A student electrolyses copper(II) sulfate solution using copper electrodes instead of inert electrodes. Explain what happens at each electrode during this electrolysis, and explain why the concentration of copper(II) sulfate solution remains constant throughout the process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
At the cathode copper(II) ions are reduced:
Cu²⁺ + 2e⁻ → Cu(s) (copper metal deposits on the electrode).
At the anode copper atoms from the copper electrode are oxidised:
Cu(s) → Cu²⁺ + 2e⁻ (copper ions enter the solution).
The rate of Cu²⁺ removal at the cathode equals the rate of Cu²⁺ production at the anode, so the total number of Cu²⁺ ions in solution remains unchanged. Hence the concentration of copper(II) sulfate stays constant throughout the electrolysis.
Cu²⁺ + 2e⁻ → Cu(s) (copper metal deposits on the electrode).
At the anode copper atoms from the copper electrode are oxidised:
Cu(s) → Cu²⁺ + 2e⁻ (copper ions enter the solution).
The rate of Cu²⁺ removal at the cathode equals the rate of Cu²⁺ production at the anode, so the total number of Cu²⁺ ions in solution remains unchanged. Hence the concentration of copper(II) sulfate stays constant throughout the electrolysis.
Examiner tips
- Use the correct half‑reactions for Cu²⁺ reduction and Cu oxidation.
- Show that the two processes are equal in rate – this gives the constant concentration.
- Mention that the solution is not consumed or produced.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing the cathode and anode reactions; writing Cu²⁺ → Cu(s) at the anode.
- Failing to state that the rates are equal, so concentration is unchanged.
- Using the wrong electrode material (e.g., inert electrodes) in the explanation.
Mark scheme (4 marks)
- At the cathode, copper(II) ions gain electrons / are reduced to form copper metal deposited on the electrode
- At the anode, copper atoms from the copper electrode lose electrons / are oxidised to form copper(II) ions which pass into solution
- The rate at which copper ions are removed at the cathode equals the rate at which copper ions are released at the anode
- Therefore the overall concentration of copper(II) sulfate solution does not change / stays constant
Key terms in this question
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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