During the electrolysis of copper(II) sulfate solution using copper electrodes, describe what happens at each electrode and explain why the concentration of copper(II) sulfate solution remains constant throughout the process.

Cambridge International IGCSE Chemistry (0620) — 4.1 Electrolysis · 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)

At the cathode copper ions are reduced:
1. Cu²⁺ + 2e⁻ → Cu(s)
Copper is deposited on the cathode.

At the anode copper is oxidised:
2. Cu(s) → Cu²⁺ + 2e⁻
The anode dissolves.

The rate of Cu²⁺ removal at the cathode equals the rate of Cu²⁺ production at the anode, so the number of Cu²⁺ ions in solution remains unchanged. Therefore the concentration of Cu²⁺ (and CuSO₄) stays constant throughout the electrolysis.

Examiner tips

  • Use the correct half‑reactions for each electrode.
  • Show that the electron flow is the same at both electrodes.
  • Explain that equal rates keep the ion concentration constant.
  • Keep the answer concise and use the exact terminology.

Common mistakes

  • Confusing the cathode and anode reactions.
  • Failing to state that the ion concentration remains constant.
  • Using incorrect electron counts in the half‑reactions.

Mark scheme (4 marks)

  1. At the cathode, copper is deposited / copper ions gain electrons
  2. At the anode, copper dissolves / the anode loses mass / copper atoms lose electrons
  3. The rate at which copper is removed from solution at the cathode equals the rate at which copper enters solution from the anode
  4. Therefore the number of copper(II) ions in solution stays the same / concentration of Cu²⁺ ions does not change

Key terms in this question

electrolysis · copper(II) sulfate solution · concentration

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