4.13 Extracting aluminium by electrolysis
5 exam questions worth 7 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Why is aluminium oxide dissolved in molten cryolite?
Choose one answer.
Correct Not quite: the answer is To lower the melting point
Mark scheme
D
Model answer
D: cryolite lowers the melting point, so less energy is needed.
Explain why the carbon anodes must be replaced regularly.
Mark scheme
- oxygen is produced at the anode (1)
- it reacts with the (hot) carbon to form carbon dioxide, so the anodes burn away (1)
Model answer
Oxygen forms at the anodes and reacts with the hot carbon to make carbon dioxide, so the anodes are gradually used up.
Explain why aluminium cannot be extracted by heating aluminium oxide with carbon.
Mark scheme
- aluminium is more reactive than carbon (1)
Model answer
Aluminium is more reactive than carbon, so carbon cannot remove the oxygen from aluminium oxide.
Give two reasons why extracting aluminium is expensive.
Mark scheme
any two from:
- large amounts of electricity (1)
- energy to keep the electrolyte molten / high temperature (1)
- anodes must be replaced (1)
Model answer
It uses huge amounts of electricity and energy to keep the electrolyte molten, and the carbon anodes have to be replaced.
Higher tier.
Write the half equation for the reaction at the cathode.
Mark scheme
- \mathrm{Al^{3+}} + 3\mathrm{e^-} → Al (1)
Model answer
\mathrm{Al^{3+}} + 3\mathrm{e^-} → Al
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