After having discussed the concept of oxidation number, let us now define oxidation and reduction in terms of oxidation number. Oxidation is defined as a chemical process in which oxidation number of the element increases. On the other hand, reduction is defined as the chemical process in which oxidation number of the element decreases. For example, let us consider the reaction between hydrogen sulphide and bromine to give hydrogen bromide and sulphur.
In the above example, the oxidation number of bromine decreases from 0 to -1, thus, it is reduced. The oxidation number of S increases from – 2 to 0. Hence, ~S is oxidised.
Let us now define oxidising and reducing agents in the light of the concept of oxidation number:
Oxidising agent is a substance which undergoes decrease in the oxidation number of one or more of its elements.
Reducing agent is a substance which undergoes increase in the oxidation number of one or more of its elements. In the above example, H2S is reducing agent while Br2 is oxidising agent.
OXIDATION HALF AND REDUCTION HALF REACTIONS
Every redox reaction can be split up into two half reactions. The part of the redox reaction which represents loss of electrons or increase in oxidation number, is called oxidation half reaction while the other part which represents gain of electrons or decrease in oxidation number, is called reduction half reaction. Some examples are given below:
(i) The reaction: Zn + Cu2+ àZn2+ + Cu, can be split up into two half equations as
(i) The reaction: Sn2+ + 2Hg2+ à Sn4+ + Hg2 can be split up into half reactions as