Magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen gas. Describe what happens during this reaction and explain why magnesium is able to react with hydrochloric acid whilst copper does not.

AQA GCSE Chemistry (8462) — 4.4.2 Reactions of acids · Describe and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Magnesium displaces hydrogen from the acid, so the acid supplies H⁺ ions and the magnesium gives up two electrons to form Mg²⁺ ions. The reaction produces MgCl₂ in solution and H₂ gas.

Magnesium is higher in the reactivity series than hydrogen, so it can oxidise hydrogen ions to H₂. Copper is lower in the series; it has a lower tendency to lose electrons and form Cu²⁺, so it cannot displace hydrogen from hydrochloric acid.

Examiner tips

  • Use the phrase ‘displaces hydrogen’ to show the displacement reaction.
  • Mention that Mg forms Mg²⁺ ions – the formation of a positive ion is key.
  • Compare the reactivity of Mg and Cu to hydrogen using the reactivity series.
  • Keep the answer concise – 4 marks can be earned with two short points.

Common mistakes

  • Writing that magnesium reacts with acid but not mentioning hydrogen displacement.
  • Confusing the order of the reactivity series (placing copper above hydrogen).
  • Failing to state that Mg forms Mg²⁺ ions or that Cu has a lower tendency to form positive ions.

Mark scheme (4 marks)

  1. Magnesium displaces hydrogen from the acid / magnesium is more reactive than hydrogen
  2. Magnesium atoms form positive ions (Mg²⁺) during the reaction
  3. Copper is less reactive than hydrogen / copper is lower in the reactivity series than hydrogen
  4. The reactivity of a metal is related to its tendency to form positive ions / copper has a lower tendency to form positive ions than magnesium

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