During the electrolysis of copper sulfate solution, copper is deposited at one electrode and oxygen gas is produced at the other. Explain what happens at each electrode during this process, including the movement and behaviour of ions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student sets up an electrolysis cell using copper sulfate solution as the electrolyte and two inert carbon electrodes connected to a power supply.
Model answer (4 marks)
At the cathode (negative electrode) the Cu²⁺ ions from the copper sulfate solution are attracted because they are cations. They gain two electrons (reduction) according to
Cu²⁺ + 2e⁻ → Cu(s)
and copper metal is deposited on the electrode.
At the anode (positive electrode) the negatively charged species – mainly OH⁻ ions produced from the dissociation of water – are attracted. They lose electrons (oxidation) to form oxygen gas:
4OH⁻ → O₂ + 2H₂O + 4e⁻
Thus Cu²⁺ ions are reduced at the cathode and oxygen is evolved at the anode while the ions move toward the electrode of opposite charge.
Cu²⁺ + 2e⁻ → Cu(s)
and copper metal is deposited on the electrode.
At the anode (positive electrode) the negatively charged species – mainly OH⁻ ions produced from the dissociation of water – are attracted. They lose electrons (oxidation) to form oxygen gas:
4OH⁻ → O₂ + 2H₂O + 4e⁻
Thus Cu²⁺ ions are reduced at the cathode and oxygen is evolved at the anode while the ions move toward the electrode of opposite charge.
Examiner tips
- Use the correct electrode names (cathode, anode) and the charge of the ions (Cu²⁺ cation, OH⁻ anion).
- Show the half‑reactions and the electron transfer to demonstrate reduction/oxidation.
- Mention that copper is deposited as a solid at the cathode and that oxygen gas is released at the anode.
Common mistakes
- Confusing the cathode with the anode or vice versa.
- Forgetting to state that Cu²⁺ is reduced to Cu(s).
- Using the wrong anion (e.g., sulfate) instead of hydroxide for the oxygen evolution reaction.
Mark scheme (4 marks)
- Copper ions (Cu²⁺) are positively charged ions (cations) that move towards the cathode (negative electrode)
- At the cathode, copper ions gain electrons (reduction), forming copper metal which is deposited
- Sulfate / hydroxide ions (anions) are negatively charged and move towards the anode (positive electrode)
- At the anode, water molecules / hydroxide ions lose electrons (oxidation) and oxygen gas is produced
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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