Oxidising agent, reducing agent: which gives, which takes?

A reaction unlike any other

When you dip an iron nail into a solution of copper sulphate (containing Cu²⁺ ions), the nail gradually becomes covered in a reddish deposit of metallic copper, and the blue solution fades. Something invisible to the naked eye has taken place: electrons have moved from the iron to the copper ions. This is a redox reaction.

Key definitions

• A reducing agent is a chemical species capable of donating one or more electrons. Metallic iron (Fe), by donating electrons, is the reducing agent.
• An oxidising agent is a chemical species capable of accepting one or more electrons. The Cu²⁺ ion, by accepting electrons, is the oxidising agent.
• The reducing agent is oxidised (it loses electrons), whilst the oxidising agent is reduced (it gains electrons).

A useful mnemonic: “OIL RIG” (Oxidation Is Loss, Reduction Is Gain). Above all, remember: losing electrons = oxidation, gaining electrons = reduction.

One reaction, two halves

There is never oxidation without an associated reduction: the electrons released by the reducing agent are immediately accepted by the oxidising agent. The redox reaction is the result of two coupled half-reactions, which occur simultaneously.

Common mistake

Do not confuse ‘being oxidised’ with ‘turning into an oxide’: being oxidised means losing electrons, which does not necessarily involve the formation of an oxide. For example, when an Fe²⁺ ion becomes Fe³⁺, it oxidises, without any oxygen atoms being involved in the transformation.