CHAPTER 3; OXIDATION AND REDUCTION

A substance REDOX A substance
that reduced that oxidized
Chemical reaction where oxidation and
another reduction occur simultaneously another
substance substance

Oxidation Reduction
(act as reducing agent) (act as oxidizing agent)

Gain oxygen 2Mg + O2 ---> 2MgO Loss oxygen CuO + H2 ---> H2O
Loss Hydrogen H2S + Cl2 ---> HCl + S Gain Hydrogen Br2 + 2HI ---> 2HBr + I2
Loss electron Na ---> Na+ + e Gain electron Cl2 + 2e → 2Cl-
Decrease Oxd. No Zn + CuSO4 ---> ZnSO4 + Cu
Increase Oxd. No Zn + CuSO4 ---> ZnSO4 + Cu
(0) (+2) Oxidising agent (+2) (0)
Cu is oxidising agent
EXAMPLE 1 as it causes Mg to Rule 1
be oxidized. the oxidation number for
Magnesium is atom and molecule is zero
oxidized + Cu Example:
Mg, Ca, C, Cu, Zn, Cl2, O2, H2
(gain of oxygen)

Oxidation

Mg + CuO → MgO Rule 2

Reduction The sum of oxidation number

Reducing agent Copper(II) of all elements in polyatomic
Mg is reducing agent oxide is reduced
as it causes CuO to (loss of oxygen) ions is equal to the charge
be reduced.
of the ions

Example:

NH4+ SO42- MnO4- Cr2O72-

+1 -2 -1 -2

EXAMPLE 1 NaCl is an ionic Rule 3
Na → Na+ + e (loss electron) The sum of oxidation
compound. It exist numbers of all elements in
Oxidation as Na+ ions and Cl- the compound is zero
Example:
2Na + Cl2 → 2NaCl ions H2SO4 KMnO4 SO2 K2Cr2O7

Reduction 0 00 0

Cl2 + 2e → 2Cl- (gain electron) Rule 4

Calculate the oxidation numbers for the underlined elements. the oxidation number for
monoatomic ion is equal to
(i) SO2 [compound] (ii) S2O32- [polyatomic ion] its charge
Example:
1(x) + 2(-2) = 0 2(x) + 3(-2) = -2 H+ Fe2+ Cr3- O2- N3-
+1 +2 -1 -2 -3
x- 4 = 0 2x - 6 = -2

x = +4 x = (-2 + 6) /2

Type 1: Redox reaction involves Fe2+ ion and ion Fe3+ ixon = +2
Thus; Oxidation number of S in SO2 Thus;

= +4 Oxd. number of S in S2O32- = +2

Oxidation (loss e-) Others oxidizing agents Others reducing agents
that can replaced bromine that can replaced zinc:
Reducing Fe3+ water: - Magnesium, Mg
agent - Chlorine water, Cl2 - Sulphur dioxide, SO2
Oxd. - Acidified potassium - Hydrogen sulphide,
Fe2+ agent
manganate(VII), H2S
KMnO4 - Sodium sulphide
- Acidified potassium
dichromate(VI), solution, Na2SO3

Reduction (gain e-)

Change of iron (II) ions to iron (III) ions

Observation Bromine Observation
water
Iron (II) solution change Brown colour of bromine water
colour from green to brown decolourised.

FeSO4

Bromine water oxidizes Fe2+ ions Ionic equation: Electron released are accepted by
to Fe3+ ions 2Fe2+ + Br2 → Fe3+ + Br- bromine molecules
Causing Fe2+ release one electron Its undergo reduction to form
bromide ions, Br-
Fe2+ → Fe3+ + e
(Oxidation) Br2 + 2e → Br-
(reduction)
Fe2+ acts as reducing agent because it
reduced Br2 to Br- Br2 acts as oxidizing agent because it
oxidized Fe2+ to Fe3+ ions

Observation Change of iron (III) ions to iron (II) ions Observation

Zn powder dissolved in Zinc Iron (III) solution change
solution Fe2(SO4)3 powder colour from brown to green

Zinc atom lose electrons to Fe3+ Fe2(SO4)3 Zinc powder reduced Fe3+ ions to
Its undergo oxidation to form zinc Fe2+ ions
ions, Zn2+ Ionic equation: Fe3+ accept one electron from
2Fe3+ + Zn → 2Fe2+ +Zn2+ Zinc
Zn → Zn2+ + 2e-
(oxidation) Fe3+ + e → Fe2+
(Reduction)
Zinc acts as reducing agent because it
reduced Fe3+ to Fe2+ ions Fe3+ acts as oxidising agent because
it oxidized Zn to Zn2+

Metal X + Salt Y Observation: Type 2: Displacement of metal from its salt solution
Metal dissolve
Observation: /become thinner K Na Ca Mg Al Zn Fe Sn Pb H Cu Ag Au
Metal dissolve
/become thinner Salt X + Metal Y MORE ELECTROPOSITIVE
More a electropositive metal will displace a
metal less electropositive metal from its
solution

The copper(II) ions are Ionic Equation: Mg2+ + Cu Zinc is more electropositive than
taken out from the solution to Mg + Cu2+ copper ( zinc is in higher position
form copper metal . than copper in electrochemical
series).
Cu2+ (aq) + 2e → Cu (s)
Zinc displaced copper from
Copper(II) ions are reduced. copper(II) sulphate solution.
The blue colour of copper(II)sulphate
solution fades. Zn(s) → Zn2+(aq) + 2e

Zinc is oxidised

Type 3: Displacement Halogen from its halide solution F2 Cl2 Br2 I2

A more reactive halogen displaces a less reactive halogen LESS ELECTRONEGATIVE
from its aFq2ueouCslh2alidBers2olutioIn2. /REACTIVE
The more reactive halogen has a higher tendency to gain
electron.

Chlorine Bromine Bromine
water water water

Potassium Potassium Potassium
bromide bromide bromide

Chlorine act as oxidizing agent Bromine act as oxidizing agent
Chlorine undergo reduction Bromine undergo reduction

Cl2 + 2KBr 2KCl + Br2 Br2 + 2KI 2KBr + I2 Br2 + 2KI 2KBr + I2

Bromine ion undergo oxidation Iodide ion undergo oxidation Bromine cannot displace
Potassium bromide act as Potassium iodide act as reducing chlorine from sodium
reducing agent agent chloride
Observation: Observation: Bromine is less
electronegative than chlorine
Yellow solution of chlorine Brown solution ofbromine
decolorized decolorized
Colourless solution of potassium Colourless solution of potassium
bromide turn to brown bromide turn to brown