Oxidation and reduction

Oxidation and reduction always happen together, so these are called redox reactions. At first they are described with oxygen: a substance that gains oxygen is oxidised and one that loses oxygen is reduced. The fuller idea uses electrons: oxidation is loss of electrons and reduction is gain of electrons (OIL RIG). Half equations show the electrons, and at A-level and in CAPS oxidation numbers keep track of them.

Key facts

  • Oxidation is gain of oxygen or loss of electrons. Reduction is loss of oxygen or gain of electrons.
  • OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
  • In a displacement reaction the more reactive metal's atoms lose electrons (oxidised) and the less reactive metal's ions gain them (reduced): Mg + Cu²⁺ → Mg²⁺ + Cu.
  • A half equation shows one change with its electrons: Mg → Mg²⁺ + 2e⁻ (oxidation), Cu²⁺ + 2e⁻ → Cu (reduction). The charges balance on each side.
  • An oxidising agent takes electrons and is reduced; a reducing agent gives electrons and is oxidised.
  • Oxidation numbers: elements are 0, O is usually −2, H is +1, and they add up to the charge. Oxidation makes the oxidation number go up; reduction makes it go down.

Worked examples

Example 1

Iron(III) oxide is heated with carbon monoxide: Fe2O3 + 3CO → 2Fe + 3CO2. Which substance is oxidised and which is reduced?

  1. Iron(III) oxide, Fe2O3, ends up as iron, Fe: it has lost its oxygen.
  2. Losing oxygen is reduction, so iron(III) oxide is reduced.
  3. Carbon monoxide, CO, becomes carbon dioxide, CO2: it has gained oxygen, so it is oxidised.

Answer: carbon monoxide is oxidised; iron(III) oxide is reduced

Example 2

Zinc is added to copper(II) sulfate solution. Write the two half equations and say which is oxidation.

  1. Zinc is more reactive than copper, so zinc atoms give their electrons to copper(II) ions.
  2. Each zinc atom loses 2 electrons: oxidation. Zn → Zn²⁺ + 2e⁻
  3. Each copper(II) ion gains 2 electrons: reduction. Cu²⁺ + 2e⁻ → Cu
  4. Add them and cancel the electrons for the ionic equation. The sulfate ions are spectator ions. Zn + Cu²⁺ → Zn²⁺ + Cu

Answer: Zn → Zn²⁺ + 2e⁻ (oxidation); Cu²⁺ + 2e⁻ → Cu (reduction)

Example 3

What is the oxidation number of manganese in KMnO4?

  1. Potassium is in group 1, so it is +1. Each oxygen is −2.
  2. KMnO4 has no overall charge, so the oxidation numbers add up to 0. +1 + x + 4 × (−2) = 0
  3. Solve for x. x = 8 − 1 = +7

Answer: +7