AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemical changes › Electrolysis
Practise Using electrolysis to extract metals. 5 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.
Key facts
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
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
diagram
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) and form molten aluminium, which collects at the bottom of the cell.
Oxide ions (O2−) move to the anode (positive) and form oxygen.
The positive electrodes (anodes) are made of carbon.
Oxygen forms at the carbon anodes (red bubbles) and reacts with them to make carbon dioxide, so they burn away.
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.
Overall: 2Al2O3 → 4Al + 3O2
How to answer each type of question
Explain why a metal is extracted by electrolysis, and why it is expensive
3 to 4 marksGrade 5
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 answerHide 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)
Don’t lose marks
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.
More tips
Memory tricks
Cryolite cuts the melting point, so it cuts the energy bill.
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.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
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.
Grade 5
Name the products from aluminium oxideAluminium forms at the cathode and oxygen forms at the anode.
Grade 5
Explain why electrolysis is expensiveLarge amounts of energy are needed to melt the compound and to produce the electric current.
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
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