Redox reactions: oxidation and reduction

Redox covers reactions where electrons move. Oxidation and reduction always happen together: one species loses electrons while another gains them. You can spot a redox change by oxygen, by hydrogen, by electron transfer, or by a change in oxidation state.

Four ways to spot redox

A reaction is oxidation if a species gains oxygen, loses hydrogen, loses electrons, or its oxidation state rises. Reduction is the opposite of each. The electron view is the most general — remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Oxidising and reducing agents

The oxidising agent is the species that causes oxidation by taking electrons — so it is itself reduced. The reducing agent gives electrons, so it is itself oxidised. Naming the agents correctly is a common exam ask, and the trick is that the agent does the opposite of its name to itself.

Oxidation states

Oxidation state is a bookkeeping number for electrons. Elements are 0; simple ions take the charge of the ion; oxygen is usually −2 and hydrogen +1. Tracking how these change across a reaction is a reliable way to prove a redox process and to identify what was oxidised or reduced.

Tests for redox

Colour changes flag redox in the lab: acidified potassium manganate(VII) goes from purple to colourless when reduced, and potassium iodide releases brown iodine when oxidised. These appear in qualitative analysis and data questions, so link the observation to the electron change.