Oxidation and reduction reactions

Reactions in which both oxidation and reduction occur are called redox reactions. The acronym OILRIG might be familiar, reminding us that oxidation reactions involve a loss of electrons by a species and reduction reactions involve a species gaining electrons.

We can split equations such as this into half equations which show us the transfer of electrons more clearly.

Oxidising agents remove electrons from another substance (and they are reduced themselves). Non-metal elements such as the halogens (F₂, Cl₂) are very powerful oxidising agents as they are so keen to gain electrons themselves to form anions.

Reducing agents give electrons to another substance (and they are oxidised themselves).

In this example, sodium is the reducing agent, giving electrons to oxygen and sodium is itself oxidised. Oxygen is the oxidising agent, removing electrons from sodium and oxygen is itself reduced.

For non-ionic compounds e.g. NH₃ and polyatomic ions e.g. SO₄^2- writing half equations to follow electron movements during a reaction is not possible, so the concept of oxidation states or oxidation numbers was invented. We can deduce whether an element has been oxidised or reduced by following the change in oxidation number during the reaction.