4.4 Oxidation and reduction in terms of electrons

OIL RIG

Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Displacement reactions are redox reactions

When magnesium is added to copper sulfate solution:

Mg + CuSO₄ → MgSO₄ + Cu

The sulfate ions are in solution before and after: they are spectator ions. Leaving them out gives the ionic equation:

Mg + \mathrm{Cu^{2+}} → \mathrm{Mg^{2+}} + Cu

Half equations

A half equation shows what happens to one substance, with the electrons written in.

• Mg → \mathrm{Mg^{2+}} + 2\mathrm{e^-}: magnesium loses electrons, so it is oxidised
• \mathrm{Cu^{2+}} + 2\mathrm{e^-} → Cu: copper ions gain electrons, so they are reduced

The charges must balance on each side of a half equation. For Mg → \mathrm{Mg^{2+}} + 2\mathrm{e^-}: left side 0, right side (2+) + (2−) = 0.

More examples

• Zn + \mathrm{Fe^{2+}} → \mathrm{Zn^{2+}} + Fe: zinc is oxidised, iron ions are reduced
• Metal + acid (Higher): Mg + 2\mathrm{H^+} → \mathrm{Mg^{2+}} + H₂. Magnesium is oxidised and hydrogen ions are reduced. The chloride or sulfate ions are spectators.

Writing an ionic equation

1. Write the balanced symbol equation.
2. Split any dissolved ionic compounds (aq) into their ions.
3. Cross out ions that appear unchanged on both sides.