During the electrolysis of copper sulfate solution, copper is deposited at the negative electrode and oxygen is produced at the positive electrode. Explain why each product forms at its respective electrode.

AQA GCSE Chemistry (8462) — 4.4.3 Electrolysis · Explain · 4 marks · View as Markdown

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

Copper sulfate solution contains copper ions (Cu²⁺), sulfate ions (SO₄²⁻), hydrogen ions (H⁺) and hydroxide ions (OH⁻).

Model answer (4 marks)

Copper ions (Cu²⁺) are positively charged and are attracted to the negative electrode (cathode). At the cathode they gain electrons (reduction) and are deposited as copper metal.

Hydroxide ions (OH⁻) are negatively charged and are attracted to the positive electrode (anode). At the anode they lose electrons (oxidation) and form oxygen gas.

Examiner tips

  • Use the charge of the ions to explain migration to electrodes. Show the redox process at each electrode. Mention reduction at the cathode and oxidation at the anode. Keep the answer concise and use correct terminology.

Common mistakes

  • Confusing the direction of ion migration. Forgetting that copper is reduced, not oxidised. Saying oxygen comes from water instead of hydroxide ions.

Mark scheme (4 marks)

  1. Copper ions (Cu²⁺) are positively charged and move to the negative electrode (cathode)
  2. At the negative electrode, copper ions gain electrons and are reduced to form copper metal
  3. Hydroxide ions (OH⁻) are negatively charged and move to the positive electrode (anode)
  4. At the positive electrode, hydroxide ions lose electrons and are oxidised, producing oxygen gas

Key terms in this question

electrolysis · negative electrode · positive electrode

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