Chemistry, Third edition

7 · OXIDATION AND REDUCTION

Oxidizing and reducing agents

Anoxidizing agentis a substance that takes up electrons during a chemical reaction

and, in doing so, becomes reduced. A reducing agentsupplies the electrons in this

process and so becomes oxidized.

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Oxidizing and

reducing agents

Look back at the redox

ladder for sulfur in Exercise

7D:

(i) Which sulfur species

can act as both

reducing and oxidizing

agents in different

chemical reactions?

(ii)Which sulfur species

can only act as

oxidizing agents and

why?

(iii)Which sulfur species

can only act as

reducing agents and

why?

Exercise 7F

Example 7.1

When iron is heated in chlorine gas, iron(III) chloride is

produced. The ‘ordinary’ equation is:

2Fe(s)3Cl 2 (g)2FeCl 3 (s)

The redox half-equations are

Fe(s)Fe^3 (s)3e

Cl 2 (g)2e2Cl(s)

Here, iron is acting as a reducing agentbecause it supplies electrons to chlorine

and is oxidized.

Chlorine is acting as an oxidizing agentbecause it accepts electrons from iron and

isreduced.

Writing and balancing redox equations

In order to write a redox equation, it is first necessary to write the two half-reactions

that identify the oxidation and reduction processes taking place. The overall redox

equation is then obtained by adding these two half-reactions together, so that the

electrons in each half reaction cancel out.

To write the half reactions, follow these simple rules:

1.Identify the atoms that are oxidized and reduced, using the oxidation number

method.

Most of the reactions that you will come across at this stage, will occur in neutral or acid

solution, and step 2 applies.

2.Balance the half-reactions:

(i) Make sure that there are the same number of atoms of the element that is

oxidized (or reduced) on each side of the half-reaction.

(ii)If there are any oxygen atoms present, balance them by adding water molecules

to the other side of the half-reaction.

(iii)If there are hydrogen atoms present, balance them by adding hydrogen ions on

the other side of the half-reaction.

(iv)Make sure that the half-reactions have the same overall charge on each side by

adding electrons.

Note that the rules are slightly different if the reaction occurs in basic solution:

hydrogen atoms are balanced using H 2 O molecules and then the same number of

OHions are added to the opposite side of the equation to balance the oxygens.

Carry on as before, adding electrons to balance the charges.

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