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.
Why some metals are extracted by electrolysis rather than reduction with carbon, and how aluminium is extracted from a molten mixture of aluminium oxide and cryolite. Common questions ask why cryolite is used, why the anodes must be replaced and why the process is expensive.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
4
State when electrolysis is used for extractionWhen the metal is too reactive to be extracted by carbon, or when it reacts with carbon.
5
Name the products from aluminium oxideAluminium forms at the cathode and oxygen forms at the anode.
5
Explain why electrolysis is expensiveLarge amounts of energy are needed to melt the compound and to produce the electric current.
6
Explain why cryolite is usedMixing aluminium oxide with cryolite lowers the melting point, so less energy is needed.
7
Explain why the anodes must be replacedOxygen formed at the carbon anodes reacts with them to form carbon dioxide, so they wear away.
8
Write half equations for aluminium extractionAl3+ + 3e− → Al at the cathode and 2O2− → O2 + 4e− at the anode.
Notes
When electrolysis is used
A metal is extracted by electrolysis if it is too reactive to be extracted by reduction with carbon, or if the metal reacts with carbon.
The metal compound must be molten so that its ions can move.
Electrolysis is expensive: large amounts of energy are used to melt the compound and to produce the electric current.
Extracting aluminium
The electrolyte is a molten mixture of aluminium oxide, Al2O3, and cryolite.
Aluminium oxide on its own melts at over 2000 °C. The mixture with cryolite melts at a much lower temperature (about 950 °C), so less energy is needed, which saves money.
Aluminium ions (Al3+) move to the cathode (negative), gain electrons and form molten aluminium, which collects at the bottom of the cell.
Oxide ions (O2−) move to the anode (positive), lose electrons and form oxygen.
The positive electrodes (anodes) are made of carbon.
Why the anodes are replaced
At the high temperature, the oxygen produced reacts with the carbon anodes to form carbon dioxide: C + O2 → CO2.
The anodes gradually burn away, so they must be replaced regularly, which adds to the cost.
More reactive than carbon (or reacts with carbon) → extracted by electrolysis of a molten compound
Aluminium: electrolysis of molten aluminium oxide mixed with cryolite
Cryolite lowers the melting point → less energy needed
Energy is needed to melt the electrolyte and to produce the current
Carbon anodes react with oxygen: C + O2 → CO2, so they are replaced
2Al2O3 → 4Al + 3O2
Cathode (−): Al3+ + 3e− → Al grade 8+
Anode (+): 2O2− → O2 + 4e−grade 8+
How to answer each type of question
Explain why a metal is extracted by electrolysis, and why it is expensive
3 to 4 marks5
Compare the metal's reactivity with carbon.
Say that carbon cannot remove the oxygen from (reduce) the oxide.
For cost, mention energy to melt the electrolyte and energy for the current (and anode replacement).
Example. Iron is extracted by heating iron oxide with carbon. Aluminium is extracted by electrolysis. (a) Explain why aluminium cannot be extracted by heating aluminium oxide with carbon. (b) Give two reasons why extracting aluminium by electrolysis is expensive.
Show the model answer
(a) Aluminium is more reactive than carbon (1) so carbon cannot remove oxygen from / reduce aluminium oxide (1) (b) Any two from: a lot of energy is needed to melt the electrolyte (1); a lot of electrical energy is needed to produce the current (1); the carbon anodes have to be replaced (1)
Explain why a mixture with cryolite is used
2 marks6
Say what the mixture does: it has a lower melting point than aluminium oxide alone.
Link to less energy needed, so lower costs.
Example. In the extraction of aluminium, aluminium oxide is mixed with cryolite. Explain why.
Show the model answer
The mixture has a lower melting point than aluminium oxide on its own (1) so less energy is needed to melt it / energy costs are lower (1)
Explain why the positive electrodes must be replaced
3 marks7
Oxygen is produced at the positive electrode.
The electrode is made of carbon, which reacts with the oxygen at the high temperature.
Carbon dioxide forms, so the electrode wears away.
Example. Explain why the positive electrodes need to be replaced regularly during the extraction of aluminium.
Show the model answer
Oxygen is produced at the positive electrode (1) The electrode is made of carbon, which reacts with the oxygen (1) to form carbon dioxide, so the electrode burns away / gets smaller (1)
Complete the half equations
2 marks8
Balance the atoms first (two oxide ions make one O2 molecule).
Add electrons so the total charge is the same on both sides.
Example. Complete the half equations for the electrolysis of aluminium oxide. Al3+ + ....e− → Al ....O2− → O2 + ....e−
Show the model answer
Al3+ + 3e− → Al (1) 2O2− → O2 + 4e− (1)
Shortcuts and memory tricks
Cryolite cuts the melting point, so it cuts the energy bill.
Electron check: Al3+ needs 3e−; two O2− ions give up 4e−.
Where marks are lost
Saying cryolite lowers the boiling point, or that it is a catalyst.
Saying the anodes are replaced because they melt or dissolve: they react with oxygen to form carbon dioxide.
Saying aluminium forms at the anode.
Explaining the cost as 'it uses electricity' without saying large amounts of energy are needed to melt the compound too.
Exam technique
Use the word 'energy' in cost answers: melting the electrolyte and producing the current both need large amounts of energy.
The anode question has three linked points: oxygen forms, it reacts with the carbon, carbon dioxide forms so the anode wears away.
Call the positive electrode the anode and say it is made of carbon.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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.