Aluminium is extracted from its ore by electrolysis of molten aluminium oxide. Explain what happens at each electrode during this process, including the type of reaction occurring at each electrode.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Aluminium is one of the most widely used metals in the world. It cannot be extracted by reduction with carbon because it is too reactive, so electrolysis of molten aluminium oxide is used instead. The electrodes used are inert.
Model answer (4 marks)
Aluminium ions (Al³⁺) are positively charged and migrate to the cathode. At the cathode electrons are supplied from the external circuit, so Al³⁺ + 3e⁻ → Al (metal) – a reduction reaction.
Oxide ions (O²⁻) are negatively charged and move to the anode. At the anode electrons are removed, so 2O²⁻ → O₂ + 4e⁻ – an oxidation reaction, producing oxygen gas.
Oxide ions (O²⁻) are negatively charged and move to the anode. At the anode electrons are removed, so 2O²⁻ → O₂ + 4e⁻ – an oxidation reaction, producing oxygen gas.
Examiner tips
- Use the correct charge symbols (Al³⁺, O²⁻) and state the direction of ion movement. Include the half‑reactions with electron transfer. Mention that the cathode is negative and the anode is positive. Keep the answer concise and to the point.
Common mistakes
- Writing the wrong ion charge or forgetting the + or – sign. Confusing the cathode with the anode or vice versa. Omitting the electron transfer in the half‑reaction or not specifying that oxygen gas is produced.
Mark scheme (4 marks)
- Aluminium ions (Al³⁺) are positively charged ions / cations that move to the cathode (negative electrode)
- At the cathode, aluminium ions gain electrons / are reduced to form aluminium metal
- Oxide ions (O²⁻) are negatively charged ions / anions that move to the anode (positive electrode)
- At the anode, oxide ions lose electrons / are oxidised to form oxygen gas
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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