Practise Using electrolysis to extract metals. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Aluminium is extracted from aluminium oxide by electrolysis. What is the positive electrode made from?
carbon (graphite)
In the manufacture of aluminium, the electrolyte is a molten mixture of aluminium oxide and cryolite. Name the gas formed when the positive electrode wears away.
carbon dioxide
Write the half equation for the reaction at the negative electrode.
Al3+ + 3e− → Al
Write the half equation for the reaction at the negative electrode.
Na+ + e− → Na
The aluminium oxide is mixed with another compound so that the electrolyte melts at a lower temperature. Name this compound.
cryolite
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Aluminium is extracted from aluminium oxide by electrolysis.
(a) Explain why aluminium is extracted by electrolysis and not by reduction with carbon.[1]
(b) What is the positive electrode made from?[1]
(c) Name the product formed at the negative electrode.[1]
(d) Why is extracting metals by electrolysis expensive? Tick (✓) one box.[1]
Large amounts of energy are needed to melt the compound and to produce the electric current.
The electrodes are made of a precious metal.
The products are all gases.
Aluminium oxide is very rare.
Show the answer and mark scheme
(a)
aluminium is more reactive than carbon
(b)Answer: carbon (graphite)
carbon / graphite
(c)Answer: aluminium
aluminium
(d)Answer: Large amounts of energy are needed to melt the compound and to produce the electric current.
Question 2Medium5 marks
In the manufacture of aluminium, the electrolyte is a molten mixture of aluminium oxide and cryolite.
(a) Explain why a mixture of aluminium oxide and cryolite is used as the electrolyte, rather than aluminium oxide alone.[2]
(b) Explain why the positive electrodes must be replaced regularly.[2]
(c) Name the gas formed when the positive electrode wears away.[1]
Show the answer and mark scheme
(a)
the mixture has a lower melting point than aluminium oxide
so less energy is needed (to melt it), which reduces costs
(b)
oxygen is produced at the positive electrode
the oxygen reacts with the carbon electrode to form carbon dioxide, so the electrode wears away / burns away
(c)Answer: carbon dioxide
carbon dioxide
Question 3Hard7 marks
Aluminium is manufactured by electrolysis. The electrolyte is aluminium oxide dissolved in molten cryolite. The electrodes are made of carbon.
(a) Write the half equation for the reaction at the negative electrode.[1]
(b) Explain how aluminium is manufactured by electrolysis. Include the reasons for the conditions used and what happens at each electrode.[6]
Show the answer and mark scheme
(a)Answer: Al3+ + 3e− → Al
Al3+ + 3e− → Al
(b)
electrolysis is used because aluminium is more reactive than carbon
aluminium oxide is dissolved in molten cryolite, which lowers the melting point, so less energy is needed
the electrolyte is molten so the ions are free to move
Al3+ ions are attracted to the negative electrode, where they gain electrons to form aluminium (reduction)
molten aluminium collects at the bottom of the cell and is tapped off
O2− ions are attracted to the positive electrode, where they lose electrons to form oxygen (oxidation), 2O2− → O2 + 4e−
the oxygen reacts with the carbon positive electrodes to form carbon dioxide, so these electrodes must be replaced regularly
large amounts of electrical energy are needed, so the process is expensive
Marked with levels of response: the full level descriptors are in the app.