6 CHEMICAL ENERGETICS
• Fossil fuels . .1111111.11.1,1;.,,. , . . .
usedto ma ke bitumenfor
Coal, o il and natural gas arc all cxampks o f fossil surfac in g roa ds
fuels. The term foss il focl s is d erived fro m the fa ct
that th ey arc fo rmed fro m d ead plants and animals b Usesofthedifferentfractionsobtainedfromcrudeoil.
whi ch were foss ilised over 2 00 milli on years ago
during the carbo niferous era. Figure 6.5 Piece of coal sho'Ning a fossilised leaf.
Coal was produced by the actio n o f pressure and
heat o n dead wood fro m an cient fo rests which o nce
grew in the swampland in man y parts o f th e world
und er the prevailing weather conditi on s o f that tim e.
Wh en d ead trees fell into the swamps they were
buried by mud. This prevented aerobic d ecay (which
takes place in the presence o f oxygen ). Over millio ns
o f years, du e to m m 'cm cnt o f the Earth's crust as well
as to changes in climate, the land sank and the decaying
wood becam e covered by '-·ven m ore layers o f mud and
sand . Anaerobic d ecay (which takes place in the absence
of oxygen ) occurred , and as time passed the g radually
forming coal becam e m o re and m o re compressed as
o ther m aterial was laid d own above it ( Fig ure 6 .5 ).
Fossil fuels
Figure 6.6 Cutting coal today is extremely mechanised. Figure6.7 Oil production in the North Sea.
O ver millio ns o f years, as the layers o f fo rming
coal were pll sh ed d eeper and the pressure and
temperature increased , th e fin al conversio n to coal
too k place (Fig ure 6 .6 ). DiffCre nt types o f coal were
fo rm ed as a result o f different pressures being applied
during its fo rmation. For example, anthracite is a
hard coal with a high carbon content, typical o f coal
produ ced at g reater d epths. Table 6.1 sh ows some
o f the different types o f coal alo ng with their carbo n
contents.
Tabl e 6.1 Thedifferentcoaltypes.
Type of coal Carbon content/% Figure 6.8 Natural gas and oil are trapped under non-porous rock.
Anthracite 90
Bituminous coal 60 Questions
Lignite 40
Peat 20 1 Coal, oil and natural gas are all termed 'fossil fuels'. Why is
the word 'fossil' used in this context?
Oil and gas were fo rmed during th e sam e perio d
as coal. It is believed that o il and gas were form ed 2 a Name the process by which plants convert carbon
fro m th e rem ains o f pl ants, anim als and bacteria that dioxide and water into glucose.
o nce lived in seas and lakes. This m aterial sank to the
botto m o f these seas and lakes and becam e covered in b What conditions are necessary for this process to occur?
mlld , sand and silt which thicken ed with tim e . 3 Draw a flow diagram to represent the formation of coal, oil
An aerobic d ecay took place and , as the mud layers or gas.
built up, hig h temperatures and pressures were
created which converted th e material slowly into oil
and gas. As rock fo rmed , earth m ovements ca LJ sed it
to bu ckle and split, and the o il and gas were trapped
in fo lds beneath layers o f no n-porous rock or cap-
rock (Fig ures 6 .7 and 6 .8 ).
6 CHEMICAL ENERGETICS
• What is a fuel? Fi gure 6.9 A Hongkong Electric Co. Power Station, Hong Kong, China.
A foci is a substan ce which can be co nveni ently u sed
as a source o f energy. Fossil fud s release energy in the
form o f heat when they undergo combustion.
fossil foci + oxygen ----> carbon dioxide + water + energy
For example, natural gas burns readily in air
(Chapter 14, p. 220).
methane+ oxygen ----> carbo n + water+ energy
dioxide
CH 4(g) + 202(g) - C02(g) + 2H20(1)
It sho uld be noted that natural gas, like crud e o il ,
is a mixture o f hydrocarbo ns su ch as m ethane ( main
constituent), ethane and propane, and may also
contain som e sulfur. Th e sulfor content varies fr om
source to source. Nah1ral gas obtained fro m th e
N o rth Sea is quite low in sulfur.
The perfect foci would be:
• cheap
• available in large quantities
• sak to sto re and transport
• easy to ig nite and burn , causing no pollution
• capable o f releasing large am ounts o f en ergy.
Solid fuels are safer than volatile liquid fuel s like
petrol and gaseous fuels like natural gas.
Question t r ansfo rmer
Figure6 .10 Thewayin w hichfuelsareusedtoproduceelectricity.
1 'We have not yet found the perfect fuel. Discuss this
statement. Other maj or u ses o f the foss il fuel s arc:
How are fossil fu els used ? • as major feed stock (raw material ) fo r the
chemi cals and pharmaceuticals indu stries
A major use o f fossil fuel s is in the productio n o f
electricity. Coal, o il and natural gas arc burned • or d o mesti c and industrial heating and cooking
in power statio ns ( Figure 6.9) to h eat water to • as fuel s for vario u s fo rms o f vehicle transport.
produ ce steam , which is then u sed to drive large
turbines ( Figure 6.10). A high propo rtio n o f the Question
electricity generated worldwide is d erived fr o m
fossil fuel s. H owever, it sho uld be n o ted that th e 1 'Fossil fuels are a major feedstock for the chemical and
relative importan ce o f the three maj or fossil fuel s is pharmaceutical industries.' With reference to Chapters 14
chang in g . Coal and o il arc becoming less important and 15, give examples which support this statement.
while natural gas is increasin gly important.
In a power statio n, the turbine drives a generato r
to prod uce electricity which is then fed into the
natio nal grid (Figure 6.10 ). The natio nal g rid is
a system for distributing electricity th rough o ut a
c o u n t r y.
Alternative sources of energy
• Alternative sources ?
of energy
°ur"a"n'i"um'oa~t om
Fossil fods arc an c.xampk of non -renewable
resources, so called because they arc no t being Q Q neutron
replaced at the same rate as they arc being u sed up. 00
Fo r example, we have approximately 60 years' supply O fragments
of crud e o il remaining, dependin g o n th e way in
which it is extracted and th e rate at which it is u sed,
maybe mo re (Table 6.2 ). It is important to use
n on-ren ewable fud s very carefully and to con sider
alternative, renewable sources o f energy for u se n ow
and in the future.
Table 6.2 Estimates of how long our fossil fuels will last.
Fossil fuel Estimated date it is expected to run out Figure6.12 Chain reaction inuranium-235fission.
2110
Oil 2080
Cool 2500
Nuclear power three further n eutron s. These three neutron s split
three more atoms ofuranium-235 , which produces
Calder Hall power statio n in C umbria, UK, o n the nine neutrons and so o n. This initiates a chain
site o f th e present-day nuclear power complex at reaction (Fig ure 6.12 ).
Sdlafidd ( Fig ure 6.11 ), opened in 1956 and was
the first nuclear reacto r in the world to produce In a reacto r the fi ssio n process cann ot be allowed
electricity o n an indu strial scale. to get out o f control as it d ocs in an ato mic bomb.
To prevent this, boron control rods can be pushed
Figure 6.11 The nuclear power complex at Sellafield, Cumbria, UK w as into difti:rcnt positions in th e reactor to absorb
thefirsttoproduceelectricityinthev,,orld. some of the neutrons which arc produced and so
slow d own th e chain reactio n. If this is d o n e, the
Nuclear reactors harness the energy fro m the fi ssio n energy released from the reactio n is obtain ed in a
of uranium-235. Nuclear fission occurs when the m o re controlled way. The energy is u sed to prod uce
unstable nu cleus of a radioactive isoto pe sp lits up , steam, which in turn is used to generate electricity
forming smaller atoms and producing a large a.mount ( Fi gure 6.10, p. 92).
o f en ergy as a result. Scientists believe that the en ergy
com es fro m the conversio n o f some o f the mass o f H owever, there arc problems. The main problem
the isotope. associated with a nuclear power stati o n is that the
reactor produces hig hly radi oactive waste materials.
This fi ssio n process begins wh en a neutron hits an These waste materials arc difficult to store and cann o t
atom of uranium-235 , ca using it to split and produce be disposed o f very easily. Also, leaks of radioactive
material have occurred at vario u s sites th rou gh o ut
the world. Accidents at a small number o f nuclear
power statio ns, su ch as Three Mile Island in the US
( 1979) and C hern o byl in the Ukraine ( 1986) have
led to a g reat deal o f concern about their safety. More
recently, in March 2011 a maj or nuclear accident
happened at the Fukushima nucl ear power plant in
no rth ern Japan. On this occasion th e accident was
no t caused as a result of the plant itself undergoing a
problem, but as a result o f an earthquake near Japan
that gave rise to a tsunami. This damaged essential
6 CHEMICAL ENERGETICS
equipment at the plant which was u sed to en sure the Biomass and biog as
temperatures were maintain ed at a cor rect level. \.York
is still goin g o n at th e plant to rem ove the nuclear \Vh en any bi o logical material , whether plant o r
material safely. animal, is converted into en ergy, this en ergy is call ed
biomass energy. It can be taken fr om animal or
plant materials in different ways:
• by burning it, for example wood ( Fig ure 6.15 )
Figure 6.13 Workers wearing protective suits and masks at the Figure 6.15 Biomass energy is produced by burning wood.
Fuku shima power plant.
• by pressing o ut oils that can b e burned
Hydroelectric power • by fermenting it to produ ce fuel s su ch as ethano l
H ydroelectri c power (HEP) is electrici ty generated or methane.
fro m th e cncrgy of fallin g water (Fi gure 6 . 14). It is
an excellent en ergy source, and electricity has been At least 50% o f the world 's po pulatio n rely o n wood
generated in thi s way fo r som e tim e. Fo r example, as their main en ergy source.
the H oover Dam in the US provid es HEP fo r
the surrounding area. It is a very cheap source o f In India there arc milli on s o f methane generators .
electricity. Once you have built th e po wer statio n , M ethane gen erated by the digestio n o f animal waste
the energy is abso lutely free. In some m o untain o us is called bioga.s. The bi ogas produ ced is u sed fo r
areas of the world , such as the Alps, HEP is cooking, heating and lig hting. The by-product o f
the main source o f electricity. One o f the main this process is an excellent fertiliser.
advantages o f this system is that it can be qui ckly
u sed to supplement th e natio nal g rid at tim es o f Som e countries have already experimented with
hig h demand. A disadvantage o f HEP schem es is ethano l as a fuel for cars. Up to 20% o f ethano l
that they o ften require valleys to be fl ood ed and can be add ed to petrol witho ut th e need to adju st
communiti es to be moved. the carburetto r. Brazil , which has few o il reserves,
produces ethanol by fermentatio n (breakdown
by en zym es ) o f sug ar cane and g rain , and uses it
as a petro l additive ( Fig ure 6.1 6 ). Th e Brazilian
governm ent has cut d own its petrol imports by up
to 60% throug h using this alco ho l/petrol mixture.
Figure 6.14 A hydroelectric pov,,er sta tion. Figure6.16 lnBra zilcarsuseanethanoVpetrolmixtu re.
Chemical energy
Hydrogen - t he fu el of th e future • Chemical energy
It is widely believed th at hyd rogen will play an We o btain o ur energy need s fro m the combustio n o f
impor tant part as a fuel in the futur e. It is alread y fuels, su ch as hydrocar bon s, fr o m the combustio n o f
bein g triall ed by m ajo r m o tor manufacturers food s and fr om many o ther chemical reactio ns.
as an altern ative to foss il fu els su ch as petro l
( Fig ure 6 .1 7) . Co m bustio n
Wh en natural gas burn s in a plentiful supply of air it
prod u ces a large amo unt o f energy.
methane+ oxygen------> carbon + water + heat
dio xid e energy
CH4(g) + 202(g) ------>C02(g) + 2H20 (t) + heat
energy
During this process, the complete combustion
o fm ctham:, heat is g iven o ut. It is an exothermic
reacti on. If o nly a limited supply o f air is available
then the reactio n is n ot as exothermic and the
poison ou s gas carbo n m o n oxid e is produ ced.
methane + oxygen------> carbon + water + heat
mo no xid e energy
2C H4(g)+ 302(g) ------> 2C O(g) + 4H20 (1) + heat
energy
This is the incomplete combustion o f m ethane.
The energy chan ges that take place durin g a
ch emical reactio n can be shown by an energy level
diagram. Fig ure 6 .18 shows the cncrgy level diagram
fo r the complete combustio n o f mcthan c.
Fi gure6 .17 Thiscarispaweredbyhydrogeng as. 1
H yd rogen burn s cleanly to form water: progress of reaction
Fi gure 6.18 Energy level diagram for the complete combustion of
h ydrogen + oxygen -----+ wate r methane.
2H 2(g) + 0 2 (g) - 2H2 0 (!)
When an y reacti o n occurs, the ch emical b ond s
H en ce virtually n o exh au st pollutio n is created. Also, in the reactants have to be broken - this requires
hyd rogen can be o btained in an alm ost unlimited energy. This is an cndo thermic process . \,Vh cn the
supply fro m water by elect rolysis. H owever, at n cw bo nds in the products arc form ed , energy is
present it is a costly alternati ve. given o ut ( Figure 6 .19). This is an exothermic
p rocess. The bon d energy is defined as the amo unt
Questions o f energy in kilo joulcs ( kJ ) associatcd with the
breakin g o r making o f o ne m ole o f chemical bo nds
1 Draw up a table showing t he alternat ive sources of energy in a m olecular clem ent o r compound.
along with t heir advantages and disadvantages.
2 What is meant by t he terms:
a non-renewable energy sources?
b ren ewable energy sources?
3 Use your research skills to obtain technical information
about the operation of Pelamis Wave Energy Converter.
6 CHEMICAL ENERGETICS
.. ••••i{ ~+;?' G @@G®
BREA K BONDS
MAKE NEW BONDS
gives o ut en ergy
~H
progress of react ion
Figure 6.19 Breaking and forming bonds during the combustion of methane.
Usin g th e data in T able 6 .3, whi ch tells u s how Energy difference
mu ch en ergy is need ed to break a ch emical bo nd
and h ow much is g iven o ut w hen it fo rm s, we can energy required energy given o ut wh en
calculate h ow much energy is involved in each stage.
to break bo nds bonds arc made
~ 2734 - 3462
Tabl e 6.3 Bondenergydata. ~ -728 kJ
Bond Bond energy/ kJmol- 1 The negative sig n sh ows that the chemicals arc
C-H 435 losing en ergy to th e surroundings, that is, it is an
exothermic reactio n . A positive sign would indicate
O=O 497 that th e chemi cals arc gaining energy fro m th e
C=O 803 surroundings. This type o f reaction is called an
H--0 464 endo thermic reactio n.
347
c-c The energy stored in the bonds is called the
C--0 358 enthalpy and is given the sym bol H . The change in
energy going fr om reactan ts to prod ucts is called the
Bond breaking change in enthalpy and is shown as 11H (prono unced
Breakin g 4 C-H bo nds in methane requires
4 X 4 35 ~ ] 74Qkj ' ddta H ' ). 11 H is called the heat o f reaction .
Breaking 2 O=O bonds in oxygen rcquin:s Fo r an exothermic reactio n 11 H is negative and fo r
2 X 497 ~ 994kj
an en dothermic reaction 11 H is positive.
Total= 2734kJ of energy
W hen fu els, su ch as m ethan e, arc burned they
Making bonds
M akin g 2 C=O bo nds in carbon diox ide gives out require energy to start the chemical reactio n . This is
2 X 803 ~ ] 6Q6 kj
known as the activation en ergy, EA ( Figure 6.20 ).
M aking 4 0-H bo nds in water gives o ut
4 X 464 ~ ] 856 kj In the case of m ethane reacting with oxygen , it
Total= 3462 kJ of energy is some o f the energy involved in the initial bond
breaking ( Figure 6 .2 0 ). The value o f the activation
en ergy will vary fr o m foci to fu el.
Changes of state
activation Enthalpy of neutralisation (molar
energy(EA) heat of neutralisation)
1 This is th e enthalpy ch ange that takes place when
l m o l o f h ydrogen ions (H+(aq )) is n eutralised.
progress of reaction
Figure6.20 Energyleveldiagramformethane/oxygen. W (,q ) + OH-(,q) - H20 (1) 11H ~ - 57 kJ m o J- 1
End othermic reactions arc much less comm o n than This p rocess occurs in the titratio n o f an alkali b y
exothermic o nes . In this type o f reactio n energy is an acid to produ ce a neutral so lutio n (C hapter 8,
absorbed fro m the surroundings so that the en ergy p . 124).
o f the produ cts is greater than that o f the reactants .
The reaction between nitrogen and oxygen g ases Questions
is end othermic (Fig ure 6.21 ), and th e reactio n is
fa voured b y high temperatures . 1 Using the bond energy data given in Table 6.3:
a Calculate the enthalpy of combustion of ethanol, a fuel
nitrogen+ oxygen---,. nitrogcn ( n ) oxide added to petrol in some countries.
b Draw an energy level diagram to represent this
N2 (g) + 0 2(g) - 2NO(g) +!1H combustion process.
c How does this compare with the enthalpy of combustion
progress of reaction o-4f 8h5e3ptkaJnme o(C1-71H?14), a major component of petrol, of
d How much energy is released per gram of ethanol and
Figure6.21 Energyleveldiagramfornitrogen/oxygen. heptane burned?
Di ssolvin g is o ft en an end othermic p rocess . Fo r 2 How much energy is released if:
example, wh en amm onium nitrate dissolves in a 0.5 mole of methane is burned?
water the temperature o f the water fall s, indicating b 5 moles of methane are burned?
that energy is being taken fro m the surroundin gs. c 4g of methane are burned?
Photosynthesis and th ermal d ecom positio n arc (A.: C = 12; H = 1)
other examples of end oth ermi c processes.
3 How much energy is released if:
In equations it is usual to express the !J.H valu e in a 2 moles of hydrogen ions are neutralised?
units o fkJ m o l-1. For example : b 0.25 mole of hydrogen ions is neutralised?
c 1 mole of sulfuric acid is completely neutralised?
CH4(g) + 202(g) - C0 2(g) + 2H 20 (1)
!J.H= -728kJ mo l- 1 • Changes of state
This !J.H value tells us that when l mo le of m ethane In Ch apter 1, p. 4 , we discu ssed th e melting and
is burned in oxygen, 7 28 kJ o f en ergy arc released. boiling o fa substance. The heating curve for water
This valu e is called th e enthalpy of combustion (or is shown in Fig ure 1. 11 o n p. 5. For ice to melt
molar heat of combustion) of methane. to produ ce liquid water, it must absorb energy
fro m its surroundings. This energy is used to break
d own the weak fo rces between the water mo lecules
(interm olecular forces) in the ice. Thi s energy is
called the enthalpy of fusion and is given the symbo l
L\.Hfusion· Similarly, when liquid water ch anges into
steam, the en ergy required fo r this process to occur
is called the enthalpy of vaporisation and is given
the sym bol L\ll.,ap· Figure 6.22 sh ows an energy
level diagram representing both the fu sion and the
vapo risatio n processes .
6 CHEMICAL ENERGETICS
vaporisation steam zinc dissolves in the dilute sulfuric acid , prod ucing
zinc ions (Zn2+(•q)) and releasing two elect rons.
Zn (s) _,. Zn2+(aq ) + 2e-
+ 2.28kJg- 1 Th csc electro ns produ ced at th e zinc elect rod e
4 1.04kJmo l- 1 cause it to beco m e th e n egative terminal. Th ey
(&1-/vap) fl ow t o th e positive coppe r terminal th rou g h
th e external circuit via the bulb , w hich g lows.
fusion liquid water Bubbles o f h yd rogen arc seen when th e electrons
arri ve at the copper elect rod e. The hyd roge n gas
ice J ;~J~~~~~1 is produ ced fr o m th e h yd rogen io ns in the acid ,
w hich co llect the electrons appearin g at the copper
(6.H1uJ electrode.
progress of reacti on
2H'(,q) + 2e- - H2(g)
Fi gure 6.22 Energy level diagram for the fusion and vaporisation of water.
Slowly, the zinc electro d e di ssolves in th e acid and
Quest ions th e bulb will th en go o ut. If th e zin c is replaced
by a m o re reactive m etal, su ch as m agn esium , then
1 Describe the energy changes which take place when the the bulb g lows m ore bri g htly. Mag n esium loses
processes described in this section, with water, are reversed . elect ro ns m o re easil y as it reacts fas ter with the
dilute acid.
2 Using the knowledge you have obtained from Chapter 1,
p. 5, give a full definition of the enthalpy of fusion and Th e differen ce in th e reactivity bet ween th e two
enthalpy of vaporisation for water. m e tals used in the ce ll crea tes a particular vo ltage
readin g o n th e vo ltm eter sh own in Fig ure 6 .23 b.
• Cells and batteries Th e m ore the two m e tals differ in reactivity, th e
larger is th e vo ltage shown and delivered by th e
A chemical cell is one which produces d cctrical energy cell. This meth od ca n be used t o co nfirm th e
fro m chemical energy. The electrical energy is produced o rd er o f reactivity o f th e m etals (C hapte r 10,
as a result o f the transfer of electro ns which takes place p. 150 ) . O the r t ypes o f ch e mi ca l cell in co mm o n
during the chemical reaction via a rcdox process. use arc dr y cells used in radi os , to rch es , and so
o n , and lead - acid accumul at o rs u se d in m o t o r
A simple type of chemi cal cell is that shown in vehicles . These arc co nvenie nt and po rta b le
Figure 6.23a . In this cell the m ore reactive m etal energy sources.
a A simplechemical cell. b Thevoltagereflectsthedifferenceinreactivityofthemetals.
Fi g u r e 6 .23
Cells and batteries
Fu el ce lls the transfer o f 0 2 , H 2 and water throu gh th em
(Figure 6.25 ). As 0 2 gas is passed into the cathod e
Scientists h ave fo und a mu ch m o re effi cient way regio n o f the cell it is reduced:
o f ch ang ing chemical energy into el ectrical energy,
using a fu el cell (Figure 6.24 ). Fuel cel ls arc like The OH- ion s fo rmed arc rem oved fro m the fo ci
the ch emical cells in the previo u s section , except cell b y reactio n with H 2 :
that the reagents arc supplied continu o usly to the
elect rodes . The reagents are usually hyd rogen and The electro n s produced by this process pass around
oxygen . The foci cell principl e was fir st discovered an external circuit to the cathode.
by Sir William Grove in 1839.
(- ) external circuit (+)
H,O
H, in -----.. ---o,in
anode made OS- cath ode made
from a porous from a porous
material, material with
impregnated cobalt oxide
with platinum or platinum
catalyst catalyst
Figure 6.24 The space shuttle's computers use electricity produced by H,O
fuel cells. oot
W hen he was elect rolysing water and h e switched electrolyte of
o ff the power supply, he no ticed that a current still NaOH(~
flowed but in the reverse directio n. Subsequ ently, Figure6.25 A diagrammat icviewofafuelcell.
the process was explained in terms o f the reactio ns
at the electrod es' surfaces o f the oxygen and Quest i ons
hyd rogen gases which had b een produ ced during
the elect rolysis. 1 Describe how simple chemical cells can be used to confirm
the order of reactivity of the metals in the reactivity series.
The hyd rogen foci cells used by N ASA in th e
US space programme arc a.bout 7 0% effi cient and , 2 The fuel cell was discovered during electrolysis experiments
since the o nly produ ct is water, they arc pollutio n with water. It is the reverse process which produces the
free. The aqueous Na OH electro lyte is kept within electricity. Write a balanced chemical equation to represent
the cell by electrod es whi ch arc porous, allowing the overall reaction taking place in a fuel cell.
•
6 CHEMICA L ENERGETICS
Check li st • Enthalpy of vaporisation The enthalpy change that
takes place when one mole of liquid is changed to one mole
After studying Chapter 6 you should know and understand the of vapour at the same temperature.
following terms.
• Exothermic reaction A chemical reaction that releases
• Aerobic decay Decay which takes place in the presence heat energy into its surroundings.
of oxygen.
• Fossil fuels Fuels, such as coal, oil and natural gas,
• Anaerobic decay Decay which takes place in the absence formed from the remains of plants and animals.
of oxygen.
• Hydrocarbon A substance which contains atoms of
• Bond energy An amount of energy associated with a carbon and hydrogen only.
particular bond in a molecular element or compound .
• Non-renewable energy sources Sources of energy,
• Chain reaction A nuclear reaction which is self-sustain ing such as fossi l fuels, which take millions of years to form and
as a result of one of the products causing further reactions. which we are using up at a rapid rate.
Nuclear fi ssion The disintegration of a radioactive
• Chemical cell A system for converting chemical energy to nucleus into two or more lighter fragments. The energy
electrical energy. released in the process is called nuclear energy.
Combustion A chemical reaction in which a substance Oil refining The general process of converting the
reacts rapidly with oxygen with the production of heat and mixture that is collected as crude oil into separate fractions.
light. These fractions, known as petroleum products, are used
Endothermic reaction A chemical reaction which absorbs as fuels, lubricants, bitumens and waxes. The fractions
heat energy from its surroundings. are separated from the crude oil mixture by fractional
Enthalpy Energy stored in chemical bonds, given the distillation.
symbol H.
• Organic compounds Substances whose molecules
• Enthalpy change Given the symbol t,.H, it represents the contain one or more carbon atoms covalently bonded with
difference between energies of reactants and products. another element (including hydrogen, nitrogen, oxygen, the
halogens as well as phosphorus, silicon and sulfur).
• Enthalpy of combustion The enthalpy change which
takes place when one mole of a substance is completely • Renewable energy Sources of energy which cannot be
burned in oxygen. used up or which can be made at a rate faster than the rate
of use.
• Enthalpy of fusion The enthalpy change that takes place
when one mole of a solid is changed to one mole of liquid
at the same temperature.
• Enthalpy of neutralisation The ent halpy change which
takes place when one mole of hydrogen ions is completely
neutralised.
Q Chemical energetics
• Additional questions b Name each o f th e frac tio n s A t o H and g ive a
use fo r each.
1 a St ate w hich o f th e fo llowing processes is
e ndo the rmic and whic h is exothe rmic. c W hy d o the fra ctio ns com e fr o m the
( i) Th e breaking of a chemical bo nd. frac tio na tin g column in this o rde r?
(ii) Th e forming of a chemical bond.
d What is the connectio n between your answer to
b The t a ble below sh ows the b ond en e rgy data fo r c and the size o f the m o lecules in each fra ction ?
a series o f covalent bond s.
( i) U se the in fo rm atio n given in the table to e Which o f th e frac tio ns w ill be the m ost
calculate the overall enthalpy c han ge fo r the flammabl e?
combustion o f ethan o l produ cing carbo n
dio xide and water. 3 Explain the fo llowing .
( ii) Is th e process in (i) endo thermic or a H yd roelectric power is a relatively cheap source
exothe rmic? of electricity.
b G eoth erm al e ne rgy is a no n -po llutin g fo rm o f
Bond Bond energy/ e ne rgy.
kJmo1- 1 c A disadvantage o f wind power is that it cau ses
C- H 435 no ise po llutio n.
O=O 497 d The by-prod uc t fro m the process b y w hich
C=O 803 me tha ne is gen era ted by the digestion o f animal
H----0 464 wast e is an excelle nt fertiliser.
C- C 347 e The fi ssio n o furanium -235 in a nuclear react o r
C----0 358 is an example o f a c hain reactio n.
f Tidal- and wave -gene rated electricity has a maj o r
2 C rud e oil is a mi xture of hyd rocarbo ns. 1l1c enviro nmental disadvantage.
refinin g of crud e oil producr.:s fractio ns which
arc m o re u scfol t o u s tha n c rud e oil itself. Each 4 One o f th e first prac tical c he mical cells was the
fr.ictio n is composed of hyd rocarbo ns whi ch Daniell cell invented by Jo hn Daniell in 1836. A
have boiling points within a specific range of diag ram o f thi s type o f cell is sh own below.
temperature. The separation is carried o ut in a
frac tio nating column , as shown below. (+ )
fraction copper
A
•8 copper(11)
sulfate
a W hich separatio n techniqu e is used to separate so l u t i o n
th e frac tio n s?
porous
pot
It is capa ble o f gene rating ab out 1.1 volts a nd
was used t o o perate small electrical item s su ch as
d oorbells.
a The elect rod e reactio n ta king place at a copper
ano de is:
Cu2+(aq) + 2 e- --,. Cu (s)
Write an electrod e equatio n for the p rocess
taking place at the cath ode.
6 CHEMICAL ENERGETICS
b W hich way would the electro n s fl ow in the wire Th e burner was weigh ed initially and after th e
connected t o the voltmet e r - fro m 'copper t o d esired temperature rise had b een o btain ed.
zin c' o r 'zinc to copper'?
Volume o f water in glass beaker = 200 cm 3
c Why sh o uld coppcr(n ) sulfatc crystallise at the Mass o f ethano l burner at start = 85.3 g
bo tto m of the o uter container? Mass o f ethano l burner at end = 84.8 g
d What is the fun ctio n o f the porou s pot? Temperature rise o f water = 12 cc
e There arc problem s associated with the Daniell ( Density o f water = 1 g cm-3 )
cell whi ch have led to it being replaced by other H eat energy = m ass o f x 4 .2 x temperature
types o f cell. Give t wo reasons why Daniell cells given t o water/g J g - 1 cc- 1 rise/cc
arc n o lo nger in u se to day. water
5 This questio n is abo ut endo thermic and a C alculate the m ass o f ethano l burned.
e xo thermic reacti o ns. b C alculate the am ount o f heat produced, in joules,
a Explain the meaning of the terms endo thermic
and exothermic. in this experiment by the ethan ol burning.
b (i) Draw an energy level diagram fo r the reaction: c Con vert your an swer to b into kilojo ules.
d Calculate the am ount of heat produced by l g of
NaOH(,q) + H Cl(,q) - NaCl(,q) + H2 0 (!)
11H = - 57 kJ m o J- 1 ethanol burning.
e What is the mass o f 1 m o le o f ethan o l
(ii) Is this reactio n endo thermic or exothermic?
( iii ) C alculatc th e energy change associated with (C2 H;OH )?
(A.c H ~ l ; C ~ 12; 0 ~ 16)
thi s reactio n if 2 m o les of sodium hyd roxide f H ow much heat wo uld be produced if 1 m ole
were neutralised b y excess h yd rochlo ric acid. o f ethano l had been burned ? (This is the heat o f
c (i) Draw an energy level diagram for the reaction : combu stio n o f ethan ol. )
g Compare your valu e with the actual valu e o f
2H20 (!) - 2H2(s) + 0 2(s) 137 1 kJ m o l- 1 and suggest two reasons for the
11H = +5 7 5 kJ m o l- 1 difference in valu es .
h Write a balan ced ch emical equatio n to represent
(ii) Is this reactio n endo thermic or exothermic? th e combustion o f ethan o l.
( iii ) C alculate th e energy change for this reactio n
7 Th e fo llowing results were o btain ed fr om a
if only 9 g o f water were converted into neutralisatio n reactio n betv.reen l m o l dm- 3
hyd rogen and oxygen. hyd rochlo ri c acid and 1 m o l dm-3 sodium
hyd roxide. This experiment was carri ed o ut to
6 The fo llo wing results were o btained fr o m an measure the heat o f neutralisatio n o f hydrochlo ric
experiment carried o ut to m easure the enthalpy o f acid. The temperature rise which occurred durin g
combustion (heat o f combustio n ) o f ethano l. The the reactio n was recorded.
experiment involved heating a known volume o f
water with the flam e fro m an ethano l burner. Volume o f sodium h yd roxid e used = 50 cm 3
Volume o f acid u sed = 50 cm 3
thermometer Temperature rise = 5 cc
( Density o f water = l g cm - 3 )
r -- ---,.,- metal
calorimeter H eat energy = mass o f x 4.2 x temperature
f-,11---,0 - 11 - - clamp given o ut water/g 1g - 1 cc- 1 rise/cc
.- - draught during
heat shield reactio n
a Write a balanced ch emical equatio n fo r the
reactio n .
b W hat mass o f so lutio n was warm ed during the
reactio n ?
Additional questions
c H ow much heat energy was produced during th e 9 'Propagas' is used in some central heating systems
reaction? where natural gas is not available. It burns
according to the following equation:
d H ow many moles of hydrochloric acid were
involved in the reaction? C3H 8(s) + 502(s) -----> 3COi(g) + 4H20 (1)
!J.H = - 2220kJ mo[-1
e H ow much heat would be produced if 1 mole of
hydrochloric acid had reacted? (This is the heat a What arc the chemical names for 'propagas' and
of neutralisation of hydrochloric acid. ) natural gas?
f The heat of neutralisation of hydrochloric acid b Would you expect the heat generated per mole
is -57kJ mol- 1• Suggest two reasons why there of'propagas' burned to be greater than that for
is a difference between this and your calculated natural gas? Explain your answer.
value.
c What is 'propagas' obtained from?
8 Write down which factors arc most important d Calculate:
when deciding on a particular foci for the purpose
given: (i) the mass of'propagas' required to produce
:i foci for a cigarette lighter 5550 kJ of energy
b foci for a camping stove
c foci for an aeroplane (ii) the heat energy produced by burning
d foci for an underground transport system 0.5 mole of'propagas'
e foci for a space craft
f foci for domestic heating. (iii)thc heat energy produced by burning 11 g of
'propagas'
(iv) the heat energy produced by burning
2000dm3 of'propagas'
(Ar: H= l ; C = 12; 0= 16. One mole of any
gas occupies 24 dm3 at room temperature and
pressure. )
0 Chemical reactions
Factors that affect the rate of a reaction Enzymes
Surface area Checklist
Concentration Additional questions
Temperature
Ligh t
Catalysts
Fig ure 7. 1 sh ows som e slow and fa st reacti on s. The As new techniqu es have b een d evelo ped , the
two pho tographs o n the kft sh ow examples o f slow processes u sed within the ch emical industry have
reaction s. Tiu: ripening of appks takes place over a becom e m ore complex . Th erefore, ch emists and
number o f weeks, and th e making and maturing o f chemical eng ineers have increasingly looked for ways
cheese may take mo nth s. The burning o f so lid fod s, to control the rates at which ch emical reactio n s take
such as coal , can be said to in volve chemical reactio ns place. In d o ing so , they have discovered that there
taking place at a m edium speed or rate . The other arc five main ways in which yoll can alter the rate o f a
cxamplt: shows a fast reacti on. The chemicals inside chemical reacti o n. These ideas arc no t o nly incredibly
n plosivcs, such as TNT, react very rapidly in reacti on s u sefol to indll stry bllt can also be applied to reactio n s
which arc over in seconds or fra ctio n s o f second s. which occur in the sch ool laborato ry.
Figure 7.1 Some slow (ripening fruit and cheese making), medium (coal fire) and fast (explosion) reactions.
Factors that affect the rate of a reaction
• Factors that affect the
rate of a reaction
• Surface area o f the reactants.
• Concentration o f the reactants and gas pressure.
• Temperature at whi ch the reaction is carried o ut.
• Lig ht.
• U se o f a catalyst, including en zym es .
Co llisio n t heory
Fo r a chemical reacti o n to occur, reactant particl es
need to collide with o ne ano ther. N o t every collisio n
results in the formati o n o f p rodu cts. Fo r produ cts
to be fo rmed, th e collisio n has to have a certain
minimum am o unt o f en ergy associated with it.
This minimum am o unt o f en ergy is kn own as the
activation energy, Ea (Figure 7 .2 ). Collisio ns which
result in the formati on o f products arc kn own as
su cccssfol collisio ns.
~~ --------- -=~~;:!ion Figure 7.3 The powdered limestone {left) reacts faster with the acid than
the limestone in the form of lumps.
e' ------- ------- overall
~ reactants The rates at which the t wo reactio ns occur can be
energ y found by m easurin g either:
chang e
pro ducts • the volume o f the carbon dioxide gas which is
produced, or
progress of reacti o n
• the loss in mass o f the reacti on mixture with tim e.
Figure7.2 Energyleveldiagramshowingactivationenergy.
These two m eth ods arc generally used for measuring
Surface area the rate o f reactio n for processes involving the
fo rmati o n o f a gas as o ne o f the produ cts.
In C hapter 13, we shall sec that limesto ne (calcium
carbonate) is a substance which can be u sed to The apparatus shown in Fig ure 7 .4 (p. 106) is u sed
n eutralise soil acidi ty. Powdered lim esto n e is u sed to m easure the loss in mass o f the reactio n mixture.
as it neutralises th e acidity fa ster than iflumps The mass o f the conical fla sk plu s the reactio n
o flim csto n c arc used. Wh y d o you think this is mixture is measured at regular intervals. The to tal
the case? loss in mass is calculated fo r each reading o f the
balance, and this is plo tted ag ainst time . Som e sample
In th e labo rato ry, the reacti o n bet ween acid resul ts fro m exp eriments o f this kind have been
and limesto n e in th e fo rm o flump s o r powd er plo tted in Figure 7 .5.
can be o bserved in a simple test -tube ex periment.
Fig ure 7 .3 sh ows the reactio n bet wee n dilute Th e reacti o n is gen e rally at its fa stest in th e
hyd rochl oric acid and limesto ne in lump and fir st minute . This is indicated by th e slo pes o f th e
powd ered fo rm. cun'cs durin g this time. Th e stee pe r th e slo pe, th e
fa ster th e rate o f reacti o n. Yo u can see fr o m th e
hydrochloric + calcium ----+ calcium + carbon + water t wo traces in Fi g ure 7 .5 that the rate o f reacti o n
acid carbonate chlorid e dioxide is greater with the powdered limesto ne than the
lump fo rm.
2HC l(aq) + CaC03(s) ----+ CaC l2(aq) + C O i(g) + H 20 (1)
7 CHEMICAL REACTIONS
1 ·-· "d ~ ----
lin est' "' V /
~V
/ l/ 21 m 0
V-~ 1
I lim!estc ne
] /
//
V
tim e/min
Figure 7.5 Sample r=lts for the limestone/acid experiment.
An increase in the surface area of a solid reactant
results in an increase in the number o f collisions, and
this results in an increase in the number o f succcssfi.tl
collision s. Therefore, the increase in surface area of
the limest one increases the rate of reaction.
Figure 7.6 A powder has a larger surface area.
In certain industri es the large surface area o f fine
powders and dusts can be a problem. For example ,
th ere is a risk o f ex pl osio n in fl ourmills and mines,
where the large surface area o f the fl o ur o r coal
dust can - and has - resulted in explosio ns th rou gh
a reactio n with oxygen gas in the air when a spark
has been created by machinery o r th e workfo rce
(Figure 7.7). On 2 6 September 19 88, two sil os
containin g wheat exploded at the Jamaica Flo ur Mills
Plant in Kingsto n, Jam aica, killing three workers, as a
result o f fin e dust explo ding.
The surface area has been increased by powdering the Figure 7.7 The dust created by this cement plant in India is a potential
lim esto ne (Fig ure 7 .6 ). The acid particles now have an hazard.
increased amo unt of surface o f limesto ne with which
to collide. The produ cts of a reaction arc fo rmed
when collisions occur between reactant particles.
Factors that affect the rate of a reaction
Questions Some sample results o f experim ents o f tl1is kind
have been plo tted in Fig ure 7 .9 . You will note fro m
1 What apparatus would you use to measure the rate of the graph that when the most concentrated sodium
reaction of limestone with dilute hydrochloric acid by thiosulfatc solutio n was used , the reaction was at its
measuring the volume of carbon dioxide produced? fastest . This is sh own by th e sh ortest time taken for
the cross to be obscured.
2 The following results were obtained from an experiment of
the type you were asked to design in question 1.
Total r:mm:
v olume 0.01 0.015 0.02 0.025 0.03
ofC02 concentrati on of sodium thi osulfa te/mo l dm-3
gas/cm 3
a Plot a graph of the total volume of Co2 against time.
b At which point is the rate of reaction fastest?
c What volume of Co2was produced after 1 minute
15 seconds?
d How long did it take to produce 30cm3 of C02?
Concent rati on Figure7.9 Sampledataforthesodiumthio5ulfatelacidexperimentat
differentconcentrationsofsodiumthiosulfate.
A yellow precipitate is produced in the reacti o n
between sodium thiosul fa tc and hyd rochl oric acid. From the d ata sh own in Figure 7.9 it is possib le to
produ ce a different graph which directly sh ows tl1 c
sodium + hrdrochloric ----,. sodium + sulfur + sulfur + water rate o f the reactio n against con centratio n rath er
than tl1e time taken fo r the reaction to occur against
thiosulfate acid chloride dioxide concentratio n. To d o this, tl1c times can be converted
to a rate u sing:
Na2S20.i(aq) + 2H Cl(aq ) ----,. 2NaCl(aq ) + S(s) + S02(g) + H 20(l)
ratc = - - - - -
The rate of this reaction can be followed by recording reactio n time (s)
the time taken fo r a g iven am ount of sulfur to be
precipitated . This can be do ne by placing a conical fl ask This would give tl1c graph shown in Figure 7.1 0 (p . 108 ).
containing the reaction mixture o n to a cross o n a p iece
o f paper (Figure 7.8 ). As the precipitate of sulfur forms, As di scu ssed earlier, tl1e produ cts o f the reaction
the cross is obscured and fin ally disap pears fro m view. arc fo rmed as a result o f tl1e collisio n s between
The time taken fo r this to occur is a measure of the reactant particles. There arc m o re particles in a
rate o f this reactio n . To o btain sufficient info rmation m o re concentrated solutio n and tl1c collision rate
about tlu: effect of changin g the concentratio n of the b et ween reactant particles is hig her. Th e m ore o ften
reactants, several experimen ts of this type must be the particles collid e, tl1c greater the chance they
carried o ut, using diftC:rc nt con ce ntration s o f sodium have o f havin g sufficient energy to overcom e tl1e
thiosul fa tc or hydrochlo ric acid. activatio n en ergy o f the reactio n , and o f a su ccessful
collisio n occurring . This m ean s tliat the rate o f a
ch emical reactio n will increase if the concentratio n
o f reactants is in creased , becau se tl1cre arc m o re
particles per unit volume.
In reaction s involving o nly gases, fo r exam ple the
H aber process (C hapter 11 , p . 177 ), an in crease in
the overall pressure at which the reacti o n is carried
o ut increases the rate o f the reactio n. The increase
in pressure results in th e gas particl es bein g pu shed
closer together. Thi s means that th ey collide m o re
o ften and so react fas ter.
Figu re7 .8 Theprecipitateofsulfurobscuresthecross.
7 CHEMICAL REACTIONS
0 .035 ,,, As the t e mperature inc reases, the react ant p articles
inc rease their kin etic e ne rgy and they m ove fa ste r.
0.0 3 I/ The fast er m ovem e nt results in m ore collisio n s
~ 0.025 b etween the particles. Som e o f these extra collisio ns,
17 which result fro m the te mperature in crease, will be
i 0.02 , P' su ccessful collisio n s. This cau ses the reaction ra te t o
in c rea se.
'5 0.015 ,?
Questions
~ 0.01
1 Explain why potatoes cooked in oil cook faster than those
e cooked in water.
0 .005
2 Devise an experiment to study the effect of temperature on
0 the reaction between magnesium and hydrochloric acid.
0.01 0.015 0.02 0.025 0.03
3 Explain why food cooks faster in a pressure cooker.
concentrati on of sodium thi osulfate/mo l dm- 3
Figure 7.10 Graph to show the rate of reaction against concentrat ion.
Question
1 Devise an experiment to show the effect of changing
the concentration of dilute acid on the rate of reaction
between magnesium and hydrochloric acid .
Tem pe rature Li g ht
Wh y d o you think foo d is stored in a refri gerator? Some c he mical reactio n s arc affect ed by light. W he n
The reason is that the rate of d ecay is slower at lower particl es ab sorb lig ht e ne rgy, the en e rgy can be u sed
te m peratures. This is a gen eral fe ature o f the m ajo rity t o break bo nds, overcoming the activatio n e n ergy
o f chemi cal p rocesses . o f the reaction s a nd cau sing a che mical reacti o n to
occur fa st er (sec p. 105 ). Reactio n s w hich occur
The reactio n between sodium thiosulfatc and as a result o f the ab sorption o fli ght arc known as
hyd rochloric acid can also be u sed to study the effect photoc hemic.'ll re:ictions. The a bsorpti o n o f lig ht
of te mperature o n the rate o f a reactio n . Figure 7. 11 in these reactio n s causes bo nds to break, p rodu cing
sh ows som e sample results o f ex pe rime n ts with reactive particles kn own as radicals. These radicals
sodium thiosulfate a nd h ydrochloric acid ( at fi xed are resp on sible fo r ma ny o f the ch e mical reactio n s
concentratio ns ) carried out at diffe re nt t e mpera tures. which h appe n in the strat osph ere (sec p. 22 1 ).
Yo u can sec fr om the g ra ph that the rate o f the
reactio n is fast est at hig h te mpe ratures . Ozone depleti on
140 \ Ozone, 0 3 , is an impo rtant gas fo und in the
strat osph ere. It is important because it a bsorbs hi gh -
120 \ e nergy ultraviolet radia tio n . If this radiatio n were
t o reach the surface of the Ea rth it would lead to an
~ \ inc rease in skin can cers and oth er m edical probl e m s.
[100 [\ In 1985 largc holes were discovered in the ozone
layer over Antarctica, A ustralasia a nd Europe
•'c 80 \ ( Figure 7. 12 ). Scie ntists think that these h o les
h ave partly been produ ced by ch emicals called
E chl oroflu orocar bon s or C F Cs. C FCs were developed
as refri gerants in frid ges back in th e 195 0 s. They
~ 60 h a,'c also been u sed in air conditio ning system s, in
aerosol can s (as prop el lants) a nd in th e m anu facture
"'- 1---. o f expa nded plastics su ch as po lystyre n e. C FCs
arc ver y unreactivc m o lecules. Th ey escape into
20 the atmosphere and , because o f their inertness,
re m ain without furth e r reaction until they reach th e
10 20 30 40 50 60
temperaturef'C
Figure 7.11 Sampledataforthesodiumthiosulfatelacidexperimentat
different temperatures.
Factors that affect the rate of a reaction
Figure7.1 2 Diagramshov.ingthehole intheozonelayer agreements arc adh ered to then the ozone layer will
overAnta11:tica recover by 2050.
stratosphere and th e ozone layer. l.n the stratosph ere, Pho to synth esis is a very impo rtant reaction
the high -energy ultravio let radiatio n causes a (C hapter 11 , p. 172 ) which occurs only wh en
ch emical bond in the C FC mo lecule to break and a sunlig ht fall s on lea,,cs containing th e g reen pi g me nt
chl orine radi cal to split o ff fro m the C FC m olecule . chl orophyll. The chlorophyll acts as a catalyst
For example : and the rate o f photosynthesis d epends o n th e
intensity o f the light. An oth er chemical reactio n
C F2C l2 ~ C F2C l + C l that takes place only in lig ht is that which occurs in
pho tographic film. This is a transparent plastic strip
This chlorin e radical then reacts with the o zo ne, coated with emulsio n: a layer o f gelatin throug h o ut
d estroying it : which arc spread many millio ns o f tiny crystals o f
silver halid es, in particular, silver bromide (AgBr).
Cl(g) + 0 3(g) - O Cl(g) + 0 2(g) The emulsio n used is similar fo r both black-and -
white and colo ur film.
One o f the worrying things abo ut this process is
that, although the above equati on indicates that When light hits a silver b romid e crystal, silver
the chlorine radical has also been destroyed , it has cations (Ag+) accept an electro n (reductio n ) fr om
been found that it is then regen erated by a furth er the b ro mid e ions (Br- ), which arc ox idised . H ence,
reaction. It has consequently b een fo und that a silver ato m s and b ro min e ato m s arc produ ced in the
single chlorine radical is capabl e o f dest roying emulsio n.
hundred s o f th o usand s o f ozon e m o lecules.
silver io n + elect ron ---> silver ato m
This is n ot the only probkm with C FCs . They arc
also sig nificant 'greenho use g ases' (sec C hapter 13, Ag+ + c- Ag
p. 212 ).
The m ore lig ht that fall s on the film , th e g reater th e
O zon e depictio n and greenho use effects have am o unt o f silver d eposited. Places where th e m ost
becom e su ch serious problems that an internatio nal silver is d eposited are darkest wh en the negative is
agreem ent, kn own as the Mo ntreal Protocol on d evelo ped and li ghtest o n th e ph o tographic print.
Substances that D eplete the O zone Layer, was
agreed in 1987 . Quest ion
Research is now going ahead, with some success, 1 Devise an experiment to show how sunlight affects
to produ ce safer alternatives to C FCs . At present, the rate of format ion of silver from the silver salts silver
better alternatives, called hyd rochloroflu orocarbo ns chloride and silver bromide.
(H C FCs ), have been develo ped. Th ese have lower
ozone d epictio n effects and arc n ot effective Cat a lyst s
greenho use g ases. It is believed that if the
O ver 9 0% o f industri al processes use catalysts. A
catalyst is a substance whi ch can alter the rate o f a
reaction witho ut being chemically changed itself. In
the laboratory, the effect o f a catalyst can be o bserved
using the d ecompositio n o f hydrogen peroxide as an
example .
hydrogen peroxide ---> water + o xygen
2H2 0 2 (, q) - 2H,0 (1) + 0 2(g)
The rate o f decompositio n at room temperature is
very slow. 1l1erc arc substances , however, whi ch will
speed up this reactio n, o n e being mangancsc(1v )
oxide . When black manganese(!\-') o xide powder
7 CHEMICAL REACTIONS
is ad ded to hyd rogen perox id e solutio n, oxyge n is A catalyst increases the rate o f a chemical reactio n by
p roduced rapidly. The rate at whi ch this occurs can providing an alternative reaction path which has a
be fo ll owed by m easuring the vo lum e o f oxygen gas lower activatio n e nergy. A catalyst d ocs no t increase the
produced with time. num ber of collisions between the reactant particles b ut
only causes m ore o f the collision s to b ecome successful
Som e sample results fro m expe rime nts o f this typt: collision s, so increasing the rat e o f the reactio n.
have been plotted in Fig ure 7. 13. At th e end of the
reactio n , th e m angancsc(rv) oxid e can be filtered If the activatio n e nergy is lowered by using a
o ff and u sed again. The reacti on can proceed even catalyst the n , on co lli sio n , m o re particles w ill go
fas te r by inc reasing the am o unt an d surface a rea of o n to p rod uce produ ct s at a g iven te m perature
the cat alyst . This is becau se the activity o f a catalyst ( Fig ure 7. 14).
involves its surface. N ot e that , in gaseous reactio ns,
if dirt o r impurities arc present o n the surface o f th e without
catalyst , it w ill n ot act as efficie ntly; it is said t o have cat alyst
been 'poison ed' . Therefore, the gaseou s reactants
mu st be pure. ,, L, -
1 :: progress of reaction
; 120 Figure 7.14 Energyleveldiagramsho'Ningactivationenergy,withand
withoutacataly,t.
[ 100
Catalytic converters
0 80
European regulatio n s st ate th at all n ew cars have
60 t o be fitted w ith catalytic con ve rte rs as pa rt o f their
exh au st syste m (Fi g ure 7 .1 5). Car exha ust fum es
"'l~l~~il~~l~~ij]540 contain pollutant gases such as carbo n m o noxide
( CO ) fo rmed fro m the inco mplete com bu stio n o f
] 20 hyd rocarbo ns in the fu el, and nitrogen (u ) oxide
( N O ) fo rmed by the reactio n o f nitrogen gas and
0 oxygen gas fro m the air. The followin g reactio ns
0 50 100 150 200 250 300 350 proceed o f their own accord but ver y slowly unde r
the co nditio ns inside an exhau st .
t ime/s
Figure 7.13 Sample data for differing amounts of Mn02 catalyst.
C hemi sts h ave fo und that: car bon m o n ox ide + oxygen ----,. car bon diox ide
• a sm all am o unt o f cat alyst will produce a large 2CO(g) + o , (s) - 2CO , (g)
am ount o f chemical ch ange
nitrogcn (n) + carbon ----,. nitrogen + carbon
• cat alysts re m ain unch anged ch e mically after a
reactio n has taken place, b ut th ey can ch ange ox id e m o n ox ide dioxide
physically. For example, a fin e r m an gan ese(1v)
oxide powder is left behind afte r th e decompositio n 2 NO(g) + 2CO(g) - N2(g) + 2C0 2(g)
o f h yd rogen peroxide
• catalyst s arc specific to a particu lar che mical reactio n.
Some exam ples o f chemical p rocesses and the
catalyst s used a rc sh own in Table 7 .1 .
Tabl e 7.1 Examplesofcatalysts. Catalyst The cat alyst in th e con verte r speed s up th ese
Iron reaction s con sid era bly. In these reaction s, the
Process Vanadium(v) pollutants arc converted to carbon dioxide and
Haber process - for the manufacture of ammonia oxide nitrogen , w hic h arc na turally present in th e air.
Contact process~ for the manufacture of sulfuric acid Platinum The re m oval o f oxides of nitrogen is important
Nickel becau se they cause respiratory disease . They arc also
Oxidation of ammonia to give nitric acid in volved in the produ ctio n o f pho t och emical sm ogs
Hydrogenation of unsaturated oils to form fats in the ( Figure 7. 16 ) which occur worldw ide in m ajo r
manufacture of margarines cities, especially in the summe r. lt sh o uld be
Enzymes
b Asectionthroughacatalyticconverter.
n o ted , h owever, that the catalytic converter can
o nly be used with unleaded pet rol and that, due
to impurities being deposited o n the surface o f the
catalyst , it becomes po isoned and has to be replaced
every five o r six years.
Figure 7.1 6 The haze is due to pollution caused mainly by cars v.it hout Quest ions
catalyst exhaust systems.
1 Using a catalysed reaction of your choice, devise an
experiment to follow the progress of the reaction and
determine how effective the catalyst is.
2 Why do some people consider catalytic converters not
to be as environmentally friendly as suggested in their
advertising material?
3 Unreacted hydrocarbons such as octane, C8H18 (from
petrol), also form part of the exhaust gases. These gases
are oxidised in the converter to carbon dioxide and water
vapour. Write an equation for the oxidation of octane.
• Enzymes catalyst . Th ere arc literally hundred s o f different
kinds o f en zym e. En zym es all have an active site .
Enzymes arc protein m o lecules produ ced in living The active site is a particular shape and locks into a
cells. They are catalysts which speed up hundred s o f correspo nding shape in a reactant m o lecule . When
different ch emical reactio ns going o n inside living this has happened , the en zym e can work to break up
cells. These bio logical catalysts arc very specific in the reactant (Fig ure 7 . 17 ).
that each chemical reacti o n has a different en zym e
7 CHEMICAL REACTIONS
Figure 7.17 Theenzymemolecules(red, pink, greenandblue)lock on There were problems associated with the earl y
exactly to a particular reactant molecule (yellow ). Once the enzyme is bi ological washing powd ers . So m e custo m ers suffered
locked on, the reactant molecule breaks up into pieces. fro m skin rash es, b ecause they were allergic to th e
en zym es (Figure 7 .19 ). This problem has been
For example, hyd rogen peroxid e is produ ced within overcome to a certain extent by ad vising that extra
o ur bodies. H owever, it is cxtrcmdy damaging and rinsing is required. Also, many manufacturers have
mu st be d ecomposed very rapidly. C ata.lase is the placed warnin gs on th eir packets , indicatin g that
enzym e which converts hyd rogen peroxid e into the powd er contains en zym es which may cause skin
harmless water and o xygen within our livers: rash es.
hydrogen peroxide ~ water + oxygen
2H 2 0 2 (,q ) ~ 2H,0(1) + 0 2 (s) Figure7.19 Anallergicreactiontoabiologicaldetergent.
Althou gh many chemical catalysts can work under Other industrial processes also make u se o f enzym es .
vario us co nditio ns o f temperatun: and pressure as
well as alkalinity or acidity, bio logical catalysts o perate • In th e manufacture o f baby foo d s, en zym es called
o nly under very particular conditi o ns. Fo r example, pro teases arc used to ' pre-digest' the protein part
they operate over a very narrow tcmpcratun: range o f the baby fo o d. This is because young babie s a.re
and if the temperature becomes too hi gh , th ey unable to digest protein m o lecules.
become ino perative. At temperatures above abo ut
45 °C, they d enature . This m eans that the sp ecific • The en zyme isom erasc is u sed to con vert glu cose
shape o f the active site o f the en zym e m o lecule syrup to fru ctose syrup. Fru ctose syrup is mu ch
changes du e to the breaking o f b ond s. This m eans sweeter than glucose syrup and can be used as a
that th e reactant m o lecules arc n o lon ger able to fit sweetener in slimming foo d s as less is n eeded.
into the active site .
A hu ge multimilli o n -p ound industry has grown
up around th e u se o f enzym es to produ ce new
materials. Bio logical washing powd ers (Figure 7 .18 )
contain en zym es to break d own stains such as sweat,
blood and egg, and they d o this at the relatively low
temperature o f 40 °C . This reduces energy costs,
becau se the washin g water d ocs n ot n eed to be
heated as much.
Enzymes
• In the prod uction o f yoghurt, milk is initially
heated to 9 0 °C fo r 15- 3 0 minutes to kill any
bacteria in the milk. After cooling to 4 0 °C, a
starter culture o f Lactobncillm bacteria is add ed
and the mixture incubated at 40 °C fo r eig ht h o urs
(Figure 7. 20). Th e bacteria ferment the lactose
in the milk to lactic acid, which causes the milk
protein to become solid.
• In cheese making, milk is initially h eated to
kill bacteria and th en cooled. A starter culture
o f Streptococcm bacteria is th en add ed , which
coagulates the milk into curds and whey
(Fig ure 7 .21 ). Th e curds arc put into steel or
wooden drums and pressed and allowed to dry.
In industry, en zym es arc used to bring about
reacti ons at no rmal temperatures and pressures that
would o therwise require expensive conditio ns and
equipment. Su ccessful processes using en zym es need
to ensure that:
• the enzyme is able to fun ctio n fo r lon g peri od s o f
time by o ptimising the environment
• the en zyme is kept in place by trapping it o n
the surface o f an inert solid (som e processes
imm o bilise the en zym es when th e process is
complete)
• continu o us processes occur rath er than batch
p rocesses .
Questions
1 When using biological washing powders what factors
have to be taken into consideration?
2 Enzymes in yeast are used in the fermentation of glucose.
Why, when the temperature is raised to 45 °C, is very
little ethanol actually produced compared with the
amount formed at room temperature?
Figure 7.20 Yoghurt is incubated in these tanks, and allowed to mature.
7 CHEMICAL REACTIONS
Checklist • Enzymes Protein molecules produced in living cells.
They act as biological catalysts and are specific to certain
After studying Chapter 7 you should know and understand the reactions. They operate only within narrow temperature and
following terms. pH ranges.
Reaction rate A measure of the change which happens
• Activation energy The excess energy that the reactants during a reaction in a single unit of time. It may be affected
must acquire to permit the reaction to occur. by the following factors:
• surface area of the reactants
• Catalyst A substance which alters the rate of a chemical • concentration of the reactants
reaction without itself being chemically changed. • the temperature at which the reaction is carried out
light
• Catalytic converter A device for converting dangerous • use of a catalyst.
exhaust gases from cars into less harmful emissions. For
example, carbon monoxide gas is converted to carbon
dioxide gas.
Q Chemical reactions
• Additional questions 3 a Which o f th e following reactio n mixtures
will produ ce hyd rogen m o re quickly at room
1 E xplain the fo llowing statem ents. t e m p er a t u re?
a A car exh au st pipe will rust much fa ster if the car (i) zinc g ranules+ dilute nitri c acid
is in constant u se. (ii) zinc powd er + dilute nitric acid
b Vegetables cook faster wh en th ey arc ch o pped
up. b Give an explanatio n o f your answer to a.
c Indu strial processes becom e m o re econ omically c Suggest two o ther meth ods by which the speed
viable if a catalyst can be fo und fo r the reactio ns
in vo lve d. of this reacti on can be altered.
d In fireworks it is u su al fo r the ingredi ents to b e
powd e re d . 4 A fla sk containing dilute hydrochloric acid was
e T om at oes ripe n fas te r in a g reenho use . placed o n a dig ital balance. An excess o flim csto nc
f The reacti o n between zinc and dilute chippings was added to this acid , a plug o f cotto n
hyd rochloric acid is slower than the reactio n wool was placed in th e neck o f th e fl ask and
between zinc and concentrated hyd rochl oric the initial m ass was record ed. Th e mass o f th e
acid . apparatus was recorded every two minutes. At
the end o f the experiment the loss in mass o f the
2 A stud ent perfo rm ed t wo experiments to establish apparatus was calculated and the fo llowing results
how effective man gan csc(rv) oxid e was as a catalyst were o btained.
for the decomposition o f hyd rogen peroxide .
Th e results below were o btain ed by carrying out Ti me /m in 02 10 12 14 16
these experim ents with t wo different quantities
o f mangancsc(1v) oxid e. 1l1c volum e o f the gas Loss in mass/g O 2.1 3.0 3.1 3.6 3.8 4.0 4.0 4.0
produced was recorded against time.
a Plo t the results o f the experiment.
118 133 146 b Which o f the results would appear to be
162 174 182 incorrect? Explain your an swer.
c Write a balanced chemical equatio n to represent
a Draw a diagram o f the apparatus you could u se
to carry o ut th ese experimen ts. the reactio n taking place.
d Why did the mass o f the flask and its co nten ts
b Plo t a graph o f the results.
c Is the mangancsc(rv) o xide acting as a catalyst in d ecrease?
e Why was the plu g o f cotto n wool u sed ?
this reaction ? E xplain your answer. f H ow d ocs th e rate o f reaction change during
d ( i) At which stage d ocs the reactio n proceed
this reactio n ? Explain this using particle theory.
m ost quickly? g H ow lo ng did the reactio n last?
(ii) H ow can you tell this fr o m your g raph ? h H ow lo ng did it take fo r half o f the reactio n to
(iii)In term s o f parti cles, explain why th e
occur?
reactio n is quickest at the po int you have
chosen in (i) . 5 a What is a catalyst?
e W hy d ocs th e slo pe o f the graph become less b List the properties o f catalysts.
steep as the reactio n proceeds? c Name the catalyst used in the fo llowing
f What volume of gas has been produced when processes:
u sin g 0 .3 g o fman gancsc(1v) oxid e after 50 s? (i) the Contact process
g H ow lon g did it take fo r 60 cm 3 o f gas to be (ii) the Haber process
produ ced wh en th e experiment was carried o ut (iii) the hyd rogenatio n o f unsaturated fats .
u sin g O.S g o f the man gancsc(rv) ox ide? d Which series o f m etallic elements in the Perio dic
h Write a balan ced ch emical equatio n fo r the T able (p. 136) d o the catalysts you have named
decompositio n o f hyd rogen p eroxide. in c belo n g to?
e VVhat arc the conditi o ns u sed in the industrial
processes named in c? The fo llowing referen ces
will help you: C hapters 11 , 12 and 14.
7 CHEMICAL REACTIONS
6 This questio n concerns the reactio n o f coppcr(n ) 7 Euro pean regulati on s state that a.11 new cars have
carbonate with dilute hyd rochlo ric acid. The to b e fitted w ith catalytic con verte rs as part o f their
equatio n for the reaction is: e xhaust syst e m.
a Why arc these regulati on s necessary?
CuC0 3(s) + 2HCl(,q) - CuCl2(,q) + C0 2(,q) + H,0(1) b W hich gases arc re m oved by catalyti c converte rs?
c W hich m e ta.ls arc o fte n u sed as catalyst s in
a Ske tc h a graph to show the rate o f p roduc tio n catalytic con verte rs?
o f carbo n dio xide when an excess o f dilute d W hat d ocs the term ' po isoned ' m ean with
hydrochloric acid is add ed. Th e reactio n lasts respect t o catalyst s?
40 s and produces 60 cm3 of gas. e The latest con verte rs w ill also re m ove unburnt
pe t rol. An equation fo r this t ype o frcactio n is:
b Find o n your g raph the part which sh ows:
(i) w he re the reactio n is at its fast est 2c,H1.<s) + 210,(g) -14CO,(g) + 14H, O (g)
(ii) when the reactio n has sto pped.
(i) C alculate th e mass of carbon dioxid e
c C alculate the mass o f coppc r(n ) carbo na te p roduced b y 1.96 g o funburnt fuel.
used to produce 60 cm 3 o f carbon dioxid e .
(Ar: C = 12; 0 = 16; Cu= 63.5. On e mole of a (ii) Con vert this mass o f carbo n dioxid e into a
gas occupies 24 dm 3 at roo m temperature and volume measured at rtp.
pressure (rtp ).)
(iii) If the average car produces 7. 84 g o f
d Sketch a furth er graph using the same axes t o unburnt fu el a day, calculate th e volume o f
show what happen s to th e rate at w hich th e gas carbon diox ide prod uced b y the catalyti c
is p roduced if: con verte r measured at rtp . (A,: H = l ;
(i) th e con centratio n o f the acid is d ecreased C = 12; 0 = 16. One m ole o f an y gas
(ii) th e te mpe rature is inc reased. occupies 24dm 3 a.t rtp .)
8 Suggest practical m ethods by which the rate
o f reactio n can be investig ated in each o f the
fo llo wing cases:
a mag nesium reacting w ith hyd rochl o ric acid
b nitrogen m on oxide reacting with oxygen.
@ Acids, bases and salts
Acids and alkalis Crystal hydrates
Theories of acids and bases Calculation of water of crystallisation
The relative st rengths of acids and bases Solubility of salts in water
Neutralising an acid Titration
Checklist
Formation of salts Additional questions
Methods of preparing soluble salts
Methods of preparing insoluble salts
More about salts
Testing for different salts
• Acids and alkalis
All the substances shown in Figure 8. 1 contain an acid
o f o ne sort o r ano ther. Acids arc certainly all around
us. W hat properties do these substances have which
m ake you think that they arc acids or contain acids?
Figure 8.2 Some common alkaline substances.
Figure 8.1 What do all these foods have in common? It would be too dangerous to taste a liquid to find o ut if
it was acidic. C hemists use substances called indic..'lto rs
The word acid m ean s 'sour' and all acid s possess this which change colo ur when they are added to acids
property. Th ey arc also: or alkalis. M any indicators arc d yes which have been
extracted fro m natural sources, for example litmus .
• soluble in water
• corrosive. M ethyl orange, a commo n indicator used in
titration s (sec p . 129) is pink in an acid solution but
Alk.-,Jis are very diftCren t fro m acid s. Th ey are the changes to show a yellow colo ur in an alkaline solutio n.
chemical 'opposite' of acids. Some other indicato rs arc sh own in Table 8 .1 , alo ng
with the colours they turn in acids and alkalis.
• Th ey will n:m ove th e sharp taste fro m an acid .
• Th ey have a soa py fee l. Tabl e 8.1 lndicatorsandtheircoloursinacidandalkalinesolution.
Some commo n alkalin e substan ces are sh own in Indicator Colour in acid Colour in alkaline
Figure 8 .2. Blue litmus solution solution
Red Blue
Methyl orange Pink Yellow
Methyl red Red Yellow
Phenolphthalein Colourless Pink
Red litmus Red Blue
8 ACIDS, BASES AND SALTS
These indicators tell che mist s whe the r a substance An o ther way in which th e pH o f a sLJbstance can be
is acid or alkaline (Figure 8 .3). To obtain an id ea m easured is by u sing a pH meter (Fi g ure 8 .5 ). Th e
o f h ow acidic or alkaline a substance is, we use p H elect rod e is placed into the solution and a pH
ano the r indicato r known as a unive rsal in dicato r . reading is given o n the digital d isplay.
This indicator is a mixture of many o ther indicators.
The colour shown by this indicator can be matched ,--11111111111-i•
against a pH scale. The pH scak was d eveloped by a
Scandinavian chemist called S0rc n S0rcn son. The pH
scale runs from below O to 14. A substance with a pH
o f less than 7 is an acid. O ne with a pH of g reater than
7 is alkaline. One with a pH of 7 is said to be neither
acid nor alkalin e, that is neutral. Water is the most
comm on example o f a neutral substance. Figure 8 .4
shows the universal indicato r colour range alon g with
everyday substances with their particular pH va!LJes.
Figure 8.5 A digital pH meter.
Theo ries of acid s a nd bases
There have been many attempts to d efin e th e
differen ce between acids and bases . The first real
attempt took place in 1777, wh en the Frenchman
Anto ine Lavoisier ( Fig ure 8.6 ) suggested that acid s
were substances that contained oxygen.
lime oven cleaner,
d ilute so dium
hydro xid e
o range juice, solution bl eac h,
vineg ar t oothpaste washing
(ethano ic soda.aq ueous
ac id) ammonia
a The pH scale.
lmmmmm
i e8.4b Universal indicator in solution, showing the colour range. Figure 8.6 Antoine Lavoisier (1714-1794) at work in his laboratory.
Acids and alkalis
It was no t lo ng after this that the ' hyd ro-halic' acids Th ese ideas we re rather limiting since they only
(H C l, HBr, and so o n ) were discovered and they applied to aqueou s solution s. There were situatio ns
had n o o xygen present in them . This p rodu ced a wh ere acid- base reactio ns were taking place in
m o dified th eory in 18 10 when the English ch emist solvents other than water, o r even in no solvent
Sir Humphry Davy (1778- 1829 ) su ggested that all at all. This probl em was addressed in 1923 by the
acid s contain hyd rogen as the impo rtant clement; Danish chemi st Jo hann es Br0n stcd ( 1879- 1947 )
however, it was pointed o ut that there were many and the En glish ch emist Th o mas Lowry ( 1874-
hyd rogen -containing substan ces that were no t acid s. 19 36 ) wh en they independently proposed a mo re
The G erman chemist Justu s van Liebig ( Fig ure 8. 7) general d efiniti o n o f acids and bases, and the study
then mad e the next u seful proposal abo ut acids o f acids and bases took a great step fo rward. This
( 1838 ) when he suggested that acids were substances theory became known as the Br0nsted- Lowry
that can react with m etals to prod uce hyd rogen gas. theory of acids and bases.
The Br0 nst ed-Lowry t heory
This theory d efin ed:
• an acid as an H + ion (o r p roto n ) d on o r
• a base as an H + ion (o r pro to n ) accepto r.
The theory explains wh y a pure acid behaves
differently fr o m its aqueous solutio n , since for an
acid to behave as an H + io n d o n o r it must have
an o th er substance present to accept the H+ ion. So
the water, in the aqueou s acid solutio n , is beha,,in g
as a Brnn sted-Lowry base and accepting an H + io n .
G enerally:
HA(,q) + H 2 0 (!) - H,O+(,q ) + A-(,q )
acid base
Figure 8.7 Justusvon Liebig {1803--1873), w ho defined acids more closely. If a substan ce can behave b oth as a Brnn sted-
Lowry acid and as a Brnnsted - Lowry base then it
Thi s theor y was fo llowed in 1884 by the first really is called amphoteric. Water has this ability. As well
comprehen sive theory o f acids and bases, produced by as reacting with acids (ab ove ) it can also react with
the Swedish chemist Svante Arrhcnius ( 1859- 1927). Br0 n sted- Lowry bases su ch as amm o nia in th e
H e su ggested that since these acid solutio ns were fo llowing way to form the base OH- :
elect rolytes (sec C hapter 5 ) their solutio ns contained
many io ns. According to Arrhenius' theory, acids NH3(, q) + H 20 (!) - NH ; (, q) + OW(,q )
produ ce hyd rogen io ns (H+) when they dissolve in
water, wh ereas bases prod L1cc hyd roxide io ns (OH- ). b ase acid
It was thus recognised that water plays an The reaction bet ween hyd rogen chlo ride gas and
impo rtant part in the acidity o f a substance. This ammo nia can be d escribed as an acid- base reactio n
led to the suggestio n that the h yd rogen ion canno t under this theory. The hyd roge n chloride m o lecule
exist alon e in aqueous soluti o n. This was prompted acts as a pro to n d o no r and th e amm onia m olecule
by the fa ct that gaseou s hyd rogen chl oride, H C l(g), acts as the proto n acceptor (Figure 8.8).
is no t acidic but when it dissolves in water an acidic
so lutio n is p roduced. HCl(g) + NH3(g) - NH;c1-(, )
acid base
8 ACIDS, BASES AND SALTS
solutio n s arc good condu ctors of electricity and they
react q uickly with m etals, bases and m etal car bonates .
W hen st ron g acids are neu trali sed by strong alkalis
the fo llowing reactio n takes place between h yd rogen
io ns and hyd roxide io ns.
A weak acid such as ethan oic acid, which is fo und in
vinegar, produces few hyd rogen io ns when it dissolves
in water com pared with a stron g acid of the sam e
concentratio n . It is only partially ionised . Its solutio n
has a high er pH than a stron g acid , but still less than 7.
ethan oic acid ~ hyd rogen ion s + ethan oate io n s
W (,q)
Figure 8.8 The hydrogen chloride molecule (from concentrated Th e ~ sign m ean s that the reaction is reversible.
hydrochloric acid) acts as a hydrogen ion donor. The ammonia molecule This m ean s that if the cthan oic acid m o lecule breaks
{from concentrated ammonia) acts as a hydrogen ion acceptor. d own to g ive hyd rogen io ns an d ethan oate io ns then
they will react together to re -form the ethan oic acid
The relative strengths of acids m olecule. The fact that fewer ethano ic acid m o lecules
and bases dissociate com pared with a st rong acid, and that the
reaction is reversible, means that few hyd rogen ion s
The relative strength o f an acid is found by comparing are present in the solutio n. O ther examples o f weak
on e acid with ano ther. 1l1c strength o f any acid acids arc citric acid , fo und in oranges and lem o ns,
dep ends upon how m any m o k:cu lcs dissociate (o r carbonic acid, found in soft drinks, sulforous acid
ionise ) when tht: acid is dissolved in water. The (acid rain ) ( Figu re 8.9) and ascorb ic acid (vitamin C ).
relative stren gth o f a base is fo und by comparing o ne
base with an other an d is again dependent u pon the
dissociation o f the base in aqu eous solutio n.
Strong and weak acids
A typi cal strong acid is hyd rochlo ric acid. It is fo rmed
by dissolving hyd rogen chlo ride gas in water. In
hyd rochloric acid the ion s fo rm ed separate compk tdy.
hydrogen chloride ~ hydrogen ions + chloride ions
H Cl(g)
For hyd rochloric acid all th e hyd rogen chlo ride Figure 8.9 Sulfurousacidisfoundinacidrain.ltisaweakacidand
mo lecules break up to fo rm H + io n s and c 1- ion s. is oxidised to sulfuric acid (a strong acid). Acid rain damages the
An y acid that behaves in this way is termed a s trong
acid. Bo th sulforic acid and nitric acid also behave in environment quite badly.
this way and are therefore also termed stro ng acids.
All these acids have a hig h con centration o f hyd rogen
io ns in solutio n ( H +(aq)) and have a low p H . Their
Acids and alkalis
Solutio ns of weak acids arc poorer conducto rs of In th e laboratory, if you wish to neutralise a
electricity and have slower reacti on s with metals, comm on acid su ch as hyd rochloric acid you can u se
bases and metal carbonates. an alkali such as sod ium hydroxide. If the pH o f the
acid is measured as som e sodium hyd roxid e solution is
All acids when in aqueou s solutio n prod uce add ed to it, the pH increases . If equ al volum es o f the
hydrogen io ns, H+(aq ). T o say an acid is a st rong same concentratio n o f h yd rochlo ric acid and sodium
acid d ocs no t m ean it is concentrated. The strength hyd roxide arc added to on e an o ther, the resulting
o f an acid tells you h ow easily it dissociates (io nises) soluti on is found to have a pH o f 7. The acid has been
to produ ce hyd rogen io ns. Th e concent ra tion o f neutralised and a n eutral solutio n has been fo rm ed .
an acid indicates the propo rtio n s o f water and acid
present in aqueou s soluti o n. It is important to hydrochloric + sodium sodium +
emphasise that a strong acid is still a st rong acid acid hydroxide chlo ride
even when it is in dilute soluti on and a weak acid is NaCl(aq ) + H 20 (t)
still a weak acid even ifit is concentrated. H Cl(aq ) + NaOH(aq )
St rong and weak bases As we have sh own , when bo th hyd rochlo ri c acid and
sodium hyd roxide disso lve in water the io n s separate
An alkali is a base which produces hydroxide ions, completely. We may th erefore write:
OH-(aq), when dissolved in water. Sodium hydroxide
is a strong alk.1li because when it dissolves in water its You will no tice that certain io n s arc unchan ged o n
lattice breaks up completely to produce ions. eith er side o f th e equatio n. They arc called spectator
ions and arc u sually taken o ut of th e equatio n . The
sodium hydroxide ~ sodium ions + hydroxide ions equ ation n ow becom es:
N,OH (, ) ~ No'(, q) + OH-(, q) H' (,q) + OW (,q ) - H2 0 (I)
These substances which arc strong alkalis produ ce This type o f eqmtio n is kn own as an ionic equation.
large quantities o f hyd roxide io ns and have a hig h The reaction b etween any acid and alkali in aqueou s
pH. Other comm on strong soluble bases include solutio n can be summarised by this io nic equatio n . It
po tassium hyd roxid e. sh ows the ion which causes acidity ( H +(aq )) reacting
with the io n which cau ses alkalini ty (OH-(aq )) to
A weak alkali , su ch as amm onia, produces fewer produ ce neutral water ( H 2 0 (1)).
hyd roxide io n s when it dissolves in water than a
stro ng soluble base o f the same con centratio n. It Questions
is only partially io nised. It has a lower pH than a
st ron g base, but still above 7 . 1 Complete the following equations:
ammonia + ,vater ~ ammonium ions + hydroxide ions
a CH3COOH + NaOH --->
b H1S04 + KOH ---->
The ammo nia m o lecules react with the water c NH3 + HBr --->
mo lecules to form ammo nium io ns and hydroxide In each case name the acid and the base. Also in parts
io n s. H owever, fewer amm onia m o lecules d o this
so o nly a lo w concentratio n o f h yd roxide io n s is a and b write the ionic equation for the reactions.
prod u ced.
2 Explain the terms 'concentration' and 'strength' as applied
Neut ralisin g an acid
to acids.
A comm on situation involving neutralisatio n o f
an acid is when you suffer fr o m indigesti o n. This 3 Explain what part water plays in the acidity of a solution .
is cau sed by a build -up o f acid in your sto mach.
N ormally you treat it by taking an indigestio n remedy 4 Alongside the names of various chemicals below are shown
containing a substance which will react with and
neutralise the acid. their respective pH values in aqueous solution.
potassium hydroxide pH 13
hydrogen bromide pH 2
calcium hydroxide pH 11
sodium chloride pH 7
hydrogen chloride pH 2
magnesium hydroxide pH 1O
citric acid pH4
8 ACIDS, BASES AND SALTS
Which of the substances is/are:
a a strong acid?
b aweakacid?
c astrongalkali?
d aweakalkali?
e a neutral substance?
In each case write a chemical equation to show the
molecules/ions present in solution.
5 a Write a chemical equation to represent the neutralisation
of sulfuric acid by sodium hydroxide.
b Reduce this to an ionic equation.
c Account for any difference you see between the ionic
equation you have written and the one shO\rVn on p. 121
for the reaction of hydrochloric acid and sodium hydroxide.
• Formation of salts
In the example on p. 12 1, sodium chloride was a Silverbromideisusedin photography.
produced as part o f the neutralisation reaction.
Com pounds formed in this way arc known as n o nnal
salts. A no rmal salt is a compound that has been formed
when all the hyd rogen io ns o f an acid have been replaced
by metal ions or by the ammonium ion ( NH/).
N orm al salts can be classified as those which arc
soluble in water or those which arc insoluble in water.
Th e followin g salts are soluble in cold water:
• all nitrates
• all common sodium, potassium and ammonium salts
• all chlorid es except lead , silver and m ercury
• all sulfatcs except lead , barium and calcium.
Salts arc very u scfol substances, as you can sec fro m
Table 8. 2 and Fig ure 8 .1 0 .
Tabl e 8.2 Useful salts.
Salt u.. b lron(1)sulfateistheiron-containingsubstanceintheseironand
multivitamin tablets.
Ammon ium chloride In torch batteries FigureB.10 Someusesofsalts.
Ammon iumnitrate lnfertilisers If the acid b eing neutralised is hyd rochloric acid ,
salts called chlo rides arc fo rmed . Oth er types o f salts
Calcium carbonate Extraction of iron. making cement, glas.s making can be fo rmed with o ther acid s. A summary o f the
diff"crcnt types of salt alo n g with the acid they have
Calcium chloride lntheextractionof sodium,dryingagent bee n fo rmed fro m is sh own in Table 8 .3 .
(anhydrous)
Calcium sulfate For making plaster board s, plaster casts for
injured limbs
lron(1)sulfate In 'iron'tablets
Magnesiumsulfate lnmedicines
Potassium nitrate lnfertiliserandexplosivesmanufacture Tabl e 8.3 Typesofs.altandtheacidstheyareformedfrom.
Silver bromide In film photography Acid Type of salt Exampl e
Carbonic acid Carbonates Sodium carbonate (Na2C03)
Sodium carbonate Glass making, softening water, making modern Ethanoicacid Ethanoates Sodium ethanoate (CH 3COONa)
wash ing powders Hydrochloric acid Chlorides Potassiumchloride(KCI )
Nitric acid Nitrates
Sodium chloride Making hydrochloric acid, for food flavouring, Sulfuricacid Sulfates Potassiumnitrate(KN03)
hospital saline, in the Solvay process for the Sodium sulfate (Na2S0.J
manufacture of sodium carbonate
Sodium stearate In some soaps
Tin(11) fluoride Additive to toothpaste
Formation of salts
Methods of preparing soluble salts The excess magnesium is rem oved by filtrati o n
( Figure 8. 11 ).
There arc fo ur general metho d s o f preparing soluble
salts: The m agn esium nitrate solutio n is evapo rated
slowly to form a saturated solutio n o f th e salt
Acid + metal ( Figure 8. 12 ).
This meth od can o nly be u sed with the less reactive The h ot co ncentrated magn esium nitrate solutio n
m etals. It would be very d an gerou s to u se a reactive produ ced is tested b y dippin g a cold g lass rod into it.
m etal such as sodium in this typ e o f reactio n. The lf salt crystals fo rm at the end o f th e rod the so lutio n
m etals usually u sed in this m etho d of salt preparatio n is ready to crystallise and is left to cool. An y crystals
arc the MAZIT m etals, that is, m agn esium , produ ced o n cooling arc filtered and dri ed between
aluminium, zinc, iron and tin . A t ypi cal experimental clean tissues.
m ethod is given below.
E xcess magnesium ribbon is added to dilute nitric
acid. During this addition an cftCrvcsccn cc is o bserved
du e to the productio n o f hyd rogen gas. ln this reactio n
the hyd rogen io ns fro m the nitric acid gain electro ns
fr om the metal atom s as the reaction proceeds.
hyd rogen io n s + elect ro ns ---+ h yd rogen g as
( fr o m m etal) H2(g)
2H' + 2c-
H ow would you test the gas to sh ow that it was
hydrogcn 1 W hat would be the name and fo rmula o f the
compound produced during the test you suggested ?
magnesium + nitric ----, magnesium + hydrogen
acid nitrate
FigureB.12 Thesolutionofmagnesiumnitrateisconcentratedby
slow evaporation.
Acid + carbonate
This m eth od can be used with any metal carbonate
and any acid , providing th e salt prod uced is soluble.
The typical experimental procedure is similar to that
carried o ut fo r an acid and a m etal. Fo r example,
coppcr( n ) carbo nate would be add ed in excess to
dilute nitric acid . Effervescence would be o bserved
due to the produ cti o n o f carbon di oxide .
H ow would you test the gas to show it was carbon
dioxid e? Write an equatio n to help you explain what
is happening during the test you have ch osen.
coppcr(n) + nitric ---> coppcr(n ) + carbon + water
niu:ne dioxide
carlx m a t e acid
Figure 8.11 The excess magnesium is fi ltered in this way.
8 ACIDS, BASES AND SALTS
M etal carbo nates contain carbo nate io ns, CO / - .
In this reactio n the car bonate io n s react with the
hyd rogen ions in th e acid .
carbon ate + hyd rogen----> carbo n + water
ions ions di oxid e
c o ,'-(,q) + 2W(,q ) - CO,(g) + H,O(i)
Acid + alka li (so lu ble base) Figure 8.13 The acid is added to the alkali until the indicator just
changes colour.
Thi s m etho d is gen erally used fo r pre paring the
salts o f ver y reactive metals, su ch as p o tassium o r The solutio n which is produ ced can then be
sodium. It wo uld certainl y be too d an gerou s to evapo rated slowly to o btain the salt. For example,
add the m etal directly to th e acid. In this case, we
solve th e problem indirectly and use an alkali which h yd rochl oric + sodium ---+ sodium + water
contains th e particular reactive m etal wh ose salt we acid hyd roxid e chlorid e
wish to prep are. H Cl(,q) + N,OH (,q ) - N,Cl(,q ) + H2 0 (!)
Metal oxid es arc basic. Jv' ktal oxid es and As previo usly discu ssed o n p. 11 6, this reactio n can
h yd roxides that dissolve in water to produ ce OH-(aq) best be described by the io nic equ atio n:
io ns arc kn own as alkalis , or soluble bases. If the
m etal oxide or hyd rox id t: d ocs no t dissolve in water it W (,q) + OH -(,q ) - H2 0 (1)
is known as an inso luble base.
Acid + in so lu b le base
A base is a substan ce which neutralises an acid , This metho d can be used to prepare a salt o f an
produ cing a salt and water as th e o nly produ cts. If unrcactivc m etal , su ch as lead o r copper. In th ese
the base is soluble th e term alkali can be used , but cases it is no t possible to use a direct reacti o n o f the
there arc several bases whi ch arc insolublc. It is also a m etal with an acid so the acid is neutralised using th e
substan ce which accepts a hyd rogen io n (sec p. 11 9 ). particular m etal oxide (Figu re 8 . 14).
In gen eral, m ost m etal o xides and hyd rox id es
(as well as ammo nia solutio n ) arc bases . Som e
examples o f soluble and insoluble bases arc sh own in
Table 8.4 . Salts can be fo rmed b y this meth od o nly if
the base is solu bk.
Tab le 8.4 Examples of soluble and insoluble bases.
Sol uble bas es(al ka li s) Insoluble bas es
Sodium hydroxide (NaOH) lron(11)oxide(Fe20Jl
Potassium hydroxide {KOH) Copper(11) oxide (CuO)
Calcium hydroxide (Ca(OH)i) Lead{1)oxide (Pb0)
Ammon ia solution {NH 3{aq)) Magnesiumoxide(MgO)
Becau se in this neutralisatio n reactio n bo th FigureB .14 Citric acid has been usedontheright-handsideofthispiece
reactants arc in soluti on , a special techniqu e call ed of copper metal to remove the oxide coating on its surface, giving it a
titration is required. Acid is slowly and carefull y shinier appearance.
added to a m easured volum e o f alkali usin g a
burcttc (Figure 8 .1 3 ) until the indicator, usu ally
ph en o lphthalein, chan ges co lo ur.
An indicato r is used to sh ow when the alkali has
been neutralised completely by th e acid. This is called
the end -point. Once you know wh ere th e end -
po int is, you can add th e sam e volume o f acid to the
measured volume o f alkali but this time without the
indicator.
Formation of salts
The meth od is generally the same as that for a • All po tassium , ammo nium and sod ium salts arc
metal carbonate and an acid , tho ug h some warming solubl e in water.
of the reactants m ay be necessary. An exam ple o f
su ch a reactio n is the n eutralisation o f sulfuric acid • Al l carbon ates arc insoluble, except those o f
by coppcr(11 ) oxid e to produ ce coppcr( n ) sulfa tc po tassium , amm onium and sodium.
(Fig ure 8 .1 5 ).
An insoluble salt, su ch as barium sulfatc, can be
m ad e by precipitatio n. In this case, solutions o f th e
t wo ch osen soluble salts arc mixed (Figure 8. 16).
To produ ce barium sulfa tc, barium chloride an d
sodium sulfatc can be used . The barium sulfatc
precipitate can be filtered o ff, washed with distilled
water and dried. The reactio n that has occurred is:
+ sodium ---+ barium + sodium
sulfatc sulfatc chloride
+ Na2S04(,q) - B,S0 4(s) + 2NaCl(,q )
sulfuri c + coppcr(n) coppcr(11 ) + water
acid oxide sulfa te
H 2S04(aq ) + Cu O (s) CuS04(,q) + H20 (!)
Metal ox.ides contain the oxide io n, 0 2- The io ni c
equ ati on for this reaction is therefore:
2W (,q ) + 0 2-(s) - H2 0 (!)
CuO(s) + 2W (,q ) - Cu2+(,q) + H20 (!)
Methods of preparing insoluble salts Figure 8.16 When barium chloride solution is added to sodium
sulfate a white precipitate of barium sulfate forms.
The meth ods described above can be used to
m ake a soluble sa lt , o n e th at is soluble in wa ter. The io nic equ atio n fo r this reaction is:
If a sa lt th at is in solu ble in wa ter n eed s to be
prepared , a different techni q u e is need ed . Before This m ethod is som etimes known as d o uble
we d escribe the techniqu e, it is first n ecessar y deco mposition and m ay be summari sed as follows :
to learn which sa lts arc so luble and which arc
in so luble . This ca n be d o n e usin g th e fo llowing soluble salt + soluble salt _,. insoluble salt + soluble salt
ru les o f solubili ty:
(AX ) (BY) (BX ) (AY)
• All nitrates arc solub le in water.
• All chlorides, bromides and io dides arc soluble in It sho uld be n o ted that even salts like barium sulfa tc
dissolve to a ver y small extent. For example, l litre
water, except those o flead an d silver. o f water will dissolve 2.2 x I0-3 g o f barium sulfatc
• All sul fa tes arc soluble in water except barium , at 25 °C. This substan ce and substan ces like it arc
calcium and lead sul fa tcs. said to be sparing ly soluble .
8 ACIDS, BASES AND SALTS
More about sa lts Testing for different sa lts
You have already seen on p. 122 in Tabk 8.2 that Som etimes we want to analyse a salt and find o ut
salts arc useful substances. Some o f the salts sh own in wh at is in it. There arc simple chemical tests which
that table occur naturally and arc mined , fo r example all ow us to id entify the anio n part o f the salt. These
calcium sulfatc (gypsum) and calcium carbonate arc o ften called spot tests.
(limesto ne). M an y o f the others must be made by the
chemical industry, fo r exampl e ammonium nitrate, Testing fo r a su lfat e (So/- )
iron(n ) sulfatc and silver b romide.
Yo u have seen that barium sulfatc is an insoluble
With acid s su ch as sulforic acid, which has two salt (p . 125 ). Therefore, if you take a solutio n o f a
replaceable h yd rogen ion s per m o lecule , it is su spected sulfatc and add it to a solutio n o f a soluble
possible to replace o nly one o f these with a metal barium salt (such as barium chl orid e ) then a white
io n. The salt produ ced is called an acid salt. An precipitate o f barium sulfatc will be produ ced.
acid salt is on e in whi ch n ot all of th e replaceable
hyd rogen ion s o f the acid have been replaced by barium io n + sulfatc io n ---,. barium sulfatc
metal io n s o r the ammo nium io n. So m e examples o f
acid salts arc shown in Table 8.5. Ba2+(aq) + so/ -(aq ) BaS 0 4(s)
Tables 5 Examples of acid salts A fi:w d rops of dilute hydrochloric acid arc also added to
this mixture. If the precipitate docs not dissolve, then it is
Ac id Type of acid salt Exa m p le
barium sulfatc and the unknown salt was in fact a sulfatc.
Carbonic acid Hydrogeoc:arbonate Sodium hydrogencarbonate
(H2CO~ (NaHC01) lfthc precipitate docs dissolve, then the unknown salt
Potassium hydrogensulfate
(KHS04) may have been a sulfitc (containing the so/- io n ).
Sulfuricacid Hydrogensulfate Testi ng fo r a ch lo ri de (Cl- ). a brom ide (B r- )
(H2SO.J o r an iod ide (I - )
Sodium h yd rogcncarbonatc is the acid salt u sed as Earli er in this chapter you saw that silver chlo ride is
the raising ag ent in the bakin g o f cakes and som e an insoluble salt ( p . 12 5) . Therefore, if you take a
bread , and is o ften called baking soda ( Fig ure 8.17). solutio n o f a su spected chl oride and add to it a small
volum e o f dilute nitric acid , to make an aqueous
acidic solution , fo llowed by a small am o unt o f a
solutio n o f a soluble silver salt (su ch as silver nitrate ),
a white precipitate o f silver chloride will be p rodu ced.
chlorid e io n + silver io n ---,. silver chlorid e
CJ-(,q) + Ag+(,q ) AgC l( s)
If left to stand , the precipitate goes grey (Figure 8 .1 8 ).
Fi gure 8.1 7 Sodium hydrogencarbonate is used as a raising agent in Figure 8.18 If left to stand the white precipitate of silver chloride goes
bread and muffins. grey. Thisphotochemicalchangeplaysanessentialpart inblackand
white photography.
Crystal hydrates
In a similar way, a b romi de and an io did e will react to Questions
produ ce either a cream precipitate o f silver bromid e
(AgB r) or a yellow precipitate o f silver iodid e (Agl ) 1 Complete the word equations and write balanced chemical
(Fig ure 8 .1 9 ). equations for the following soluble salt preparations:
a magnesium + sulfuric acid ->
I b calcium carbonate + hydrochloric acid _,.
c zinc oxide + hydrochloric acid _,.
Figure 8.19 AgCI, a white precipitate, AgBr, a cream precipitate, and d potassium hydroxide + nitric acid _,.
Agl,ayellowprecipitate. Also write ionic equations for each of the reactions.
An altern ative test fo r io did e io ns is the ad d itio n o f 2 Lead carbonate and lead iodide are insoluble. Which two
kad nitrate solution to the io dide which results in a soluble salts could you use in the preparation of each
b right yellow precipitate o f lead iod ide, Pbl 2 . substance?Write
a a word equation
Testing for a carbonate b a symbol equation
c an ionic equation
If a sm all am o unt o f an acid is added to som e o f the to represent the reactions taking place.
suspected carbo nate (either so lid o r in so lutio n ) then
effervescen ce occurs. ! f it is a carbo nate then carbo n 3 An analyt ical chemist working for an environmental health
di oxid e gas is produced , which will turn lim cwater organisation has been given a sample of water which
' milky' (a cloudy white precipitate o f calcium is thought to have been contaminated by a sulfate, a
carbo nate fo rms, sec Ch apter I 3, p. 21 5 ). carbonate and a chloride.
a Describe how she could confirm the presence of these
carbonate + hyd rogen --,. carbo n + water three types of salt by simple chemical tests.
b Write ionic equations to help you explain what is
io ns io ns dioxide happening during the testing process.
co,'-(,q ) + 2W(,q ) - CO, (g) + H, O (i) • Crystal hydrates
Testing for a nitrate Som e salts, such as sodium chlo ride, copper
carbo nate and sodium nitrate, crystallise in th eir
By using Dcvard a's alloy (4 5%Al, 5% Zn, 50% Cu ) anh yd rous fo rms (witho ut water) . H o wever, man y
in alkalin e soluti on , nitrates arc reduced to ammo nia. salts produ ce hydrates wh en th ey crystallise fr om
The ammo nia can be id entified using damp indicator solutio n . A h ydrate is a salt which incorporates
paper, which turns blue. water into its crystal structure. This water is referred
to as water of crysta llisation. The sh ape o f the
In the reacti o n the nitrate io n is redu ced , as oxygen crystal hydrate is very mu ch d ependent o n the
is rem oved fro m the nitrogen atom , and it gains presen ce o f water of crystallisation. Som e cxampks
hyd rogen to fo rm ammo nia , NH 3 . The gain o f o f crystal hydrates arc given in Table 8 .6 and sh own
hyd rogen is also a d efiniti o n o f redu ctio n. in Fig ure 8 .20.
Table 8.6 Examplesofcrystalhydrates. Formula
Na,504. lOH,o
Salt hydrate
Cobalt{11) chloridehexahydrate
Copper(11) sulfate pentahydrate
lron(1)sulfate heptahydrate
Magnesiumsulfateheptahydrate
Sodiumcarbonatedecahydrate
Sodium hydrogensulfate monohydrate
Sodiumsulfatedecahydrate
8 ACIDS, BASES AND SALTS
Figure 8.20 Hydrate crystals (left to right): cobalt nitrate, calc ium nitrat e Becau se the colo ur chan ge o nly takes place in the
and nickel sulfate (lop) and manganese sulfat e, copper sulfate and pn:sence o f water, the reactio n is used to test fo r th e
chromium potassium su lfate (bottom). presence o f water.
\.Yhcn many hydrates arc heated the water o f These processes g ive a simple example o f a
crystallisatio n is driven away. For example, if crystals o f reversible reactio n:
coppcr(n) sulfatc hydrate (blu e) arc heated strongly,
they lose their water of crystallisatio n. Anhydrous C uS0 4 ( , ) + 5H20 (1) c,=' C uS0 4 .5H 20 (, )
coppcr(n ) sulfatc rem ains as a white powder:
Some crystal hydrates effloresce, that is they lose some
copper(n) sulfate -----> anhyd rous copper(n ) + water or all o f their water of crystallisatio n to the atmosphere.
For example, when colourless sodium carbonate
p entahydrate sulfatc decahydrate crystals arc left out in the air they become
coated with a white powd er, which is the mon ohydratc
C uS04.SH 20 (s) -----> CuS04(s) + SH20 (g) (Figure 8.22 ). 1l1e process is called efflorescence.
\¥ hen water is added to anhyd rous coppcr(n) sulfatc N, 2C0 3.10H20 (, ) - N,2C0 3.H20 (, ) + 9H20 (g)
the reverse process occu rs. It turns blu e and the
pcntahydratc is produced (Figure 8.21 ). This is an ••
extremely cxothnmic process.
Figure 8.22 A white powder forms on the surface of sodium carbonate
C uS0 4( , ) + 5H20 (1) - C uS0 4.5H 20 (, ) decahydratewhenit isleftinthe air.
\Vith som e substances, no t n ecessarily salt hydrates,
the reverse o f effi o rcscencc occurs. For example,
if anhyd rous calcium chl o rid e is left in the air,
it absorbs water vapour and eventually fo rm s a
very concentrated solutio n. This process is called
deliquescence, and substances which behave like this
arc said to be deliquescent. So lid sodium hydroxide
will deliqu esce .
There are some substances which , ifl eft o ut in
the atmosph ere, absorb m o isture but do no t change
their state. Fo r example, concentrated sulfuri c acid , a
colo urless, viscou s liquid, absorb s water vapo ur fr om
the air and becomes a soluti o n. Substan ces whi ch d o
this arc said to be hygroscopic.
Calculation of water of crystallisation
So metimes it is necessary to work o ut th e
percentage, by mass, o f water o f crystallisatio n in a
h ydrated salt. The m ethod is the same as that used in
C hapter 4 , p. 65 , but this tim e the ' H 2 0 ' is treated
as an clement in the calculatio n.
Figure 8.21 Anhydrous copper(•) sulfate is a white powder which turns
blue when water is added to it.
Titration
Examp le • Titration
C alculate the percentage b y mass o f water in the On p. 124 you saw that it was possible to prepare a
soluble salt b y reacting an acid with a soluble base
s~salt hydrate MgS0 4 .7 H 20. (A r: H = l ; 0 = 16; (alkali ). The metho d used was that o f titration .
Titratio n can also be used to find the concentratio n
Mg ~ 24; 32) o f the alkali used. In the laboratory, the titratio n o f
h yd rochlo ric acid with sodium hyd roxide is carried
M, for MgS04 .7H 20 o ut in the fo llowing way.
~24+ 32 + (4 x 16) + (7 x 18) 1 25 cm 3 o f sodium hyd roxide solution is pipetted
~ 246 into a conical fla sk to which a few drops o f
pheno lphthalein indi cator have been add ed
The mass o f water as a fractio n o f the to tal m ass o f (Fig ure 8 .23 ). Pheno lphthalein is pink in alkaline
hydrate conditions but colo urless in acid.
12 6
246
The percentage o f water pn:sent
~ 126 X 100
246
~ 51.2%
Questi ons
1 Calculate the percentage by mass of water in the
following salt hydrates:
a CuS04.SHp
b Na2C03 .lOH20
c Na2S203.SH20.
(A,: H = 1; 0 = 16; Na = 23; S = 32; Cu = 63.5)
2 Devise an experiment to determine the percentage of
water of crystallisation present in a salt hydrate of your
choice .
• Solubility of salts
in water
Water is a very good solvent and will dissolve a
who le range o f solutes, including sodium chl oride
and copper( n ) sulfate, as well as oth er substances
su ch as sugar. Yo u can dissolve m ore sugar than
sodium chlorid e in l 00 cm 3 o f water at the same
temperature. The su gar is said to be m o re soluble
than the sodium chlo ride at the sam e temperature.
We say that the su gar has a greater solubility than
the sodium chloride . The solubility o f a so lute
in water at a g iven tem perature is the num ber o f
gram s o f that solute which can be dissolved in l 00 g
o f water to produ ce a saturated solutio n at that
temperatun: .
Figure8.23 Exactly25.0cm3of sodium hydroxide solution is pipetted
into a con ical flask.
8 ACIDS, BASES AND SALTS
2 A 0. 10 mo ld m-3 solution of h ydrochloric acid is
pl aced in th e burettc using a filter fimnd until it is
fill ed u p exactly to tht: zero mark (Figure 8 .24 ).
dilut e
hydrochlo ric
ac id
burette
Note: remove filter Figure 8.25 The titration is carried out accurately.
f unnel befo re titratin g
6 Th e fin al reading on the burettc at the end -p o int
is recorded and furth er ti tra.tio ns carried o ut until
con sistent results arc o btained (within 0 .1 cm 3 o f
each o ther). Som e sam ple data arc sh own below.
Volume o f sodium h yd roxid e solutio n
= 25 .0 cm3
Figure 8.24 The burette is filled up to the zero mark with a O.lOmoldm- 1 Average volume o fO. l Om o ldm -3 solutio n of
5olutkmofhydrochloricacid. hyd rochlo ri c acid add ed
= 2 1.0cm 3
3 Th e filter fimn cl is now rem oved.
The neutralisatio n reactio n which has taken place is:
4 Th e hyd rochloric acid is add ed to the sodium
hyd rox ide solutio n in small q uantities - usually no hyd rochloric + sodium ---+ sodium + water
mo re than 0 .5 cm3 at a time (Fi gu n: 8 .25 ). The
contents of the fl ask must be swirled after each acid hydroxide chlorid e
;1ddition of acid fo r thoroug h mixing.
H Cl(,q) + N,OH (,q ) - N,Cl(,q ) + H2 0 (1)
5 The acid is added until th e alkali has been
neutralised completely. This is shown by the pink From this equatio n it can be seen that 1 m ok o f
colo ur of the ind icato r just d isa ppearing. hyd rochl oric acid neutralises l m ole o f sodium
h yd roxid e.
Titration
N ow yo ll can work o ut the num ber of mo les of In the exam ple :
the acid using the fo rmula given in C hapter 4 , p. 63. M 1 = O. lOmo ldm -3
V1 = 2 1.0cm3
mo les = volllme x concentratio n
1000 Macid =l mole
M2 =unknown
~ 2 1.0 X __ll_:l_[)_ V2 = 25.0cm3
1000 Ma1kat; =l mole
= 2. 1 X 10-3
Su bstituting in the equatio n:
number of mo les num ber of mo les 0 .1 0 x 2 1. 0 = M2 x 25 .0
of h yd rochl o ric acid of sodium hyd roxide
11
Therefo re, the number of mo les of sodium Rearrangin g:
hyd roxide M _ 0. 10 x 2 1.0 x l
= 2. 1 X 10-3 l - 1 X 25.Q
2 . 1 x 10-3 mo les of sodium h yd roxid e is present in M2 ~ 0.084
2 5.0cm3 of solution.
The concentratio n of the sodium hydroxide solutio n
Therefo re, in 1 cm3 of sodium hyd roxide solutio n we is 0.084 mo ld m-3.
h ave Ano ther example of a titratio n calculatio n could
2 .1 X 10-3 involve a nelltralisatio n reactio n in whi ch the ratio
- - - mo les of the number of mo les of acid to alkali is no t 1 : 1.
The exam ple below shows how such a calculatio n
25.0 could be carried o ut.
Therefo re, in 1 litre of sodium hyd roxide solution Examp le
we have In a titration to find the concentratio n of a solutio n
of sulfuric acid, 2 5 cm3 of it were just neutralised
_2 ·_1 X_10_-3 x 1000 = 0.084 mo le by 20.1 5cm3 of a 0 .2 mo ldm-3 solution of sodium
25 .0 hydroxide. What is th e concentrati o n of th e sulforic
acid used ?
The concentratio n of sodium hyd rox ide solutio n is
0.084 moldm-3. First, write out the balan ced chemical equatio n fo r
the reactio n taking place.
You can simplify th e calculati on by substituting in
the fo ll owing mathemati cal equ ation: sulfuric acid + sodium hydroxide ----> sodium sulfate + water
M1 V1 = M2 V2 H2S0 1. + 2NaOH Na2S0 ._ + 2 H20
Macid Malkali From this balanced equatio n it can be seen that
1 mo le of sulforic acid reacts with 2 moles of sodium
w h e re: hy d r o x id e.
M 1 = concentratio n of the acid used Therefore, the nllmber of moles of sodium
V1 = volllm e o f acid used (cm3 ) hydroxide used
Macid = num ber of mo les of acid shown in the = 20 .1 5 X ___2.2_ = 4. 03 X 10-3
chemical eq uatio n 100 0
M 2 = conce ntratio n of the alkali used
V2 = volume o f the alkali used (cm3 )
Mallcali = number of moles of alkali shown in the
chemical equatio n.
8 ACIDS, BASES AND SALTS
The number of mo les of sulforic acid which will react Questions
with 4.03 x 10-3 m o les o f sodium h yd roxide
1 24.2cm3 of a solution containing 0.20moldm-3
=4.0 3 X 10-J X _!_= 2.015 X IQ-3 of hydrochloric acid just neutra lised 25.0cm3 of a
2 potassium hydroxide solution. What is t he concentration
This is the number of mo les o f sulfuric acid present of this potassium hydroxide solution?
in 25 cm 3 o f th e solutio n , so the concentrati on o f the 2 22.4cm3 of a solution containing 0.10moldm-3 of
sulfuric acid lOOO sulfuric acid just neutralised 2S.Ocm3 of a sodium
= 2.015 x 10-3 x --= 0.081 moldm-3 hydroxide solution. What is the concentration of this
25
sodium hydroxide solution?
Checklist • Solubility The solubility of a solute in a solvent at a given
temperature is the number of grams of that solute which
After studying Chapter B you should know and understand the can dissolve in 100 g of solvent to produce a saturated
following terms. solution at that temperature.
• Acid A substance which dissolves in water, producing • Strong acid An acid which produces a high concentration
W(aq} ions as the only positive ions. An acid is a hydrogen of W(aq) ions in water solution, for example hydrochloric
ion (W) donor. acid.
• Acid salt A substance formed when only some of the • Strong alkali An acid which produces a high
replaceable hydrogen of an acid is replaced by metal ions or concentration of oH-(aq) ions in water solution, for example
the ammonium ion (NH/).
sodium hydroxide.
• Alkali A soluble base which produces OW(aq) ions in • Testing for a carbonate If effervescence occurs when
water.
an acid is added to the suspected carbonate and the gas
• Base A substance which neutralises an acid, producing a produced tests positively for carbon dioxide, the substance
salt and water as the only products. A base is a hydrogen is a carbonate.
ion (W) accepter. • Testing for a chloride If a white precipitate is produced
when dilute nitric acid and silver nitrate solution are added
• Double decomposition The process by which an to the suspected chloride, the solution contains a chloride.
insoluble salt is prepared from solutions of two suitable • Testing for a sulfate If a white precipitate is produced
soluble salts. when dilute hydrochloric acid and barium chloride solution
are added to the suspected sulfate, the solution contains a
• Efflorescence The process during which a substance loses sulfate.
water of crystallisation to the atmosphere. • Titration A method of volumetric analysis in which a
volume of one reagent (for example an acid) is added to a
• Hygroscopic The ability to absorb water vapour from the known volume of another reagent (for example an alkali)
atmosphere without forming solutions or changing state, slowly from a burette until an end-point is reached. If an
for example, concentrated sulfuric acid. acid and alkali are used, then an indicator is used to show
that the end-point has been reached.
• Indicator A substance used to show whether a substance • Water of crystallisation Water incorporated into the
is acidic or alkaline (basic), for example phenolphthalein. structure of substances as they crystallise, for example in
copper(u) sulfate pentahydrate (CuS04.SH20).
• Ionic equation The simplified equation of a reaction • Weak acid An acid which produces a low concentration
which we can write if the chemicals involved are ionic of H+(aq) in water solution, for example ethanoic acid. It is
substances.
only partially ionised.
• Neutralisation The process in which the acidity or • Weak alkali An acid which produces a low concentration
alkalinity of a substance is destroyed. Destroying acidity
means removing H+(aq) by reaction with a base, carbonate of OH-(aq) in water solution, for example ammonia solution.
or metal. Destroying alkalinity means removing the QH-(aq)
by reaction with an acid. It is only partially ionised.
W(aq) + QH-(aq) -+ Hp(1)
Normal salt A substance formed when all the replaceable
hydrogen of an acid is completely replaced by metal ions or
the ammonium ion (NH/).
• pH scale A scale running from Oto 14, used for
expressing the acidity or alkalinity of a solution.
• Salt hydrates Salts containing water of crystallisation.
Q Acids, bases and salts
• Additional questions a Give th e names and fo rmulae o f substances A
to E .
1 Explain , with the aid of c.xa.mpks, what you
und erstand by the fo llowing terms: b D escri be a test which could be used to identify
a stron g acid the pn:scncc o f water.
b weak acid
c strong alkali c W hich indicator is suitable fo r the initial reactio n
d weak alkali between the hyd roxides and the dilute acids
e co ncentrated acid . shown ?
2 a Copy o ut and com plete the table, which covers d Write balanced ch emical equatio ns fo r the
th e different m eth ods o f preparing salts. reactio ns taking place in the schem e.
Met hod of Name of salt Two substa nces used e VVritc an io nic equ atio n fo r the productio n o f the
p repa rat ion prepared in th e preparation white precipitate D.
Acid + alkali Potassiumsulfate .............. and 4 ln a titratio n involving 2 4.0cm 3 potassium
h yd roxide solutio n against a solution containing
Acid + metal .............. and 1 m o ldm -3 o f sulforic acid , 2 8 .0 cm 3 o f t h e acid
dilute hydrochloric acid was found to just neutralise the alkali completely.
Acid + insoluble Magnesiumsulfate .............. and a Write a word and balanced chemical equation fo r
bare Copper .. th e reactio n.
.............. and b Name a suitable indicator fo r the titratio n and
Acid + carbonate state the colo ur chan ge you would o bserve.
c Calculate th e con centratio n of the alkali in
Precipitation Lead iodide .............. and m oldm - 3 .
d D escri be a chemi cal test which you could u se
b Write word and balan ced ch emical equatio ns fo r to id entify th e type o f salt produced during the
each reactio n sh own in your tab le. Also write reactio n.
io nic equ ation s where appropriate.
5 Explain the fo llowing, with the aid o f examples:
3 Study th e following schem e. Calcium hydroxid e a neutralisatio n
b titration
lSodium hydroxide ldU,te c soluble salt
nitric d insoluble salt.
sduUlfu, treic ac id
ac id 6 Read the fo ll owing passage and then answer the
question s which fo llow.
So lution A So lution B
Sodium carbo nate dccahydratc effloresces quite
Wat er Liquid C readily. W ith some substances, su ch as solid
sodium hydroxide, the reverse o f effl orescence
Mixed occu rs - they deliquesce. There arc some substances,
such as concentrated sulfuric acid , which when
I left o pen to th e atmosphere arc diluted - they arc
hygroscopic.
White precip itat e D
a W hat arc the m eanings o f the terms in italics1
So lution E b W hat precautio n s sh o u ld be taken to en sure
that these substan ces arc n o t involved in th e
processes d escribed above?
c W hich o f the other salts sh own in Table 8 .6 o n
p. 127 arc likely to effl oresce? Give a reason fo r
your answer.
8 ACIDS, BASES AND SALTS
7 Coppcr( n ) sul fu tc crystals exist as the pmtnhydrate, e Som etimes it is necessary to work o ut the
CuS04 .5 H 20 . It is a salt hydrate. !fit is heated
quite stron gly, the water ofcrystallisation is driven percentage b y m ass o f water o f crystallisati on as
off and the anhydrous salt rem ains.
a Explain the meaning o f the terms shown in italics. well as the num ber o f mo les o f water present in a
b D escribe the experiment you would carry
o ut to collect a sample o f the wate r g iven off h ydrated crystal.
when the salt hydrate was heated st ron gly.
Your d escriptio n sh ould include a diagram o f (i) U se the info rmatio n given to calculate
the apparatus u sed and a chemi cal equ ation to
represent the process taking place. the percentage, by m ass, o f water o f
c D escri be a ch e mical test you could carry o ut
to sh ow th at the colourless liquid g iven o ff was crystallisatio n in a sample o f h ydrated
wate r.
d D escri be o ne o the r t est you could carry o ut t o magnesium sulfate.
sh ow that the colo urless liquid o btained in this
exp eriment was pure water. M ass o f crucible = 14.20g
M ass o f crucible+
hyd rated MgS0 4 = 16.66 g
M ass afte r heating = 15.40 g
(ii) Calculate th e number o f m o les of water
o f crystallisation driven o ff during the
experiment as well as the numbe r o f m o les
o f anhyd rous salt remaining.
(A,c H ~ l ; 0 ~ 16; Mg ~ 24;S ~ 32)
(iii) U sing the info rmation you have obtained in
(ii), write d own , in th e fo rm M gS0 4 .xH 20 ,
the fo rmula o f hydrated magn esium sul fa te.
® The Periodic Table
Development of the Periodic Table Group O- the noble gases
Electronic structure and the Periodic Table Transit ion elements
Grou p I - the alkali meta ls The position of hydrogen
Group II - the alkali ne earth metals Check li s t
Additional questions
Group VII - the halogens
Displacement reactions
• Development of the H owever, oth er scienti sts had also attempted to
Periodic Table categorise th e kn own elements. In 181 7, Jo hann
D Obereiner no ticed that the ato mic weight ( now
The Perio dic Table is a vital tool u sed by ch emists call ed ato mic mass) o f stro ntium fell midway between
to predict the way in which elements react during the weights o f calcium and barium . These were
chemical reactio ns. It is a m etho d o f categorising elem ents which possessed similar chemical p roperties.
elements according to their properties . Scientists They fo rmed a triad o f elem ents. Other triads were
started to look fo r a way in which to categorise tlu: also discovered , composed o f:
known elements around 200 years ago.
chl orine, b ro min e, iodine
Th e Periodic Table was d evised in 18 69 by the
Russian Dmitri M cndck:cv, wh o was the Professor o f lithium , sodium , p otassium
C hemistry at St Petersburg University ( Figure 9 .1 ).
His perio dic table was based o n the ch emical and H e called this the 'L,w of Triads' . This encouraged
physical properties o f the 63 elements that h ad been o ther scientists to search fo r patterns.
discovered at that time.
In 1865, John N cwland s, an English chemist,
Figure 9.1 Dmitri Mendeleev(1834-1907). arran ged the 56 known elements in o rder o f
increasing ato mic weight. H e realised when he did
this that every eigl1th elem ent in the series was similar.
H L i Be B C N O F Na M g Al Si P S Cl K
He likened this to mu sic and called it th e ' Law of
Octaves' . It tCII d own, however, becau se som e o f the
weights were inaccurate and there were elem ents that
had not been discovered then .
Mendek:ev's classificati on proved to be the most
su ccessful. M endelcev arranged all the 63 known
elem ents in o rder o f increasing atomic weig ht but in
su ch a way that elements with similar pro perties were
in the same vertical column. H e called the verti cal
columns groups and the ho rizontal rows periods
( Fi gure 9.2 ). If n ecessary he left gap s in th e table.
As a scientific idea, M endeleev's periodic table
was tested by making predictio n s abo ut elem ents
that were unknown at that time but could possibly
fill the gaps. Three o f these gaps arc sh own by the
symbols * and tin Fi gure 9.2 . As new elements were
discovered , they were fo und to fit easily into th e
classificatio n. Fo r example, M end elcev predicted the
properti es o f the mi ssin g cl em ent 'eka -silicon ' ( t ).
H e predicted the colour, d en sity and melting point as
well as its ato mic weig ht.
9 THE PERIODIC TABLE
Per iod Group Th e success o fM cndclccv's prediction s showed
that his ideas were probab ly correct. His perio dic
,, table was quickly accepted by scientists as an
impo rtant summary of th e properties o f the d cm cn ts.
Na Mg A l Cl
M cnddccv's peri odic table has been m odified in tht:
Ca Ti Cr Mn Fe Co Ni light o f work carrit:d out by Rutht:rford and M oseley.
Discoveries about su b -atomic particles led them to
Cu Zn As Se Br realise that the elements sho uld be arranged by p roto n
number. In the mod ern Perio dic Tab le the 11 8 known
Fi gure 9.2 Mendeleev'speriodictable.Heleftgapsforundiscovered elem ents arc arranged in order o f increasing proto n
elements. number (Fig ure 9. 3 ). Those elem ents with similar
chemical properties art: fo und in the same column s or
In 1886 the clement we now know as germanium gro ups. There arc eight groups o f elements. The first
was discovered in Germany by C lem ens W in kler; its column is called Group I; the second Grou p II; and so
properties were almost exactly those Mcnddccv had on up to Grou p VI I. Th e fin al column in the Periodic
predicted . In all , Mcndckc,, predicted the atomic Table is called Group O (or Group VI1I ). Som e o f the
weig ht o f ten new elemen ts, o f which seven were groups have been g iven names.
eventu ally discovered - the o ther three, atomic
we ights 45, 146 and 175, do not exist! Group I: The alkali m etals
Group II: The alkaline earth m etals
G roup VII: The halogens
Group 0 : Inert gases o r no ble gases
Peri od Gro up ~
- -2 :u :ee :He
3 ~Na ::Mg IV V VI VII
·;e ·:c '. N :o ': F : Ne
:OAI :s; :: P :s ·.: c1 : Ar
::Ni "~Cu : zn ~Ga : Ge :As : se :er : Kr
- ~- - . - - -': Pd ':Ag ·~cd ·:in ':sn ·::sb ':Te ·:1 '::Xe
""'""'"
.., O meta llo ids
D reactive metals
O transitionmetals O na n-metals
D poor metals D no ble gases
Figu re 9.3 The modern Periodic Table.
Development of the Periodic Table
Figure 9.4 Transition elements have a wide range of uses, both as elements and as alloys.
The h orizontal rows are call ed periods and th ese are • a change in the stru cture o f the clement, fr o m giant
numbered 1-7 going d own the Perio dic Table . m etallic in the case o f metals (e.g. mag nesium ,
p. 5 6 , Figure 3 .4 3 ), throug h g iant covalent (e .g .
Between G roups I I and III is the block o f elem ents diam o nd , p. 52 , Fig ure 3.3 3), to simple m o lecular
known as th e transitio n elements (Figure 9 .4 ). (e .g . chl orine, p. 4 6 , Fig ure 3.21 ).
Th e Periodic Table can be divided into two as
sh own by the b old lin e that starts ben eath boron, in
Figure 9 .3 . Th e elements to the left o f this line arc
m etals (fewer than three -qu arters) and those o n the
right arc n o n-m etals (fewer than o ne -quarter). The
elem ents which lie o n this dividing line are known
as metall oids (Figure 9 .5 ). These elements behave in
som e ways as metals and in o thers as n on-metals.
If yo u look at th e properties o f the elements across
a perio d o f th e Perio dic Table you will no tice certain
trends. Fo r example, there is:
• a gradu al change fr o m m etal to no n-metal
• an increase in the number o f elect rons in the outer
en ergy level o f th e clem ent
Figure 9.5 The metalloid silicon is used to make silicon 'chips'.
9 THE PERIODIC TABLE
• Electronic structure and prevent them co min g into contact with water o r air.
the Periodic Table These three m etals have the following properties .
The nllmbcr o f elect ron s in the outer en ergy level is • 1l1ey arc good condu cto rs o f electri city and heat.
discu ssed in C hapter 3 (p. 37 ). It can be seen that • They arc soft m etals. Lithium is the hard est and
it correspo nds with the number o f the group in
th e Periodic Table in which th e clem ent is fo und. po tassium the softest .
For example, the elements sh own in Table 9 .1 have • They arc metals with low d en sities. For example ,
o n e electron in th eir outer en ergy leve l and they
arc all found in G roup I. Th e elements in G roup 0, lithium has a den sity o f0.5 3gcm-3 and po tassium
h owever, arc an exception to this rule , as they have has a d ensity o f0. 86gcm-3 .
t wo or eight elect ron s in their o uter energy level. • They have shiny surfaces when freshly cut with a
The o uter el ect ron s arc mainly respon sible fo r th e knife (Fig ure 9.6).
ch emical properties o f any clement, and , th erefore,
elements in the sam e group have similar ch emical
properties (Tables 9.2 and 9.3 ).
Tabl e 9.1 Electronicstructureof thefirstthreeelementsofGroupl.
Element Symbol Proton number Electronic structure
Li thium Na 2,1
Sod iu m 11 2,8,1
Potassium 19 2,8,8,1
Figure9.6 Freshly cutsodium.
Table 9.2 Electronic structure of the first three elements of Group II.
Element ,,Symbol Proton number Electronic structure • They have low melting poin ts. For example,
Beryllium 2,2 lithium h as a melting point o f 18 1 °C and
Magnesium Mg 12 2,8,2 po tassium has a melting po int o f 64 °C.
Calcium Ca 20 2,8,8,2
• They burn in oxygen o r air, with characteristic fl am e
Table9.3 Electronicstructureof the firstthreeelementsinGroup\111. colo urs, to fo rm white solid oxides . Fo r example,
lithium reacts with oxygen in air to fo rm white
Element Symbol Proton number Electronic structure lithium o xide, according to the following equatio n :
Fluorine f 2,7
Chlorine 17 2,8,7 lithium + oxygen ---+ lithium oxid e
Bromine ,C,l 35 2,8,18,7 4Li(,) + 0 2(g) - 2Li20 (,)
The metallic character o f the elements in a group These Group I ox.ides all dissolve in water to fo rm
increases as you m ove d own the group. This is alkaline soluti o ns o f the m etal hyd roxid e.
because electro n s becom e easier to lose as the o uter
shell electro ns becom e furth er fr om the nucleus. lithium oxid e + wat er ---+ lithium hyd roxide
There is less attractio n between the nucleu s and
th e outer shell elect ron s becau se o f the in creased Li,O(s) + H20 (!) - 2 LiOH (aq )
distance between them .
• They react vigorously with water to g ive an
• Group I - the alkali metals alkaline solution o f th e metal h yd roxide as well as
produ cing hyd rogen gas. For example:
G roup I consists o f the fi ve metals lithium , sodium ,
potassium, rubidium and caesium, and the radio active pot assium + water ---+ po tassium + hydrogen gas
clem ent fran cium. Lithium , sodium and potassium
arc commo nly available fo r use in sch ool. Th ey arc all hyd rox id e
very reactive m etals and th ey arc sto red under o il to
2 K(, ) + 2 H,0(1) - 2KO H(aq) + H,(g)
• Of these three m etals, potassium is the m ost
reactive toward s water ( Fi gure 9 .7 ), fo ll owed by
sodium and then lithium. Such gradual changes we
call trends. Trend s arc useful to ch emists as they
Group I - the alka/f metals
all ow prediction s to be m ade abo ut elemen ts we
have n ot o b served in actio n.
• They react vigorously with halogen s, su ch as
chlorine, to form m etal halid es, fo r example sodium
chl orid e ( Fig ure 9. 8 ).
sodium + chlo rine --,. sodium chlo ride
2No (s) + C l2(g) - 2N, Cl(s)
b An alka line solution is produced when potassium reacts with water. Considering the group as a whole, the forther down
Figure9.7 the group you go the more reactive the m etals become .
Francium is, therefore, the most reactive Group I metal.
Table 9. 1 sh ows the electronic stru cture of th e
first three elem ents o f G roup I. You will n otice in
each case that the outer en ergy level contains o nly
o ne elect ron. When these elem ents react th ey lose
this o uter elect ron , and in d o ing so becom e m o re
stable, beca use they obtain the electro n config uratio n
o f a no ble gas . Yo u will learn mo re abo ut th e stabl e
nature o f these gases later in this chapter (p. 143).
\.Yh en , fo r example, the element sodium reacts
it loses its o uter elect ron. This requires energy to
overcome the electrostatic attractive forces between th e
o uter electro n and the positive nucleu s (Figu re 9 .9 ).
The words you are searching are inside this book. To get more targeted content, please make full-text search by clicking here.
Cambridge IGCSE Chemistry 3rd Edition
Discover the best professional documents and content resources in AnyFlip Document Base.
Search