 95% lustre, resistant to corrosion etc.
Magnalium Cu 5% Light, tough and strong Light instruments, balance beam,
Al 95% pressure cookers etc.
Duralumin Mg 5% Light, tough, ductile, resistant to Making aeroplanes automobile parts
Al 95% corrosive action pressure cookers etc.
Cu 4%
Mg 0.5%
Mn 0.5%
Alloys of Ag
Alloy Percentage composition Uses some
Coinage silver Ag = 90, Cu = 10 For making silver coins.
Silver solder Ag = 63, Cu = 30, Zn = 7 For soldering and joining metals
Dental alloy Ag=33, Hg=52, Sn=12.5, Cu=2.0, Zn=0.5 For filling teeth
Silver palladium Ag = 40, Pd = 60 Potentiometers and winding of
special instruments.
Alloys of Pb and Sn
Alloy Percentage Composition Uses
Solder Pb = 50, Sn = 50 For soldering.
Pewter Pb = 20, Sn = 80 In making cups, mugs and other utensils.
Type metal Pb = 70, Sb = 20 and Sn = 10 For making printing type.
Rose metal Pb = 22, Sn = 28, Bi = 50 For making electric fuses.
Britannia metal Sn = 90, Sb = 8, Cu = 2 For making table wares.
Alloys of Copper
Alloy Percentage Composition Uses
Cu = 80, Zn = 20 For making utensils, condenser tubes, wires parts
Brass
Bronze or Copper Cu = 80, Zn = 10, Sn = 10 of machinery etc.
bronze For making cooking utensils, statues, coins etc.
Aluminium bronze Al = 95, Cu = 5 Coins, picture frames, cheap jewellery
For making gun barrels.
Gun metal Cu = 90, Sn = 10 For making bells, gongs etc.
For electrical apparatus
Bell metal Cu = 90, Sn = 20 For making silver wire, resistance wires etc.
For making acid pumps and acid containers.
Constantan Cu = 60, Ni = 40 For making springs, electrical equipment
For making gold coins, jewellery, watch cases,
German silver Cu = 60, Zn = 20, Ni = 20 spectacle rims etc.
Monel metal Cu = 30, Ni = 67, Fe and Mn = 3
Phosphor bronze Cu = 95, Sn = 4.8, P = 0.2
Gold-copper alloy Au = 90, Cu = 10
Alloys of Iron
Name Percentage Properties Uses
Stainless
steel Fe = 73%, Cr = 18%, Resists corrosion For making utensils, cutlery and
Magnanese Ni = 8% and carbon ornamental pieces.
steel Fe = 86%, Mn = 13% Very hard, resistant to wear For Making rock drills, safes etc.
Tungsten and carbon and tear.
steel Fe = 94%, W = 5% and Retains hardness even at For making high speed cutting tools.
Invar carbon high temperatures
Fe = 64%, Ni = 36% Practically no coefficient of For making watches, meter scales,
Nickel steel expansion. pendulum rods etc.
Fe = 98–96%, Ni = 2– Resistant to corrosion, hard For making wire cables, gears, drive
Permalloy 4% and elastic. shafts etc.
Fe = 21%, Ni = 78% and Strongly magnetised by For making electromagnets, ocean
Chrome steel carbon electric current, loses cables etc.
magnetism when current is
Alnico Fe = 98–96%, Cr = 2– cut off. For making axles, ball bearings and
4% High tensile strength cutting tools such as files.
Fe = 60%, Al =12%, For making permanent magnents.
Ni = 20%, Co = 8% Highly magnetic
d and f block elements
d-Block Elements: Introduction.
A transition element may be defined as an element whose atom in the ground state or ion in common
oxidation state has incomplete sub-shell, has electron 1 to 9. It is called transition element due to fact that it is
lying between most electropositive (s-block) and most electronegative (p-block) elements and represent a transition
from them. The general electronic configuration of these element is (n − 1)1to10 ns 0 to 2 .
The definition of transition metal excludes Zn, Cd and Hg because they have complete d- orbital. Their
common oxidation state is Zn++ , Cd ++ , Hg ++ . They also do not show the characteristics of transition element.
Zn, Cd, Hg are called non typical transition element. Some exceptional electronic configuration of
transition element are : Cr = 3d 5 4s1 , Nb = 4d 4 5s1 , Pb = 4d10 5s 0 , Ag = 4d10 5s1 , Cu = 3d10 4s1 ,
Mo = 4d 5 5s1 , Ru = 4d 7 5s1 , Pt = 5d 0 6s1 , Au = 5d10 6s1 . These irregularities can be explain on the basis of half
filled and full filled stability of d-orbital.
Classification : Transition element are classified in following series :
(1) First transition series (3d) = 21 to 30 i.e. Sc to Zn .
(2) Second transition series (4d) = 39 to 48 i.e. Y to Cd .
(3) Third transition series (5d) = 57 La and 72 – 80 Hf to Hg .
(4) Fourth transition series or 6d series = 89 Ac and 104 – 112 Rf .
So there are 39 transition element at present in the periodic table.
12 Transition Elements 18
H (d-block elements) 13 14 15 16 17 He
3 4 5 6 7 8 9 10 11 12
s-block 21 22 23 24 25 26 27 28 29 30 p-block elements
elements Sc Ti V Cr Mn Fe Co Ni Cu Zn
39 40 41 42 43 44 45 46 47 48
Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
* 57 72 73 74 75 76 77 78 79 80
La Hf Ta W Re Os Ir Pt Au Hg
* 69 104 105 106 107 108 109 110
58-71 Lanthanide series (Lanthanides)
90-103 Actinide series (actinides)
Physico- Chemical Properties of d-Block Elements .
(1) Atomic radii : The atomic, radii of 3d-series of elements are compared with those of the neighbouring
s-and p-block elements.
K Ca Sc Ti V Cr Mn Fe Co Ni Cn Zn Ga Ge
227 197 144 132 122 117 117 117 116 115 117 125 135 122*
* in pm units
The atomic radii of transition elements show the following characteristics,
(i) The atomic radii and atomic volumes of d-block elements in any series decrease with increase in the atomic
number. The decrease however, is not regular. The atomic radii tend to reach minimum near at the middle of the
series, and increase slightly towards the end of the series.
Explanation : When we go in any transition series from left, to right, the nuclear charge increases gradually by
one unit at each elements. The added electrons enter the same penultimate shell, (inner d- shell). These added
electrons shield the outermost electrons from the attraction of the nuclear charge. The increased nuclear charge tends
to reduce the atomic radii, while the added electrons tend to increase the atomic radii. At the beginning of the series,
due to smaller number of electrons in the d-orbitals, the effect of increased nuclear charge predominates, and the
atomic radii decrease. Later in the series, when the number of d-electrons increases, the increased shielding effect and
the increased repulsion between the electrons tend to increase the atomic radii. Somewhere in the middle of the series,
therefore the atomic radii tend to have a minimum value as observed.
(ii) The atomic radii increase while going down in each group. However, in the third transition series from
hafnium (Hf) and onwards, the elements have atomic radii nearly equal to those of the second transition elements.
Explanation : The atomic radii increase while going down the group. This is due to the introduction of an
additional shell at each new element down the group. Nearly equal radii of second and third transition series
elements is due to a special effect called lanthanide contraction.
(2) Ionic radii : For ions having identical charges, the ionic radii decrease slowly with the increase in the
atomic number across a given series of the transition elements.
Elements (m): Sc Ti V Cr Mn Fe Co Ni Cu Zn
Lonic radius,(M2+)/pm: -- 90 88 84 80 76 74 72 69 74
Pm:(M3+)/pm: 81 76 74 69 66 64 63 - --
Explanation : The gradual decrease in the values of ionic radius across the series of transition elements is
due to the increase in the effective nuclear charge.
(3) Ionisation energies : The ionisation energies of the elements of first transition series are given below:
Elements Sc Ti V Cr Mn Fe Co Ni Cu Zn
lonisation E.
I1 632 659 650 652 716 762 758 736 744 906
I2 1245 1320 1376 1635 1513 1563 1647 1756 1961 1736
I3 2450 2721 2873 2994 3258 2963 3237 3400 3560 3838
* in kJ mol-1
The following generalizations can be obtained from the ionisation energy values given above.
(i) The ionisation energies of these elements are high, and in the most cases lie between those of s- and d-
block elements. This indicates that the transition elements are less electropositive than s-block elements.
Explanation : Transition metals have smaller atomic radii and higher nuclear charge as compared to the
alkali metals. Both these factors tend to increase the ionisation energy, as observed.
(ii) The ionisation energy in any transition series increases in the nuclear with atomic number; the increase
however is not smooth and as sharp as seen in the case of s- and p-block elements.
Explanation : The ionisation energy increases due to the increase in the nuclear charge with atomic number
at the beginning of the series. Gradually, the shielding effect of the added electrons also increases. This shielding
effect tends to decrease the attraction due to the nuclear charge. These two opposing factors lead to a rather gradual
increase in the ionisation energies in any transition series.
(iii) The first ionisation energies of 5d-series of elements are much higher than those of the 3d and 4d series
elements.
Explanation : In the 5d-series of transitions elements, after lanthanum (La), the added electrons go to the
next inner 4f orbitals. The 4f electrons have poor shielding effect. As a result, the outermost electrons experience
greater nuclear attraction. This leads to higher ionisation energies for the 5d- series of transition elements.
(4) Metallic character : All the transition elements are metals. These are hard, and good conductor of heat
and electricity. All these metals are malleable, ductile and form alloys with other metals. These elements occur in
three types e.g., face- centered cubic (fcc), hexagonal close-packed (hcp) and body-centered cubic (bcc), structures.
Explanation : The ionisation energies of the transition elements are not very high. The outermost shell in
their atoms have many vacant/ partially filled orbitals. These characteristics make these elements metallic in
character. The hardness of these metals, suggests the presence of covalent bonding in these metals. The presence of
unfilled d-orbitals favour covalent bonding. Metallic bonding in these metals is indicated by the conducting nature of
these metals. Therefore, it appears that there exists covalant and metallic bonding in transition elements.
(5) Melting and boiling points : The melting and boiling points of transition elements except Cd and Hg,
are very high as compared to the s-block and p-block elements. The melting and boiling points first increase, pass
through maxima and then steadily decrease across any transition series. The maximum occurs around middle of the
series.
Explanation : Atoms of the transition elements are closely packed and held together by strong metallic bonds
which have appreciable covalent character. This leads to high melting and boiling points of the transition elements.
The strength of the metallic bonds depends upon the number of unpaired electrons in the outermost shell of the
atom. Thus, greater is the number of unpaired electrons stronger is the metallic bonding. In any transition element
series, the number of unpaired electrons first increases from 1 to 5 and then decreases back to the zero .The
maximum five unpaired electrons occur at Cr (3d series). As a result, the melting and boiling points first increase
and then decrease showing maxima around the middle of the series.
The low melting points of Zn, Cd, and Hg may be due to the absence of unpaired d-electrons in their atoms.
(6) Enthalpies of atomization : Transition metals exhibit high enthalpies of atomization.
Explanation : This is because the atoms in these elements are closely packed and held together by strong
metallic bonds. The metallic bond is formed as a result of the interaction of electrons in the outermost shell. Greater
the number of valence electrons, stronger is the metallic bond.
(7) Oxidation states : Most of the transition elements exhibit several oxidation states i.e., they show variable
valency in their compounds. Some common oxidation states of the first transition series elements are given below in
table,
Outer electronic configurations and oxidation states for 3d- elements
Elements Outer electronic configuration Oxidation states
Sc 3d1 4s2 + 2, + 3
Ti 3d2 4s2
+ 2, + 3, + 4
V 3d3 4s2 + 2,+ 3,+ 4,+ 5
Cr 3d5 4s1 + 1, + 2, + 3, + 4, + 5, + 6
Mn 3d54s2 + 2, + 3, + 4, + 5, + 6, + 7
Fe 3d64s2 + 2, + 3, + 4, + 5, + 6
Co 3d74s2 + 2, + 3, + 4
Ni 3d84s2 + 2, + 3, + 4
Cu 3d104s1 + 1,+ 2
Zn 3d104s2 +2
Explanation : The outermost electronic configuration of the transition elements is (n - 1)d1-10ns2. Since, the
energy levels of (n-1)d and ns-orbitals are quite close to each other, hence both the ns- and (n-1) d-electrons are
available for bonding purposes. Therefore, the number of oxidation states show by these elements depends
upon the number of d-electrons it has. For example, Sc having a configuration 3d14s2 may show an
oxidation state of + 2 (only s-electrons are lost) and + 3 (when d-electron is also lost). The highest
oxidation state which an elements of this group might show is given by the total number of ns- and (n -1)
d-electrons.
The relative stability of the different oxidation states depends upon the factors such as, electronic
configuration, nature of bonding, stoichiometry, lattice energies and solvation energies. The highest oxidation states
are found in fluorides and oxides because fluorine and oxygen are the most electronegative elements. The highest
oxidation state shown by any transition metal is eight. The oxidation state of eight is shown by Ru and Os.
An examination of the common oxidation states reveals the following conclusions.
(i) The variable oxidation states shown by the transition elements are due to the participation of outer ns and
inner (n -1) d-electrons in bonding.
(ii) Except scandium, the most common oxidation state shown by the elements of first transition series is
+2. This oxidation state arises from the loss of two 4s elements. This means that after scandium, d-orditals
become more stable than the s-orbital.
(iii) The highest oxidation states are observed in fluorides and oxides. The highest oxidation state shown by
any transition elements (by Ru and Os) is 8.
(iv)The transition elements in the + 2 and + 3 oxidation states mostly form ionic bonds. In compounds of the
higher oxidation states (compound formed with fluorine or oxygen), the bonds are essentially covalent. For
example, in permanganate ion MnO4–, all bonds formed between manganese and oxygen are covalent.
(v) Within a group, the maximum oxidation state increases with atomic number. For example, iron shown the
common oxidation state of + 2 and + 3, but ruthenium and osmium in the same group form compounds in the +
4,+ 6,and + 8 oxidation states.
(vi) Transition metals also form compounds in low oxidation states such as +1 and 0. For example, nickle in,
nickel tetracarbonyl, Ni(CO)4 has zero oxidation state.
The bonding in the compounds of transition metals in low oxidation states is not always very simple.
(vii) Ionisation energies and the stability of oxidation states :The values of the ionisation energies can
be used in estimating the relative stability of various transition metal compounds (or ions). For example, Ni2+
compounds are found to be thermodynamically more stable than Pt2+, whereas Pt4+ compounds are more stable
than Ni4+ compounds. The relative stabilities of Ni2+ relative to Pt2+, and that of Pt4+ relative to Ni4+ can be
explained as follows,
The first four ionisation energies of Ni and Pt
Metal (IE1+IE2) kJmol-1, (IE3+IE4) kJ mol-1, Etotal, kJ mol-1
(= IE1 + IE2 + IE3 + IE4)
Ni 2490 8800
Pt 2660 6700 11290
9360
Thus, the ionisation of Ni to Ni2+ requires lesser energy (2490 kJ mol-1) as compared to the energy required
for the production of Pt2+ (2660 kjmol-1). Therefore, Ni2+ compounds are thermodynamically more stable than Pt2+
compounds.
On the other hand, formation of Pt4+ requires lesser energy (9360 kJ mol-1) as compared to that required for
the formation of Ni4+(11290 kJ mol-1). Therefore, Pt4+ compounds are more stable than Ni4+ compounds.
This is supported by the fact that [PtCl6]2– complex ion is known, while the corresponding ion for nickel is not
known. However, other factors which affect the stability of a compound are,
(a) Enthalpy of sublimation of the metal.
(b) Lattice and the solvation energies of the compound or ion.
(8) Electrode potentials (Eo) : Standard electrode potentials of some half–cells involving 3d-series of
transition elements and their ions in aqueous solution are given in table,
Standard electrode potentials for 3d-elements
Elements Ion Electrode reaction E°/ volt
Sc Sc3+ Sc3++ 3e- → Sc - 2.10
Ti Ti2+ Ti2++ 2e- → Ti - 1.60
V V2+ V2++ 2e- → V - 1.20
Cr Cr3+ Cr3+ + 3e- → Cr - 0.71
Mn Mn2+ Mn2++ 2e- → Mn - 1.18
Fe Fe2+ Fe2+ + 2e- → Fe - 0.44
Co Co2+ Co2+ + 2e- → Co - 0.28
Ni Ni2+ Ni2+ + 2e- → Ni - 0.24
Cu Cu2+ Cu2+ + 2e- → Cu + 0.34
Zn Zn2+ Zn2+ + 2e- → Zn - 0.76
The negative values of E° for the first series of transition elements (except for Cu2+/ Cu ) indicate that,
(i) These metals should liberate hydrogen form dilute acids i.e., the reactions,
M + 2H+ → M2+ + H2 (g) ; 2M + 6H+ → 2M3+ + 3H2(g)
are favourable in the forward direction. In actual practice however, most of these metals react with dilute acids
very slowly. Some of these metals get coated with a thin protective layer of oxide. Such an oxide layer prevents the
metal to react further.
(ii) These metals should act as good reducing agents. There is no regular trend in the E° values. This is due to
irregular variation in the ionisation and sublimation energies across the series.
Relative stabilities of transition metal ions in different oxidation states in aqueous medium can be predicted
from the electrode potential data. To illustrate this, let us consider the following,
M(s) → M(g) ∆H1 = Enthalpy of sublimation, ∆Hsub
M(g) → M+(g) + e− ∆H2 = Ionisation energy, IE
M + (g) → M + (aq) ∆H3 = Enthalpy of hydration, ∆Hhyd
Adding these equations one gets,
M(s) → M +(aq) + e−, ∆H = ∆H1 + ∆H2 + ∆H3 = ∆Hsub + IE + ∆Hhyd
The ∆H represents the enthalpy change required to bring the solid metal M to the monovalent ion in aqueous
medium, M+(aq).
The reaction, M(s) → M+(aq) +e-, will be favourable only if ∆ H is negative. More negative is the value is of
∆H , more favourable will be the formation of that cation from the metal. Thus, the oxidation state for which ∆ H
value is more negative will be stable in the solution.
Electrode potential for a Mn+/M half-cell is a measure of the tendency for the reaction, Mn+(aq) + ne– → M (s)
Thus, this reduction reaction will take place if the electrode potential for Mn+/ M half- cell is positive. The
reverse reaction, M(s) → Mn+(aq) + n e-
Involving the formation of Mn+(aq) will occur if the electrode potential is negative, i.e., the tendency for the
formation of Mn+(aq) from the metal M will be more if the corresponding E° value is more negative. In other words,
the oxidation state for which E° value is more negative (or less positive) will be more stable in the
solution.
When an elements exists in more than one oxidation
states, the standard electrode potential (E°) values can be used
in the predicting the relative stabilities of different oxidation
states in aqueous solutions. The following rule is found useful.
The oxidation state of a cation for which
∆H = (∆H sub + lE + ∆Hhyd ) or E° is more negative (for less
positive) will be more stable.
(9) Formation of coloured ions : Most of the
compound of the transition elements are coloured in the solid
state and /or in the solution phase. The compounds of
transition metals are coloured due to the presence of unpaired
electrons in their d-orbitals.
Explanation : In an isolated atom or ion of a transition
elements, all the five d-orbitals are of the same energy (they
are said to be regenerate). Under the influence of the combining anion(s), or electron- rich molecules, the five d-
orbitals split into two (or sometimes more than two) levels of different energies. The difference between the two
energy levels depends upon the nature of the combining ions, but corresponds to the energy associated with the
radiations in the visible region, (λ = 380 − 760nm) . Typical splitting for octahedral and tetrahedral geometries are
shown in fig.
The transition metals in elements form or in the ionic form have one or more unpaired electrons. When visible
light falls on the sample, the electrons from the lower energy level get promoted to a higher energy level due to the
absorption of light of a characteristic wavelength (or colour). This wavelength (or colour) of the absorbed light
depends upon the energy difference of the two levels. Rest of the light gets transmitted. The transmitted light has a
colour complementary to the absorbed colour. Therefore, the compound or the solution appears to be of the
complementary colour. For example, Cu(H2O)26+ ions absorb red radiation, and appear blue-green (blue-green is
complementary colour to red).Hydrated Co2+ ions absorb radiation in the blue-green region, and therefore, appear
red in sunlight. Relationship between the colour of the absorbed radiation and that of the transmitted light is given
in table
Relationship between the colours of the absorbed and transmitted light: the complementary colours.
Colour of the Colour of the
Transmitted light
Absorbed light Transmitted light Absorbed light Red
Blue-green Orange
IR White Blue Yellow
Indigo Yellow-green
Red Blue-green Violet White
UV
Orange Blue
Yellow Indigo
Yellow-green Violet
Green Purple
However, if radiations of all the wavelengths (or colours) except one are absorbed, then the colour of the
substance will be the colour of the transmitted radiation. For example, if a substance absorbs all colours except
green, then it would appear green to the eyes.
The transition metal ions which have completely filled d-orbitals are colourless, as there are no vacant d-
orbitals to permit promotion of the electrons. Therefore, Zn2+ (3d10), Cd2 + (4d10) and Hg2+(5d10) ions are
colourless. The transition metal ions which have completely empty d-orbitals are also colourless, Thus, Sc3+ and
Ti4+.ions are colourless, unless a coloured anion is present in the compound.
Colours and the outer- electronic configurations of the some important ions of the first transition series
elements are given bellow,
Ion Outer configuration Number of unpaired electrons Colour of the ion
Sc3+ 3d0 0 Colourless
Ti3+ 3d1 1 Purple
Ti4+ 3d0 0
V3+ 3d2 2 Colourless
Cr3+ 3d3 3 Green
Mn2+ 3d5 5 Violet
Light pink
Mn3+ 3d4 4 Violet
Fe2+ 3d6 4 Green
Fe3+ 3d5 5 Yellow
Co3+ 3d7 3 Pink
Ni2+ 3d8 2 Green
Cu2+ 3d9 1 Blue
Cu+ 3d10 0 Colourless
Zn2+ 3d10 0 Colourless
(10) Magnetic properties : Most of the transition elements and their compounds show paramagnetism. The
paramagnetism first increases in any transition element series, and then decreases. The maximum paramagnetism is
seen around the middle of the series. The paramagnetism is described in Bohr Magneton (BM) units. The
paramagnetic moments of some common ions of first transition series are given below in Table
Explanation : A substance which is attracted by magnetic filed is called paramagnetic substance. The
substances which are repelled by magnetic filed are called diamagnetic substances. Paramgnetism is due to the
presence of unpaired electrons in atoms, ions or molecules.
The magnetic moment of any transition element or its compound/ion is given by (assuming no contribution
from the orbital magnetic moment).
µs = 4S(S + 1) BM = n(n + 2) BM
where, S is the total spin (n × s) : n is the number of unpaired electrons and s is equal to ½ (representing the
spin of an unpaired electron).
From the equation given above, the magnetic moment (µs ) increases with an increase in the number of
unpaired electrons.
Magnetic moments of some ions of the 3d-series elements
Ion Outer No. of unpaired Magnetic moment (BM)
configuration electrons
Calculated observed
Sc3+ 3d0 0 0 0
Ti 3+ 3d1 1 1.73 1.75
Ti2+ 3d2 2 2.84 2.86
V2+ 3d3 3 3.87 3.86
Cr2+ 3d4 4 4.90 4.80
Mn2 3d5 5 5.92 5.95
+
Fe2+ 3d6 4 4.90 5.0-5.5
Co2+ 3d7 3 3.87 4.4-5.2
Ni2+ 3d8 2 2.84 2.9-3.4
Cu2+ 3d9 1 1.73 1.4-2.2
Zn2+ 3d10 0
0 0
In d-obitals belonging to a particular energy level, there can be at the maximum five unpaired electrons in d5
cases. Therefore, paramagnetism in any transition series first increases, reaches a maximum value for d5 cases and
then decreases thereafter.
(11) Formation of complex ions : Transition metals and their ions show strong tendency for complex
formation. The cations of transition elements (d-block elements) form complex ions with certain molecules
containing one or more lone-pairs of electrons, viz., CO, NO, NH3 etc., or with anions such as, F–, Cl–, CN– etc. A
few typical complex ions are,
[Fe(CN )6 ]4−, [Cu(NH3 )4 ]2+, [Y(H2O)6 ]2+, [Ni(CO)4 ] [ ], Co(NH3 )6 [ ]3+, FeF6 3−
Explanation : This complex formation tendency is due to,
(i) Small size and high nuclear charge of the transition metal cations.
(ii) The availability to vacant inner d-orbitals of suitable energy.
(12) Formation of interstitial compounds : Transition elements form a few interstitial compounds with
elements having small atomic radii, such as hydrogen, boron, carbon and nitrogen. The small atoms of these
elements get entrapped in between the void spaces (called interstices) of the metal lattice. Some characteristics of
the interstitial compound are,
(i) These are non-stoichiometric compounds and cannot be given definite formulae.
(ii) These compounds show essentially the same chemical properties as the parent metals, but differ in
physical properties such as density and hardness. Steel and cast iron are hard due to the formation of interstitial
compound with carbon. Some non-stoichimetric compounds are, Vse0.98 (Vanadium selenide), Fe0.94O, and titanium
nitride.
Explanation : Interstital compounds are hared and dense. This is because, the smaller atoms of lighter
elements occupy the interstices in the lattice, leading to a more closely packed structure. Due to greater electronic
interactions, the strength of the metallic bonds also increases.
(13) Catalytic properties : Most of the transition metals and their compounds particularly oxides have good
catalytic properties. Platinum, iron, vanadium pentoxide, nickel, etc., are important catalysts. Platinum is a general
catalyst. Nickel powder is a good catalyst for hydrogenation of unsaturated organic compound such as,
hydrogenation of oils some typical industrial catalysts are,
(i) Vanadium pentoxide (V2O5) is used in the Contact process for the manufacture of sulphuric acid,
(ii) Finely divided iron is used in the Haber’s process for the synthesis of ammonia.
Explanation : Most transition elements act as good catalyst because of,
(i) The presence of vacant d-orbitals.
(ii) The tendency to exhibit variable oxidation states.
(iii) The tendency to form reaction intermediates with reactants.
(iv) The presence of defects in their crystal lattices.
(14) Alloy formation : Transition metals form alloys among themselves. The alloys of transition metals are
hard and high metals are high melting as compared to the host metal. Various steels are alloys of iron with metals
such as chromium, vanadium, molybdenum, tungsten, manganese etc.
Explanation : The atomic radii of the transition elements in any series are not much different from each
other. As a result, they can very easily replace each other in the lattice and form solid solutions over an appreciable
composition range. Such solid solutions are called alloys.
(15) Chemical reactivity : The d-block elements (transition elements) have lesser tendency to react, i.e.,
these are less reactive as compared to s-block elements.
Explanation : Low reactivity of transition elements is due to,
(i) Their high ionisation energies.
(ii) Low heats of hydration of their ions.
(iii) Their high heats of sublimation.
Some Compounds of d-Block Elements.
Potassium dichromate, (K2Cr2O7)
Potassium dichromate is one of the most important compound of chromium, and also among dichromates. In
this compound Cr is in the hexavalent (+6) state.
Preparation : It can be prepared by any of the following methods,
(i) From potassium chromate : Potassium dichromate can be obtained by adding a calculated amount of
sulphuric acid to a saturated solution of potassium chromate.
2K2CrO4 + H2SO4 → potassiuKm2CdriOch7romate+ K2SO4 + H2O
potassium chromate
yellow orange
K2Cr2O7 Crystals can be obtained by concentrating the solution and crystallisation.
(ii) Manufacture from chromite ore : K2Cr2O7 is generally manufactured from chromite ore (FeCr2O4). The
process involves the following steps.
(a) Preparation of sodium chromate. Finely powdered chromite ore is mixed with soda ash and quicklime.
The mixture is then roasted in a reverberatory furnace in the presence of air. Yellow mass due to the formation of
sodium chromate is obtained.
4FeCr2O4 + O2 → 2Fe2O3 + 4Cr2O3
4Cr2O3 + 8 Na2CO3 + 6O2 → 8 Na2CrO4 + 8CO2(g)
4FeCr2O4 + 8 Na2CO3 + 7O2 → 2Fe2O3 + 8CO2(g ) + 8 Na2CrO4
sodium chromate
The yellow mass is extracted with water, and filtered. The filtrate contains sodium chromate.
The reaction may also be carried out by using NaOH instead of Na2CO3.The reaction in that case is,
4FeCr2O4 + 16NaOH + 7O2 → 8 Na2CrO4 + 2Fe2O3 + 8H2O
(b) Conversion of chromate into dichromate. Sodium chromate solution obtained in step(a) is treated with
concentrated sulphuric acid when it is converted into sodium dichromate.
2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O
sodium chromate sodium dichromate
On concentration, the less soluble sodium sulphate, Na2SO4.10H2O crystallizes out. This is filtered hot and
allowed to cool when sodium dichromate, Na2Cr2O7.2H2O, separates out on standing.
(c) Concentration of sodium dichromate to potassium dichromate. Hot concentrated solution of sodium
dichromate is treated with a calculated amount of potassium chloride. When potassium dichromate being less
soluble crystallizes out on cooling.
Na2CrO7 + 2KCl → K2Cr2O7 + 2NaCl
sod.dichromate pot.dichromate
Physical properties
(i) Potassium dichromate forms orange-red coloured crystals.
(ii) It melts at 699 K.
(iii) It is very stable in air (near room temperature) and is generally, used as a primary standard in the
volumetric analysis.
(iv)It is soluble in water though the solubility is limited.
Chemical properties
(i) Action of heat : Potassium dichromate when heated strongly. decomposes to give oxygen.
4K2Cr2O7 (s) ∆ → 4K2CrO4(s) + 2Cr2O3(s) + 3O2
(ii) Action of acids
(a) In cold, with concentrated H2SO4, red crystals of chromium trioxide separate out.
K2Cr2O7(aq) + conc.H2SO4 → KHSO4(aq) + 2CrO3(s) + H2O
On heating a dichromate-sulphuric acid mixture, oxygen gas is given out.
2K2Cr2O7 + 8H2SO4 → 2K2SO4 + 2Cr2(SO4)3 + 8H2O + 3O2
(b) With HCl, on heating chromic chloride is formed and Cl2 is liberated.
K2Cr2O7(aq) + 14HCl(aq) → 2CrCl3(aq) + 2KCl(aq) + 7H2O + 3Cl2(g)
(iii) Action of alkalies : With alkalies, it gives chromates. For example, with KOH,
K2Cr2O4 + 2KOH → 2K2CrO4 + H2O
orange yellow
On acidifying, the colour again changes to orange-red owing to the formation of dichromate.
2K2CrO4 + H2SO4 → K2Cr2O7 + K2SO4 + H2O
Actually, in dichromate solution, the Cr2O72− ions are in equilibrium with CrO42− ions.
Cr2O72− + H2O 2CrO42− + 2H +
(iv) Oxidising nature : In neutral or in acidic solution, potassium dichromate acts as an excellent oxidising
agent, and Cr2O72− gets reduced to Cr3+. The standard electrode potential for the reaction,
Cr2O72− + 14H+ + 6e− → 2Cr+3 + 7H2O is +1.31 V. This indicates that dichromate ion is a fairly strong
oxidising agent, especially in strongly acidic solutions. That is why potassium dichromate is widely used as an
oxidising agent, for quantitative estimation of the reducing agents such as, Fe2+. It oxidises,
(a) Ferrous salts to ferric salts
K2CrO7 + 4H2SO4 → K2SO4 + Cr2(SO4 )3 + 4H2O + 3[O]
2FeSO4 + H2SO4 + [O] → Fe2[SO4 ]3 + H2O × 3
K2Cr2O7 + 6FeSO4 + 7H2SO4 → K2SO4 + Cr2(SO4 )3 + 3Fe2(SO4 )3 + 7H2O
Ionic equation: Cr2O72− + 14H + + 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O
(b) Sulphites to sulphates and arsenites to arsenates.
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4 )3 + 4H2O + 3[O]
Na2SO3 + [O] → Na2SO4 ] × 3
K2Cr2O7 + 4H2SO4 + 3Na2SO3 → K2SO4 + Cr2(SO4 )3 + 3Na2SO4 + 4H2O
Ionic equation: Cr2O72− + 8H + + 3SO32− → 2Cr3+ + 3SO42− + 4H2O
Similarly, arsenites are oxidised to arsenates.
Cr2O72− + 8H + + 3 AsO33− → 2Cr3+ + 3 AsO43− + 4 H2O
(c) Hydrogen halides to halogens.
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4 )3 + 4H2O + 3[O]
2HX + O → H2O + X2] × 3
K2Cr2O7 + 4H2SO4 + 6HX → K2SO4 + Cr2(SO4 )3 + 7H2O + 3X2
where, X may be Cl, Br, I.
Ionic equation : Cr2O72− + 8H + + 6HX → 2Cr 3+ + 3X 2 + 7H 2O
(d) Iodides to iodine
K2Cr2O7 + H2SO4 → K2SO4 + Cr2(SO4 )3 + 4H2O + 3[O]
2KI + H 2O + [O] → 2KOH + I 2 ] × 3
2KOH + H2SO4 → K2SO4 + 2H2O] × 3
K2Cr2O7 + 7H2SO4 + 6KI → 4K2SO4 + Cr2(SO4 )3 + 3I2 + 7H2O
Ionic equation : Cr2O72− + 14H + + 6I − → 2Cr 3+ + 7H 2O + 3I 2
Thus, when KI is added to an acidified solution of K2Cr2O7 iodine gets liberated.
(e) It oxidises H2S to S.
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3[O]
H2S + [O] → H2O + S] × 3
K2Cr2O7 + 4H2SO4 + 3H2S → K2SO4 + Cr2(SO4 )3 + 7H2O + 3S
Ionic equation : Cr2O72− + 8H + + 3H2S → 2Cr3+ + 3S + 7H2O
(v) Formation of insoluble chromates : With soluble salts of lead, barium etc., potassium dichromate gives
insoluble chromates. Lead chromate is an important yellow pigment.
2Pb(NO3 )2 + K2Cr2O7 + H2O → 2PbCrO4 + 2KNO3 + 2HNO3
(vi) Chromyl chloride test : When potassium dichromate is heated with conc. H2SO4 in the presence of a
soluble chloride salt, the orange-red vapours of chromyl chloride (CrO2Cl2) are formed.
K2Cr2O7 + 4 NaCl + 6H2SO4 heat → 2KHSO4 + 4 NaHSO4 + 2CrO2Cl2
chromyl chloride
(orange −red vapours )
Chromyl chloride vapours when passed through water give yellow-coloured solution containing chromic acid.
CrO2Cl2 + 2H2O → 2HCl + H2CrO4
Chromic acid.(yellow solution)
Chromyl chloride test can be used for the detection of chloride ion is any mixture.
Uses : Potassium dichromate is used as,
(i) An oxidising agent
(ii) In chrome tanning
(iii) The raw meterial for preparing large number of chromium compounds
(iv) Primary standard in the volumetric analysis.
Structures of Chromate and Dichromate Ions
Chromates and dichromates are the salts of chromic acid (H2CrO4). In solution, these ions exist in equilibrium
with each other. Chromate ion has four oxygen atoms arranged tetrahedrally around Cr atom. (see Fig).
Dichromate ion involves a Cr–O–Cr bond as shown in Fig.
Potassium Permanganate, (KMnO4)
Potassium permanganate is a salt of an unstable acid HMnO4 (permanganic acid). The Mn is an +7 state in
this compound.
Preparation : Potassium permanganate is obtained from pyrolusite as follows.
Conversion of pyrolusite to potassium manganate : When manganese dioxide is fused with potassium
hydroxide in the presence of air or an oxidising agent such as potassium nitrate or chlorate, potassium manganate is
formed, possibly via potassium manganite.
MnO2 + 2KOH fused → K2MnO3 + 4H2O] × 2
potassium manganite
2K2MnO3 + O2 → 2K2MnO4 + 2 H2O
2MnO2 + 4 KOH + O2 fused → potass2iuKm2MmnaOn4ganate+ 2H2O
pyrolusite
dark green mass
−
Oxidation of potassium manganate to potassium permanganate : The potassium manganate so
obtained is oxidised to potassium permanganate by either of the following methods.
By chemical method : The fused dark-green mass is extracted with a small quantity of water. The filtrate is
warmed and treated with a current of ozone, chlorine or carbon dioxide. Potassium manganate gets oxidised to
potassium permanganate and the hydrated manganese dioxide precipitates out. The reactions taking place are,
When CO2 is passed
3K2MnO4 + 2H2O → 2KMnO4 + MnO2 ↓ +4KOH
potassium manganate potassium permanganate
2CO2 + 4KOH → 2K2CO3 + 2H2O
When chlorine or ozone is passed
2K2MnO4 + Cl2 → 2KMnO4 + 2KCl
2K2MnO4 + O3 + H2O → 2KMnO4 + 2KOH + O2(g)
The purple solution so obtained is concentrated and dark purple, needle-like crystals having metallic lustre are
obtained.
Electrolytic method : Presently, potassium manganate (K2MnO4) is oxidised electrolytically. The electrode
reactions are,
At anode: 2MnO42− → 2MnO4− + 2e−
green purple
At cathode: 2H+ + 2e− → H2(g)
The purple solution containing KMnO4 is evaporated under controlled condition to get crystalline sample of
potassium permanganate.
Physical properties
KMnO4 crystallizes as dark purple crystals with greenish luster (m.p. 523 K).
It is soluble in water to an extent of 6.5g per 100g at room temperature. The aqueous solution of KMnO4 has a
purple colour.
Chemical properties : Some important chemical reactions of KMnO4 are given below,
Action of heat : KMnO4 is stable at room temperature, but decomposes to give oxygen at higher
temperatures.
2KMnO4 (s) heat → K2MnO4 (s) + MnO2 + O2(g)
Oxidising actions : KMnO4 is a powerful agent in neutral, acidic and alkaline media. The nature of reaction
is different in each medium. The oxidising character of KMnO4 (to be more specific, of MnO − ) is indicated by high
4
positive reduction potentials for the following reactions.
Acidic medium: MnO4− + 8H+ + 5e− Mn2+ + 4H2O Eo = 1.51 V
Alkaline medium: MnO4− + 2H2O + 3e− MnO2 + 4OH− Eo = 1.23 V
In strongly alkaline solutions and with excess of MnO − , the reaction is
4
MnO4− + e− MnO42− Eo = 0.56 V
There are a large number of oxidation-reduction reactions involved in the chemistry of manganese
compounds. Some typical reactions are,
In the presence of excess of reducing agent in acidic solutions permanganate ion gets reduced to manganous
ion, e.g., 5Fe2+ + MnO4− + 8H + → 5Fe3+ + Mn2+ + 4H2O
An excess of reducing agent in alkaline solution reduces permanganate ion only to manganese dioxide e.g.,
3NO2− + MnO4− + 2OH− → 3NO3− + MnO2 + H2O
In faintly acidic and neutral solutions, manganous ion is oxidised to manganese oxidised to manganese
dioxide by permanganate.
2MnO4− + 3Mn+2 + 2H2O → 5MnO2 + 4H +
In strongly basic solutions, permangante oxidises manganese dioxide to manganate ion.
MnO2 + 2MnO4− + 4OH− → 3MnO42− + 2H2O
In acidic medium, KMnO4 oxidises,
Ferrous salts to ferric salts
2KMnO4 + 3H 2SO4 → K2SO4 + 2MnSO4 + 3H 2O + 5[O]
2FeSO4 + H2SO4 + [O] → Fe2(SO4 )3 + H2O]× 5
2KMnO4 + 8H 2SO4 + 10FeSO4 → K 2SO4 + 2MnSO4 + 5Fe 2 (SO4 )3 + 8H 2O
Ionic equation: 2MnO4− + 16H + + 10Fe 2+ → 2Mn2+ + 10Fe 3+ + 8H 2O
The reaction forms the basis of volumetric estimation of Fe2+ in any solution by KMnO4.
Oxalic acid to carbon dioxide
2KMnO4 + 3H 2SO4 → K 2SO4 + 2MnSO4 + 3H 2O + 5[O]
(COOH )2 + [O] → 2CO2 + H 2O] × 5
2KMnO4 + 3H 2SO4 + 5(COOH )2 → K 2SO4 + 2MnSO4 + 10CO2 + 8H 2O
Ionic equation : 2MnO4− + 6H + + 5(COOH )2 → 2Mn2+ + 10CO2 + 8H 2O
Sulphites to sulphates
2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5[O]
Na2SO3 + [O] → Na2SO4] × 5
2KMnO4 + 3H2SO4 + 5Na2SO3 → K2SO4 + 2MnSO4 + 5Na2SO4 + 3H2O
Ionic equation : 2MnO4− + 6H+ + 5SO32− → 2Mn2+ + 5SO42− + 3H2O
Iodides to iodine in acidic medium
2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5[O]
2KI + H 2O + [O] → I 2 + 2KOH × 5
2KOH + H2SO4 → K2SO4 + 2H2O ] × 5
2KMnO4 + 8H2SO4 + 10KI → 6K2SO4 + 2MnSO4 + 5I2 + 8H2O
Ionic equation : 2MnO4− + 16H+ + 10I − → 2Mn2+ + 5I2 + 8H2O
Hydrogen peroxide to oxygen
2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5[O]
H2O2 + [O] → H2O + O2 ↑ ×5
2KMnO4 + 3H2SO4 + 5H2O2 → K2SO4 + 2MnSO4 + 8H2O + 5O2
Manganous sulphate (MnSO4) to manganese dioxide (MnO2)
2KMnO4 + H2O → 2KOH + 2MnO2 + 3[O]
MnSO4 + H2O + [O] → MnO2 + H2SO4 × 3
2KOH + H2SO4 → K2SO4 + 2H2O
2KMnO4 + 3MnSO4 + 2H2O → 5MnO2 + K2SO4 + 2H2SO4
Ionic equation : 2MnO4− + 3Mn2+ + 2H2O → 5MnO2 + 4H+
Ammonia to nitrogen
2KMnO4 + H 2O → 2MnO2 + 2KOH + 3[O]
2NH 3 + 3[O] → N 2 (g) + 3H 2O
2KMnO4 + 2NH 3 → 2MnO2 + 2KOH + 2H 2O + N 2 (g)
Uses : KMnO4 is used,
(i) As an oxidising agent. (ii) As a disinfectant against disease-causing germs. (iii) For
sterilizing wells of drinking water. (iv) In volumetric estimation of ferrous salts, oxalic acid etc.
Structure of Permanganate Ion (MnO4–) : Mn in MnO4– is in +7 oxidation state.
Mn7+ exhibits sp3 hybridisation in this ion. The structure of MnO4– is, shown in fig.
Iron and its Compounds.
(1) Ores of iron : Haematite Fe 2O3 , Magnetite (Fe 3O4 ), Limonite (Fe 2O3 .3H 2O) , Iron pyrites (FeS2 ),
Copper pyrities (CuFeS2 ) etc.
(2) Extraction : Cast iron is extracted from its oxides by reduction with carbon and carbon monoxide in a
blast furnace to give pig iron.
Roasting : Ferrous oxide convert into ferric oxide.
Fe2O3 . 3H 2O → Fe2O3 + 3H 2O ; 2FeCO3 → 2FeO + 2CO2 ; 4FeO + O2 → 2Fe 2O3
Smelting : Reduction of roasted ore of ferric oxide carried out in a blast furnace.
(i) The reduction of ferric oxide is done by carbon and carbon monoxide (between 1473k to 1873k)
2C + O2 → 2CO
Note : The CO is the essential reducing agent.
(ii) Fe 2O3 + 3CO 673K 2Fe + 3CO2 . It is a reversible and oxothermic reaction. Hence according to Le −
chatelier principle more iron will be produced in the furnace at lower temp. Fe 2 O3 + CO → 2FeO + CO2
(it is not reversible)
(iii) FeO + C end1o0t7h3erKm→ic Fe + CO
reaction
Note : The gases leaving at the top of the furnace contain up to 28% CO, and are burnt in cowper's stove
to pre-heat the air for blast
Varieties of iron : The three commercial varieties of iron differ in their carbon contents. These are;
(1) Cast iron or Pig-iron : It is most impure form of iron and contains highest proportion of carbon (2.5–
4%).
(2) Wrought iron or Malleable iron : It is the purest form of iron and contains minimum amount of carbon
(0.12–0.25%).
(3) Steel : It is the most important form of iron and finds extensive applications. Its carbons content
(Impurity) is mid-way between cast iron and wrought iron. It contains 0.2–1.5% carbon. Steels containing 0.2–0.5%
of carbon are known as mild steels, while those containing 0.5–1.5% carbon are known as hard steels.
Steel is generally manufactured from cast iron by three processes, viz, (i) Bessemer Process which involves the
use of a large pear-shaped furnace (vessel) called Bessemer converter, (ii) L.D. process and (iii) open hearth
process, Spiegeleisen (an alloy of Fe, Mn and C) is added during manufacture of steel.
Heat treatment of steels : Heat treatment of steel may be defined as the process of carefully heating the steel
to high temperature followed by cooling to the room temperature under controlled conditions. Heat treatment of
steel is done for the following two purposes,
(i) To develop certain special properties like hardness, strength, ductility etc. without changing the chemical
composition.
(ii) To remove some undesirable properties or gases like entrapped gases, internal stresses and strains. The
various methods of heat treatment are,
(a) Annealing : It is a process of heating steel to redness followed by slow cooling.
(b) Quenching or hardening : It is a process of heating steel to redness followed by sudden cooling by
plunging the red hot steel into water or oil.
(c) Tempering : It is a process of heating the hardened or quenched steel to a temperature much below
redness (473–623K) followed by slow cooling.
(d) Case-hardening : It is a process of giving a thin coating of hardened steel to wrought iron or to a strong
and flexible mild steel by heating it in contact with charcoal followed by quenching in oil.
(e) Nitriding : It is a process of heating steels at about 700 oC in an atmosphere of ammonia. This process
imparts a hard coating of iron nitride on the surface of steel.
Properties of steel : The properties of steel depend upon its carbon contents. With the increase in carbon
content, the hardness of steel increases while its ductility decreases.
(i) Low carbon or soft steels contain carbon upto 0.25%.
(ii) Medium carbon steels or mild steels contain 0.25–0.5% carbon.
(iii) High carbon or hard steels contains 0.1 – 1.5 percent carbon.
(iv) Alloy steels or special steels are alloys of steel with Ni, Cr, Co,W, Mn, V etc., For example – stainless steel
is an alloy of Fe, Cr and Ni and it is used for making automobile parts and utensils. Tool steel is an alloy of
Fe,W, V etc.
Uses of steel: In general, steels are used for making machinery parts, girders, tools, knives, razors, household
utensils, etc. The specific use of steel depend upon the nature of metal added to iron.
Compounds of iron
(1) Oxides of Iron : Iron forms three oxides FeO, Fe2O3 (Haematite), Fe3O4 (magnetite also called
magnetic oxide or load stone).
(i) Ferrous oxide, FeO : It is a black powder, basic in nature and reacts with dilute acids to give ferrous salts.
FeO + H 2SO4 → FeSO4 + H 2O ; It is used in glass industry to impart green colour to glass.
(ii) Ferric oxide Fe2O3 : It is a reddish brown powder, not affected by air or water; amphoteric in nature
and reacts both with acids and alkalis giving salts. It can be reduced to iron by heating with C or CO.
Fe2O3 + 3C → 2Fe + 3CO ; Fe2O3 + 3CO → 2Fe + 3CO2
It is used as red pigment to impart red colour to external walls and as a polishing powder by jewellers.
(iii) Ferrosoferricoxide Fe3O4 (FeO. Fe2O3 ) : It is more stable than FeO and Fe2O3 , magnetic in nature
and dissolves in acids giving a mixture of iron (II) and iron (III) salts.
Fe3O4 + 4H 2SO4 (dil) → FeSO4 + Fe 2 (SO4 )3 + 4H 2O
(2) Ferrous sulphide FeS : It is prepared by heating iron filing with sulphur. With dilute H 2SO4 , it gives
H 2S. FeS + H 2SO4 (dil) → FeSO4 + H 2S ↑
(3) Ferric chloride FeCl3 : It is prepared by treating Fe(OH)3 with HCl
Fe(OH)3 + 3HCl → FeCl3 + 3H 2O
The solution on evaporation give yellow crystals of FeCl 3 . 6H 2O
Properties : (i) Anhydrous FeCl 3 forms reddish-black deliquescent crystals.
(ii) FeCl 3 is hygroscopic and dissolves in H 2O giving brown acidic solution due to formation of HCl
FeCl 3 + 3H 2O → Fe(OH)3 + 3HCl
(Brown)
(iii) Due to oxidising nature Fe 3+ ions FeCl3 is used in etching metals such as copper
2Fe 3+ + Cu → 2Fe 2+ + Cu2+ (aq)
(iv) In vapour state FeCl 3 exists as a dimer, Fe 2Cl6
Cl Cl Cl
Fe Fe
Cl Cl Cl
(4) Ferrous sulphate, FeSO4 , 7H 2O (Green vitriol) : It is prepared as follow ,
Fe + H 2 SO4 → FeSO4 + H 2
(i) One pressure to moist air crystals become brownish due to oxidation by air.
4FeSO4 + 2H 2O + O2 → 4Fe(OH)SO4
(ii) On heating, crystals become anhydrous and on strong heating it decomposes to Fe 2O3 , SO2 and SO3 .
FeSO4 .7H 2O heat → FeSO4 + 7H 2O ; 2FeSO4 Strong → Fe 2 O 3 + SO2 + SO3
heating
(iii) It can reduce acidic solution of KMnO4 and K 2Cr2O7
(iv) It is generally used in double salt with ammonium sulphate.
(NH 4 )2 SO4 + FeSO4 + 6H 2O → FeSO4 .(NH 4 )2 SO4 .6H 2O
Mohr's salt
Mohr’s salt is resistant to atmospheric oxidation.
(v) It is used in the ring test for nitrate ions where it gives brown coloured ring of compound FeSO4 . NO.
FeSO4 + NO → FeSO4 .NO
(5) Mohr's salt FeSO4 .(NH 4 )2 SO4 . 6H 2O : It is also known as ferrous ammonium sulphate and is a light
green coloured double salt.
Copper and its Compounds.
Ores : Copper pyrites (chalcopyrite) CuFeS2 , Cuprite (ruby copper) Cu2O, Copper glance (Cu2S) ,
Malachite [Cu(OH)2.CuCO3 ], Azurite [Cu(OH)2. 2CuCO3 ]
Extraction : Most of copper (about 75%) is extracted from its sulphide ore, copper pyrites.
Concentration of ore : Froth floatation process.
Roasting : Main reaction : 2CuFeS2 + O2 → Cu2S + 2FeS + SO2 .
Side reaction : 2Cu2 S + 3O2 → 2Cu2O + 2SO2 ; 2FeS + 3O2 → 2FeO + 2SO2 .
Smelting : FeO + SiO2 → FeSiO3 (slag) ; Cu2O + FeS → FeO + Cu2S
Note : The mixture of copper and iron sulphides melt together to form 'matte' and the slag floats on its
surface.
Conversion of matte into Blister copper (Bessemerisation) : Silica is added to matte and a hot blast of
air is passed FeO + SiO2 → FeSiO3 (slag) . Slag is removed. By this time most of iron sulphide is removed.
Cu2 S + 2Cu2O → 6Cu + SO2
Note : Blister copper : Which contain about 98% pure copper and 2% impurities (Ag, Au, Ni, Zn etc.)
Properties of copper : It has reddish brown colour. It is highly malleable and ductile. It has high electrical
conductivity and high thermal conductivity. In presence of CO2 and moisture Cu is covered with a green layer of
CuCO3 . Cu(OH)2 . 2Cu + H 2O + CO2 + O2 → CuCO3 .Cu(OH)2 . It undergoes displacement reactions with lesser
reactive metals e.g. with Ag. It can displace Ag from AgNO3 . The finally divided Ag so obtained is black in colour.
Compounds of copper
Cuprous oxide Cu2O : It is a reddish brown powder insoluble in water but soluble in ammonia solution,
where it forms diammine copper (I) ion. Cu+ + 2NH3 → [Cu(NH3 )2 ]+ . It is used to impart red colour to glass in
glass industry.
Cupric oxide CuO : It is dark black, hygroscopic powder which is reduced to Cu by hydrogen, CO etc. It
is used to impart light blue colour to glass. It is prepared by heating copper nitrate.
2Cu(NO3 )2 ∆ → 2CuO + 4 NO2 + O2
Copper sulphate CuSO4 . 5H 2O (Blue vitriol) : It is prepared by action of dil H 2SO4 on copper scrap in
presence of air. 2Cu + 2H 2SO4 +(aOir2) → CuSO4 + 2H 2O
(i) On heating this blue salt becomes white due to loss of water of crystallization.
CuSO4 . 5BHlu2eO 503K → CWuShiOte4 + 5H 2O
At about 1000 K, CuSO4 decomposes to give CuO and SO3 .
CuSO4 1000K → CuO + SO3
(ii) It gives a deep blue solution of tetrammine copper (II) sulphate with NH 4 OH.
Cu2SO4 + 4 NH 4 OH, → [Cu(NH 3 )4 ]SO4 + 4H 2O
Blue colour
(iii) With KCN it first gives yellow precipitate of CuCN which decomposes of give Cu2 (CN)2 . Cu2 (CN)2
dissolves in excess of KCN to give K 3 [Cu(CN)4 ]
2CuSO4 + 4KCN → Cu2 (CN)2 + 2K 2 SO4 + (CN)2
(iv) With KI it gives white ppt. of Cu2 I 2
4 KI + 2CuSO4 → 2K 2 SO4 + WChuite2 Ip2pt.+ I 2
(v) With K 4 [Fe(CN)6 ], CuSO4 gives a reddish brown ppt. of Cu2 [Fe(CN)6 ]
2CuSO4 + K 4 [Fe(CN)6 ] → RCedud2is[hFebr(oCwNn)p6p]t.+ 2K 2 SO4
Uses : For electroplating and electrorefining of copper. As a mordant in dyeing. For making Bordeaux
mixture (11 parts lime as milk of lime + 16 parts copper sulphate in 1,000 parts of water). It is an excellent
fungicide. For making green pigments containing copper carbonate and other compounds of copper. As a
fungicide in starch paste for book binding work.
Cupric sulphide CuS : It is prepared as follows : Cu(NO3 )2 + H 2 S → BClauckSp+pt. 2HNO3 .
Cupric chloride CuCl 2 : It is a dark brown solid soluble in water and its aqueous solution first changes to
green and then to blue on dilution.
Cuprous chloride Cu2Cl2 : It is a white solid insoluble in water and dissolves in conc. HCl due to
formation of H[CuCl 2 ] complex.
Silver and its Compounds.
Ores : Argentite (silver glance) Ag 2S, Horn silver (AgCl), Ruby silver (Pyrargyrite) 3Ag 2S. Sb2S3 .
Extraction : Cyanide process or Mac Arthus-Forrest cyanide process : This method depends on the fact that
silver, its sulphide or chloride, forms soluble complex with alkali cyanides in the silver. This implies that silver
compounds will dissolve in solution of alkali cyanides in the presence of blast of air.
4 Ag + 8 NaCN + 2H 2O + Oair2 ⇌ 4 Na[Ag(CN)2 ] + 4 NaOH
or 4 Ag + 8CN − + 2H 2O + O2 ⇌ 4[Ag(CN)2 ]− + 4OH −
Ag 2S + 4 NaCN ⇌ 2Na[Ag(CN)2 ] + Na2 S
AgCl + 2NaCN ⇌ Na[Ag(CN)2 ] + NaCl.
The reaction with the sulphide is reversible and accumulation of Na2S must be prevented. A free excess of
air is continuously passed through the solution which oxidizes Na2S into sulphate and thiosulphate.
2Na2 + 2O2 + H 2O → Na2 S2O3 + 2NaOH
Na2 S2O3 + 2NaOH + 2O2 → 2Na2 SO4 + H 2O
2Na[Ag(CN)2 ] + 4 NaOH + Zn → Na2 ZnO2 + 4 NaCN + 2H 2O + 2Ag
Compounds of silver : AgNO3 , Ag 2S, AgCl, AgBr, AgI, and AgO.
Gold and its Compounds.
Ores : Bismuthaurite ( BiAu2 ), Syvanite (AgAuTe 2 ) , Calverite (AuTe 2 ) .
Extraction : By cyanide or Mac-Arther forest cyanide process,
4 Au + 8 NaCN + 2H 2O + O2 → 4[NaAu(CN)2 + 4 NaOH]
Sodium aurocyanide
2Na[Au(CN)2 + Zn → Na2[Zn(CN)4 ] + 2Au
Refining : Anode : Crude gold; Cathode : Pure gold.
Electrolytic Solution : Gold chloride in hydrochloric acid.
Plattner chlorine extraction process,
AuCl3 + 3FeSO4 → FeCl 3 + Fe 2 (SO4 )3 + Au
AuCl2 + 3H 2S → 6HCl + 3S + 2Au
Quartation process : Refining of gold carried out by this method. It involves separation of gold and Ag by
H 2SO4 .
Gold is soft and hence for making ornaments it is generally hardened by adding Ag or Cu. The weight of
gold is expressed in terms of Carats. Pure gold is taken as 24 carats.
20 carats means, it contain 20 parts by wt. of gold in 24 parts by wt. of given alloy.
∴ percentage of gold in 20 carat gold sample = 20 × 100 = 250 = 83.33%
24 3
Properties : Gold is not affected by conc. H 2SO4 , conc. HNO3 or by strong alkalis. However it dissolves in
aqua regia to form H[AuCl4 ] ; 2Au + 3HNO3 + 11HCl → 2H[AuCl4 ] + 6H 2O + 3NOCl .
Compounds of gold
AuCl3 : It is a reddish solid soluble in water. It reacts with HCl to give H[Au(Cl)4 ] which is used in toning
process in photography. HCl + AuCl3 → H[Au(Cl)4 ]
Au2S : It is a dark brown solid insoluble in water prepared as follows :
2K[Au(CN)2 ] + H 2 S → Au2 S + 2KCN + 2HCN
Murcury and its Compounds.
Ores : Cinnabar (HgS)
Extraction : Roasting : The concentrated ore roasted at 770 K to 780 K in the pressure of air.
2HgS + 3SO2 → 2HgO + 2SO2 ; 2HgO → 2Hg + O2
Refining : By filtering impure Hg through thick canvass or chamois leather. It is then dropped into 5% HNO3 .
Compounds of Mercury
Mercuric chloride HgCl 2 (Corrosive sublimate) : It is a colourless solid, sparingly soluble in water. It forms
red ppt. of HgI 2 with KI : HgCl2 + 2KI → HgI 2 + 2KCl . With NH 4 OH it gives white ppt. of Hg(NH 2 )Cl.
HgCl 2 + 2NH 4 OH → Hg(NH 2 )Cl+ NH 4 Cl + 2H 2O .
white ppt.
Mercurous chloride Hg 2Cl 2 (Calomel) : It is a white solid insoluble in water. With NH4 OH it forms a
black mixture composed of black metallic mercury and white mercuric amino chloride, Hg(NH 2 )Cl.
Hg 2Cl 2 + 2NH 4 OH → Hg+Hg(NH2)Cl + NH 4 Cl + 2H 2O
Black mixture
It is used as purgative in medicine and it sublimes on heating.
Mercuric iodide HgI 2 : It is a yellow solid below 400K but changes to red solid above 400K.
HgI 2 400 K HgI 2
Red
Yellow
It dissolves in excess of KI forming K2 HgI 4 ; HgI 2 + 2KI → K 2 HgI 4
Alkaline solution of K 2 HgI 4 is called Nessler's reagent.
Zinc and its Compounds.
Ores : Zincite (red zinc ore) ZnO, Franklinite (ZnOFe2O3 ) , Zinc blende (ZnS), Calamine ( Zinc spar)
ZnCO3 .
Extraction : Concentration : Froth floatation
Roasting : ZnS + 3O2 1200K → 2ZnO + 2SO2 ; ZnS + 2O2 ∆ → ZnSO4
2ZnSO4 ∆ → 2ZnO + 2SO2 + O2 .
Reduction of ZnO : The oxide ore is mixed with crushed coke and heated to about 1670K in fire clay retorts
(Belgian process). The crude metal obtained called Zinc spelter.
Refining : By distillation and by electrolytic method
Anode : Spelter; Cathode : Pure zinc wire; Electrolyte : Zinc sulphate.
Note : Zinc is a volatile metal (easily vaporisable)
At ordinary temperature zinc metal is brittle but on heating at 120 – 150o C it is malleable and ductile.
Compounds of zinc
Zinc oxide ZnO : Zincite (ZnO) is also called Philospher's wool. It is white powder, become yellow on
heating and again white on cooling. It is amphoteric in nature. It is used as a white pigment under the name Zinc
white or Chinese white.
Zinc Sulphate (white vitriol), ZnSO4 .7H 2O : It is a colourless transparent crystal highly soluble in water.
It is used as an eye-lotion and for preparing double salts. On heating it looses its molecules of water as,
ZnSO4 .7H 2O 375K → ZnSO4 .H 2O 725K → ZnSO4 1075K → ZnO + SO2 + O2
Tin and Lead .
Tin
Ores of Tin : Cassiterite or tin stone (SnO2 )
• The concentrated and roasted ore is reduced with carbon to get impure tin which is purified by liquation
process.
• Tin forms two series of salts, i.e., Sn (II) and Sn (IV). Whereas Sn (II) salts are ionic Sn (IV) salts are
covalent.
• Tin dissolves in hot conc. NaOH forming Na2SnO3 and evolving H2 gas.
• Tin reacts with conc. HNO3 forming metastannic acid (H 2SnO3 ) .
• Tin is not attacked by organic acids and hence is used for tinning of utensils to resist corrosion. Tin foils are
used for wrapping cigarettes, confectionary items and for making tooth-paste tubes.
• SnO2 is an amphoteric oxide.
• Stannous chloride (SnCl2 ) acts as a good reducing agent. It reduces HgCl 2 to first Hg 2Cl2 and then to
Hg. It also reduces FeCl 3 to FeCl 2 .
• Stannic chloride (SnCl4 ) is a liquid and fumes in air due to hydrolysis. It acts as a Lewis acid and dissolves
in concentration HCl forming H 2SnCl6 .
• SnCl4 .5H 2O is called butter of tin.
• SnS dissolves in yellow ammonium sulphide.
Lead
Ores of lead : Galena (PbS), Anglesite (PbSO4) and Cerussits (PbCO3 ).
• Lead is extracted from galena. The ore is concentrated by froth-floatation process and roasted when a part
of the ore is converted into PbO and PbSO4 . The unchanged galena then brings about the reduction of PbO or
PbSO4 to Pb.
• Lead dissolves in hot conc. NaOH forming sodium plumbite (Na2 PbO2 ) and evolving H2 gas.
• Leads forms two series of salts, i.e., Pb (II) and Pb (IV) but Pb (II) compounds are more stable than Pb (IV)
compounds. Lead (II) compounds are essentially ionic while lead (IV) compounds are covalent.
• Lead is used in making bullet shots, lead accumulators, tetraethyl lead (antiknocking agent) and a number
of pigments such as red lead (Pb3O4 ) , white lead or basic lead carbonate [2 PbCO3 .Pb(OH)2 ] and lead chromate
(PbCrO4 ) .
• Litharge is PbO . It is obtained by heating Pb(NO3)2 or PbCO3 . It is an amphoteric oxide and is reduced
back to Pb by H 2 , C and CO.
• Red lead (Pb3O4 ) or Sindhur is a mixed oxide (2PbO.PbO2 ) . It acts as an oxidising agent and as such
oxidises HCl to Cl2.
• Lead dioxide (PbO2 ) is obtained either by treating Pb3O4 with conc. HNO3 or by treating lead acetate
with bleaching powder. It acts as an oxidising agent and oxidises HCl to Cl2.
• The ionic character of lead dihalides decreases the order: PbF2 > PbCl2 > PbBr2 > PbI 2 .
• PbF4 and PbCl4 are stable while PbBr4 and PbI 4 are however, unknown. The non-existence of
PbBr4 and PbI 4 is due to strong oxidising character of Pb4+ ions and reducing character of Br − and I − ions.
• PbF4 is ionic while PbCl4 is a volatile liquid.
• Lead is readily corroded by water containing dissolved air forming Pb(OH)2 which has appreciable
solubility in water. This action of water on lead called is Plumbosolvency. 2Pb + 2H 2O + O2 → 2Pb(OH)2
Whereas organic acids, NH + salts and nitrates increase while salts like carbonates, phosphate and sulphates
4
decrease Plumbosolvency. Hard water, however, has no solvent action on lead.
Lanthanides and Actinides.
Lanthanides and actinides are collectively called f-block elements because last electron in them enters into f-
orbitals of the antepenultimate (i.e., inner to penultimate) shell partly but incompletely filled in their elementary or
ionic states. The name inner transition, elements is also given to them because they constitute transition series with
in transition series (d-block elements) and the last electron enters into antepenultimate shell (n-2) f. In addition to
incomplete d-subshell, their f-subshell is also incomplete. Thus, these elements have three incomplete outer shells
i.e., (n–2), (n–1) and n shells and the general electronic configuration of f-block elements is
(n–2) f 1−14 (n − 1)d 0−10ns 2 .
(1) Lanthanides : The elements with atomic numbers 58 to 71 i.e. cerium to lutetium (which come
immediately after lanthanum Z = 57) are called lanthanides or lanthanones or rare earths. These elements
involve the filling of 4 f-orbitals. Their general electronic configuration is, [Xe]4 f 1−14 5d 0−10 6s 2 . Promethium (Pm),
atomic number 61 is the only synthetic (man made) radioactive lanthanide.
Properties of lanthanides
(i) These are highly dense metals and possess high melting points.
(ii) They form alloys easily with other metals especially iron. e.g. misch metal consists of a rare earth element
(94–95%), iron (upto 5%) and traces of S, C, Ca and Al, pyrophoric alloys contain Ce (40–5%), La + neodymium
(44%), Fe (4–5%), Al (0–5%) and the rest is Ca, Si and C. It is used in the preparation of ignition devices e.g., trace
bullets and shells and flints for lighters.
(iii) Oxidation state : Most stable oxidation state of lanthanides is + 3. Oxidation states + 2 and + 4 also
exist but they revert to + 3 e.g. Sm2+ , Eu2+ , Yb 2+ lose electron to become + 3 and hence are good reducing
agents, where as Ce4+, Pr4+, Tb4+ in aqueous solution gain electron to become + 3 and hence are good oxidizing
agents. There is a large gap in energy of 4 f and 5 d subshells and thus the number of oxidation states is limited.
(iv) Colour : Most of the trivalent lanthanide ions are coloured both in the solid state and in aqueous
solution. This is due to the partly filled f-orbitals which permit f–f transition. The elements with xf electrons have a
similar colour to those of (14 – x) electrons.
(v) Magnetic properties : All lanthanide ions with the exception of Lu3+, Yb3+ and Ce 4+ are paramagnetic
because they contain unpaired electrons in the 4 f orbitals. These elements differ from the transition elements in that
their magnetic moments do not obey the simple “spin only” formula µeff = n(n + 2) B.M. where n is equal to the
number of unpaired electrons. In transition elements, the orbital contribution of the electron towards magnetic
moment is usually quenched by interaction with electric fields of the environment but in case of lanthanides the 4 f-
orbitals lie too deep in the atom for such quenching to occur. Therefore, magnetic moments of lanthanides are
calculated by taking into consideration spin as well as orbital contributions and a more complex formula
µeff = 4S(S + 1) + L(L + 1) B.M.
which involves the orbital quantum number L and spin quantum number S.
(vi) Complex formation : Although the lanthanide ions have a high charge (+3) yet the size of their ions is
very large yielding small charge to size ratio i.e., low charge density. As a consequence, they have poor tendency to
form complexes. They form complexes mainly with strong chelating agents such as EDTA, β -diketones, oxine etc.
No complexes with π -bonding ligands are known.
(vii) Lanthanide contraction : The regular decrease in the size of lanthanide ions from La 3+ to Lu3+ is
known as lanthanide contraction. It is due to greater effect of the increased nuclear charge than that of the screening
effect.
Consequences of lanthanide contraction
(a) It results in slight variation in their chemical properties which helps in their separation by ion exchange
(b) Each element beyond lanthanum has same atomic radius as that of the element lying above it in the group
(e.g. Zr 145 pm, Hf 144 pm); Nb 134 pm, Ta 134 pm ; Mo 129 pm, W 130 pm).
(c) The covalent character of hydroxides of lanthanides increases as the size decreases from La 3+ to Lu3+ .
However basic strength decreases. Thus La(OH)3 is most basic whereas Lu(OH)3 is least basic. Similarly, the
basicity of oxides also decreases in the order from La 3+ to Lu3+ .
(d) Tendency to form stable complexes from La 3+ to Lu3+ increases as the size decreases in that order.
(e) There is a slight increase in electronegativity of the trivalent ions from La to Lu.
(f) Since the radius of Yb3+ ion (86 pm) is comparable to the heavier lanthanides Tb, Dy, Ho and Er,
therefore they occur together in natural minerals.
(2) Actinides : The elements with atomic numbers 90 to 103 i.e. thorium to lawrencium (which come
immediately after actinium, Z = 89) are called actinides or actinones. These elements involve the filling of 5 f-
orbitals. Their general electronic configuration is, [Rn]5 f 1−14 6d 0−17s 2 .
They include three naturally occuring elements thorium, protactinium and uranium and eleven transuranium
elements or transuranics which are produced artificially by nuclear reactions. They are synthetic or man made
elements. All actinides are radioactive.
Properties of actinides
(i) Oxidation state : The dominant oxidation state of actinides is +3 which shows increasing stability for the
heavier elements. Np shows +7 oxidation state but this is oxidising and is reduced to the most stable state +5. Pu
also shows states upto +7 and Am upto +6 but the most stable state drops to Pu (+4) and Am (+3). Bk in +4
state is strongly oxidising but is more stable than Cm and Am in 4 state due to f7 configuration. Similarly, No is
markedly stable in +2 state due to its f14 configuration. When the oxidation number increases to + 6, the actinide
ions are no longer simple. The high charge density causes the formation of oxygenated ions e.g., UO22+ , NpO22+ etc.
The exhibition of large number of oxidation states of actinides is due to the fact that there is a very small energy gap
between 5f, 6d and 7s subshells and thus all their electrons can take part in bond formation.
(ii) Actinide contraction : There is a regular decrease in ionic radii with increase in atomic number from Th
to Lr. This is called actinide contraction analogous to the lanthanide contraction. It is caused due to imperfect
shielding of one 5 f electron by another in the same shell. This results in increase in the effective nuclear charge
which causes contraction in size of the electron cloud.
(iii) Colour of the ions : Ions of actinides are generally coloured which is due to f – f transitions. It depends
upon the number of electrons in 5 f orbitals.
(iv) Magnetic properties : Like lanthanides, actinide elements are strongly paramagnetic. The magnetic
moments are lesser than the theoretically predicted values. This is due to the fact that 5 f electrons of actinides are
less effectively shielded which results in quenching of orbital contribution.
(v) Complex formation : Actinides have a greater tendency to form complexes because of higher nuclear
charge and smaller size of their atoms. They form complexes even with π-bonding ligands such as alkyl phosphines,
thioethers etc, besides EDTA, β-diketones, oxine etc. The degree of complex formation decreases in the order.
M 4+ > MO22+ > M 3+ > MO2+
where M is element of actinide series. There is a high concentration of charge on the metal atom in MO22+
which imparts to it relatively high tendency towards complex formation.
Coordination chemistry and organometalics
Double Salts and Co-ordination Compounds.
When solutions of two or more stable compounds are mixed in stoichiometric (simple molecular) proportions
new crystalline compounds called molecular or addition compounds are formed. These are of two types :
(1) Double salts, (2) Co-ordination or Complex compounds
(1) Double salts : Addition compounds, stable in solid state. Dissociate into ions in aqueous solution as such
give test for each constituent ion. Examples:
Double Salt Responds test for the ions
Carnalite : KCl. MgCl2 6H 2O K +, Mg2+, Cl−
Potash alum : K 2SO4 . Al2 (SO4 )3 24H 2O K +, Al3+, SO42−
(2) Co-ordination or Complex compounds : Addition compound, stable in solid state. Retain their identity even
in solution. Central metal ion form dative or coordinate bond with the species surrounding it (ligands). Examples :
Complex compound Cation Anion
[Cu(NH3)4 ]SO4 [Cu(NH3)4 ]+2 SO42−
K 2[PtF6 ] [PtF6 ]2−
2K + [Cr (CN)6 ]3−
[Co (NH 3 )6 ][Cr (CN 3 )6 ]
[Co(NH 3 )6 ]2+
Terminology of co-ordination compounds.
(1) Central metal atom or ion : A complex ion contains a metal atom or ion known as the central metal
atom or ion. it is sometimes also called a nuclear atom.
(2) Complex ion : It is an electrically charged radical which is formed by the combination of a simple
cation with one or more neutral molecules or simple anions or in some cases positive groups also.
(3) Ligands : Neutral molecules or ions that attach to central metal ion are called ligands. The donor atom
associated with the ligands supplies lone pair of electrons to the central metal atom (forming dative bond) may be
one or two more. Monodentate (one donor atom), bidentate (two donor atom), tridenatate (three donor atom) etc.
Monodentate Ligands (with one donor site)
Anionic Ligands (Negative legands)
Formula Name Formula Name
X− Halo Peroxo
Hydroxo O22− Acetato
Cyano Nitrato
: OH − Oxo CH 3COO− Thiosulphato
Amido Nitrito
CN − Sulphido NO3− Carbonato
Thiocyanato Sulphato
O 2− S 2 O32−
NH − NO −
2 2
S 2− CO32−
CNS − SO42−
Neutral Ligands
Formula Name Formula Name
CO Carbonyl : NH 3 Amminato
H2O
PH 3 Phosphine C6 H5 N : Aqua
NO Nitrosyl Pyridine (py)
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