Oxidation and reduction in terms of electrons (HT only)

Oxidation can also be described using electrons. Oxidation is the loss of electrons and reduction is the gain of electrons. A helpful reminder is OIL RIG: oxidation is loss, reduction is gain. When a metal atom forms a positive ion it loses electrons, so the metal is oxidised.

Displacement reactions show both changes at once. When iron displaces copper from copper sulfate solution, the ionic equation is Fe + Cu²⁺ → Fe²⁺ + Cu. The sulfate ions do not change, so they are left out. Zinc behaves in the same way: Zn + Cu²⁺ → Zn²⁺ + Cu.

We can split the ionic equation into two half equations. Fe → Fe²⁺ + 2e^- shows iron atoms losing electrons, so iron is oxidised. Cu²⁺ + 2e^- → Cu shows copper ions gaining electrons, so copper ions are reduced. The electrons lost by one species are the electrons gained by the other, and the charges balance on both sides.