Revision of oxidation and reduction
We use batteries throughout our day-to-day lives. Cell phones use lithium-ion batteries, cars
use lead-acid batteries (Figure Figure 1 (left)), while silver-oxide batteries (Figure Figure
1 (top right)) are used in watches. Some batteries are rechargeable (Figure Figure
1 (bottom right)), while others cannot be recharged and have to be thrown away.
Figure 1: a) A car battery, b) rechargeable AA batteries and c) watch batteries.
A battery consists of multiple electrochemical cells. And within each cell there
are electrochemical reactionstaking place. This can be seen in the lemon
battery experiment shown in Figure Figure 2. Each lemon is a cell in the battery, which
consists of three lemon cells. The reactions the copper and zinc undergo in the lemons are
electrochemical reactions, and a current is produced.
Figure 2: A current is produced by connecting lemons with zinc and copper metal. The
reactions taking place here are electrochemical reactions.
Electrochemical reactions, and electrochemical cells are covered in this chapter. Before
going into any more detail however, it is important to revise oxidation and reduction, as well
as redox reactions and how to balance them, as these concepts are very important in
electrochemistry.
Revision of oxidation and reduction
You should remember the terms oxidation and reduction from Grade 11:


Oxidation involves a loss of electrons
Reduction involves a gain of electrons.
An easy way to remember this is:
In both oxidation and reduction a transfer of electrons is involved resulting in a change in
the oxidation state of the elements.

An element or compound that loses electrons is oxidised.
e.g. Zn(s)→Zn2+(aq) + 2eAs it loses electrons it gives them away to another element or compound and the
element or compound it gives the electrons to is reduced.
This makes the compound or element which loses electrons a reducing agent.

An element or compound that gains electrons is reduced.
e.g. Cu2+(aq) + 2e-→Cu(s)
As it gains electrons it takes them away from another element or compound and the
element or compound it takes them from is oxidised.
This makes the compound or element which gains electrons an oxidising agent.
Exercise 1: Oxidation and reduction
Problem 1:
Define the following terms:
1. oxidation
2. reduction
3. oxidising agent
4. reducing agent
Practise more questions like this
Answer 1:
1. Oxidation is the loss of electrons by a molecule, atom or ion.
2. Reduction is the gain of electrons by a molecule, atom or ion.
3. The molecule, atom or ion that is reduced, or the molecule, atom or ion that causes
oxidation.
4. The molecule, atom or ion that is oxidised, or the molecule, atom or ion that causes
reduction.
Problem 2:
In each of the following reactions say whether the reactant iron species (Fe, Fe2+, Fe3+) is
oxidised or reduced.
1. Fe(s) → Fe2+(aq)+2e−
2. Fe3+(aq)+e− → Fe2+(aq)
3. Fe2O3(s)+3CO(g) → 2Fe(s)+3CO2(g)
4. Fe2+(aq) → Fe3+(aq)+e−
5. Fe2O3(s)+2Al(s) → Al2O3(s)+2Fe(s)
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Answer 2:
1. The oxidation number of Fe in:


Fe(s) is 0
Fe2+(aq) is +2
Fe(s) → Fe2+(aq)+2e−
Fe loses two electrons to become Fe2+. The iron species (Fe(s)) is oxidised.
2. The oxidation number of Fe in:
 Fe3+(aq) is +3
 Fe2+(aq) is +2
Fe3+(aq)+e− → Fe2+(aq)
Fe3+ gains one electron to become Fe2+. The iron species (Fe3+(aq)) is reduced.
3. The oxidation number of O is −2. Therefore the oxidation number of O3 in Fe2O3 is −6.
Therefore the oxidation number of Fe2 in Fe2O3 is +6.
The oxidation number of Fe in:
 Fe2O3(s) is +3
 Fe(s) is 0
Fe3+(s)+3e− → Fe(s)
Fe3+ gains three electrons to become Fe(s). The iron species (Fe3+(s)) is reduced.
4. The oxidation number of Fe in:
 Fe2+(aq) is +2
 Fe3+(aq) is +3
Fe2+(aq) → Fe3+(aq)+e−
Fe2+ loses an electron to become Fe3+. The iron species (Fe2+(aq)) is oxidised.
5. The oxidation number of O is −2. Therefore the oxidation number of O3 in Fe2O3 is −6.
Therefore the oxidation number of Fe2 in Fe2O3 is +6.
The oxidation number of Fe in:


Fe2O3(s) is +3
Fe(s) is 0
Fe3+(s)+3e− → Fe(s)
Fe3+ gains three electrons to become Fe(s). The iron species (Fe3+(aq)) is reduced.