How to identify oxidation half-reaction and reduction half-reaction in a redox equation?

explain

A redox reaction is a reaction in which there is an exchange of electrons between different species. As one specie gains electrons the other loses it and this furnishes the reaction.

Redox reactions are made up of half-reactions which shows how a specie gains electrons and what happens to it. It also shows what happens to a specie that loses electrons.

To understand and be able to identify what an oxidation and reduction half reaction is, we need to know what oxidation and reduction entails.

Oxidation is the loss of electron by an atom. It also entails addition of oxygen to a specie, removal of hydrogen from a specie. This leads to increase in oxidation number of the specie.

Reduction is just the opposite of oxidation and electrons are gained by an atom here.

To identify an oxidation or reduction half in a redox reaction, we look at the species closely and we check for the following:

1. Changes in oxidation number

2. Wether a species loses or gains electrons.

3. Addition / removal of oxygen and hydrogen.

Let's take for example:

Fe²⁺ + MnO₄⁻ → Fe³⁺ + Mn²⁺

By inspection, we look at the species involved:

Fe: Fe²⁺ → Fe³⁺

The oxidation number increases from 2 + to 3+

There is a loss of an electron for this to occur

We see that this is an oxidation half

Mn: MnO₄⁻ → Mn²⁺

The oxidation number of Mn changes from + 7 to + 2:

X + (-2x4) = - 1

X - 8 = - 1

X = 7

This implies that 5 electrons were gained.

There is also a loss of oxygen.

This makes the reaction a reduction half.

This simple way is used to identify reduction and oxidation halves.