Magnesium ribbon is added to copper sulfate solution. The magnesium displaces copper from the solution. Explain what happens during this reaction in terms of oxidation and reduction, identifying which substance is oxidised and which is reduced, and explaining why this reaction is classed as a redox reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When magnesium ribbon is placed into blue copper sulfate solution, the blue colour fades and a reddish-brown solid forms on the magnesium. The overall equation for the reaction is: Mg + CuSO₄ → MgSO₄ + Cu
Model answer (5 marks)
Magnesium is oxidised.
Magnesium atoms lose two electrons to form Mg²⁺.
Copper ions are reduced.
Cu²⁺ ions gain the two electrons lost by magnesium to form Cu(s).
The reaction is classed as a redox reaction because oxidation (loss of electrons by Mg) and reduction (gain of electrons by Cu²⁺) occur simultaneously in the same process.
Magnesium atoms lose two electrons to form Mg²⁺.
Copper ions are reduced.
Cu²⁺ ions gain the two electrons lost by magnesium to form Cu(s).
The reaction is classed as a redox reaction because oxidation (loss of electrons by Mg) and reduction (gain of electrons by Cu²⁺) occur simultaneously in the same process.
Examiner tips
- Use the word "oxidised" for Mg and "reduced" for Cu²⁺. Include the electron transfer (Mg → Mg²⁺ + 2e⁻, Cu²⁺ + 2e⁻ → Cu). State that both processes happen together to justify the redox classification.
Common mistakes
- Writing that magnesium gains electrons instead of loses them. Forgetting to mention the electron transfer. Saying the reaction is redox because only one element changes oxidation state.
Mark scheme (5 marks)
- Magnesium is oxidised
- Magnesium loses electrons (or magnesium gains oxygen / loses hydrogen — but electron loss is the key definition here)
- Copper ions are reduced
- Copper ions gain electrons (or copper loses oxygen)
- The reaction is classed as redox because oxidation and reduction occur simultaneously (in the same reaction)
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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