Magnesium ribbon is placed into copper sulfate solution. The blue colour of the solution fades and a reddish-brown solid forms on the magnesium. Explain, in terms of electron transfer, why this reaction is described as a redox reaction. In your answer, identify which species is oxidised and which is reduced.

AQA A-Level Chemistry (7405) — 3.1.7 Oxidation, reduction and redox equations · Explain · 5 marks · View as Markdown

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

Magnesium ribbon is placed into copper sulfate solution. The blue colour of the solution fades and a reddish-brown solid forms on the magnesium.

Model answer (5 marks)

Magnesium atoms lose electrons to form Mg²⁺.

The loss of electrons is oxidation of magnesium.

Copper ions (Cu²⁺) gain the electrons that magnesium loses.

The gain of electrons is reduction of copper.

Because oxidation of Mg and reduction of Cu occur at the same time, the process is a redox reaction.

Examiner tips

  • Use the words ‘lose’ and ‘gain’ to show electron transfer. Mention Mg²⁺ and Cu²⁺ to show the species. State clearly which is oxidised and which is reduced. Show that both processes happen simultaneously.
  • common_mistakes
  • :
  • Saying ‘magnesium is reduced’ or ‘copper is oxidised’ – reverse the roles. Forgetting to identify the ions (Mg²⁺, Cu²⁺) or the electron transfer.

Mark scheme (5 marks)

  1. Magnesium atoms lose electrons
  2. Magnesium is oxidised (because it loses electrons)
  3. Copper ions gain electrons
  4. Copper ions are reduced (because they gain electrons)
  5. Both oxidation and reduction occur simultaneously, so the reaction is a redox reaction

Key terms in this question

redox reaction · electron transfer

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