Oxidation and reduction in terms of electrons (HT only) — AQA GCSE Chemistry
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Oxidation and reduction in terms of electrons (HT only) explained
A displacement reaction occurs when a more reactive metal takes the place of a less reactive metal in a compound.
Read the full explanation
To write the ionic equation, show only the ions that change: the solid metal atom loses electrons to form aqueous ions, while the aqueous ion of the less reactive metal gains electrons and forms a solid atom. For example, zinc added to copper(II) sulfate solution gives Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). The zinc is oxidised because it loses electrons; the copper ion is reduced because it gains electrons. Spectator ions, such as sulfate, are omitted. In a half equation, electrons appear on the right for oxidation and on the left for reduction. Identifying which species is oxidised or reduced means tracking electron loss or gain, not simply naming the more reactive metal.
Your focus
- Write a balanced ionic equation for a displacement reaction by removing spectator ions.
- Identify the species oxidised and the species reduced in a given reaction, symbol equation or half equation.
- Explain oxidation and reduction in terms of loss and gain of electrons.
Oxidation and reduction in terms of electrons (HT only) exam tips
Marking Points
- Write the full symbol equation first, then cancel spectator ions to leave the ionic equation.
- Show the solid metal atom losing electrons to form a positive ion in the ionic equation.
- Show the metal ion in solution gaining electrons to form a solid metal atom.
- State that oxidation is loss of electrons and reduction is gain of electrons.
- Identify the species oxidised as the one that loses electrons and the species reduced as the one that gains electrons.
- Check that charge and mass balance on both sides of the ionic equation.
Examiner Tips
- 💡Underline the ions that change before writing the final ionic equation.
- 💡For each half equation, count the charge on each side to confirm electrons are placed correctly.
- 💡Use the phrase 'loss of electrons is oxidation, gain of electrons is reduction' to justify your identification.
Common Mistakes
- Including spectator ions such as sulfate in the ionic equation; the correction is to cancel ions that appear unchanged on both sides.
- Saying the more reactive metal is reduced because it 'wins'; the correction is to track electrons: the more reactive metal loses electrons and is oxidised.
- Writing electrons on the wrong side of a half equation; the correction is electrons on the right for oxidation and on the left for reduction.