Write half equations for the reaction of zinc with hydrochloric acid.
- Oxidation: Zn → Zn²⁺ + 2e⁻.
- Reduction: 2H⁺ + 2e⁻ → H₂.
Answer Zn → Zn²⁺ + 2e⁻ and 2H⁺ + 2e⁻ → H₂
GCSE Combined Science revision · Chemistry: reactions and energy
The idea
When a metal reacts with an acid, the metal atoms lose electrons (oxidation) and hydrogen ions gain electrons (reduction): Mg + 2H⁺ → Mg²⁺ + H₂. So acid–metal reactions are redox reactions. In electrolysis, reduction happens at the cathode (ions gain electrons) and oxidation at the anode (ions lose electrons).
- Write the ionic equation, leaving out spectator ions.
- Identify which species loses and which gains electrons.
- Link the cathode to reduction and the anode to oxidation.
Watch out In the reactions of magnesium, zinc or iron with dilute hydrochloric or sulfuric acid, the chloride or sulfate ions are spectators.
See it worked
Your turn
In the reaction of magnesium with an acid, the hydrogen ions are:
Which process happens at the cathode during electrolysis?
How many electrons does one magnesium atom lose when it reacts with acid?
In past papers
2 official questions in 2 papers since 2019. The latest:
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