Chemistry, Third edition

REDOX REACTIONS 101

reactions tend to be very similar. It is reasonable to expect that copper undergoes

much the same process here, as it does when it reacts with oxygen.

When copper reacts with oxygen (is oxidized) to form copper(II) oxide, the

chemical equation may be written as

2Cu(s)O 2 (g)2CuO(s)

In this reaction, a copper atom loses two electrons:

CuCu^2 2e

Each oxygen atom in an oxygen molecule gains two electrons from the copper atom

(the oxygen is reduced):

O2eO^2 

Or, since oxygen gas reacts with copper,

O 2 4e2O^2 

The overall equation can be obtained by adding the two ‘half-reactions’ together,

so that the electrons cancel out on each side:

1.First multiply the equation for the oxidation of copper by two, so that this half-

reaction contains the same number of electrons as the half equation for the

reduction of O 2 :

2Cu2Cu^2 4e

O 2 4e2O^2 

2.Then add the two half-reactions and the number of electrons on each side cancels

out:

2CuO 2 4e2Cu^2 2O^2 4e

3.The overall equation is

2Cu(s)O 2 (g)2Cu^2 (s)2O^2 (s)

Similar equations can be written for the reaction of copper with sulfur:

CuCu^2 2e(oxidation)

S2eS^2 (reduction)

Adding and cancelling electrons gives

Cu(s)S(s)Cu^2 (s)S^2 (s)

Here, a copper atom loses two electrons just as it did in the reaction with oxygen – it

is oxidized. Sulfur gains two electrons and is reduced.

Reactions such as these, led to a far more general definition of the terms oxidation

and reduction:

Oxidation occurs when electrons are lost, reduction occurs when electrons

are gained.

To help you remember:

OILRIG

Oxidationis electron loss;reductionis electron gain