Question 1 Medium
Explain why aluminium oxide must be molten for electrolysis.
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Answer: the ions are free to move
- Solid ionic compounds cannot conduct.
- When molten, the ions are free to move.
Extracting metals by electrolysis: GCSE Combined Science Chemistry practice questions
A second set of questions on extracting metals by electrolysis. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
Give one advantage of recycling aluminium cans compared with making aluminium from its ore.
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Answer: it uses much less energy
- Extraction needs a lot of electricity.
- Melting scrap needs far less energy.
Question 3 Medium
2Al₂O₃ → 4Al + 3O₂. Calculate the mass of aluminium made from 102 tonnes of aluminium oxide. Ar of Al = 27 and O = 16.
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Answer: 54 tonnes
- Mr of Al₂O₃ = 102, so moles = 102 ÷ 102 = 1.0.
- Moles of Al = 2 × 1.0 = 2.0.
- Mass = 2.0 × 27 = 54 tonnes.
Question 4 Medium
State one way in which the cost of extracting aluminium can be reduced.
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Answer: use cheap electricity
- The process uses a lot of electricity.
- Using cheap electricity or recycling reduces the cost.
Question 5 Hard
Write a half equation for the formation of aluminium at the cathode.
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Answer: Al³⁺ + 3e⁻ → Al
- Aluminium ions are 3+.
- They gain three electrons.
Question 6 Hard
Write a half equation for the formation of oxygen at the anode in aluminium extraction.
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Answer: 2O²⁻ → O₂ + 4e⁻
- Two oxide ions make one O₂ molecule.
- Four electrons are lost.
Question 7 Hard
Write a balanced equation for carbon in an anode reacting with oxygen to make carbon dioxide.
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Answer: C + O₂ → CO₂
- Carbon and oxygen make carbon dioxide.
- The equation balances as written.
Question 8 Hard
Write a half equation for the formation of sodium from sodium ions in the electrolysis of molten sodium chloride.
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Answer: Na⁺ + e⁻ → Na
- Sodium ions are 1+.
- They gain one electron.
Question 9 Hard
2Al₂O₃ → 4Al + 3O₂. Calculate the mass of aluminium oxide needed to make 54 tonnes of aluminium. Ar of Al = 27 and O = 16.
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Answer: 102 tonnes
- Moles of Al = 54 ÷ 27 = 2.0.
- The ratio Al₂O₃ : Al is 1 : 2, so moles of Al₂O₃ = 1.0.
- Mass = 1.0 × 102 = 102 tonnes.
Question 10 Hard
Explain why electrolysing aluminium chloride solution does not give aluminium.
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Answer: hydrogen is made at the cathode because aluminium is more reactive than hydrogen
- Aluminium is more reactive than hydrogen.
- So hydrogen ions are discharged at the cathode instead.
Question 11 Hard
Give two reasons why extracting aluminium by electrolysis is expensive.
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Answer: it needs a lot of electricity and the carbon anodes must be replaced
- The mixture must be kept molten.
- A very large current is needed, and the anodes burn away.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
Which metals are extracted by electrolysis rather than by heating with carbon?
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Metals more reactive than carbon, such as aluminium, magnesium and calcium.
Check 2
How is iron extracted from iron oxide?
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By heating the oxide with carbon, which removes the oxygen.
Check 3
What is reduction in terms of oxygen?
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Loss of oxygen from a compound.
Check 4
What is the ore of aluminium?
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Bauxite, which is purified to give aluminium oxide.
Check 5
Why is aluminium oxide dissolved in molten cryolite?
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To lower the melting point, so less energy is needed.
Check 6
At about what temperature does the aluminium cell run?
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About 950 °C.
Check 7
What are the electrodes made of in the aluminium cell?
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Carbon (graphite).
Check 8
What forms at the cathode in the aluminium cell?
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Molten aluminium, which sinks and is tapped off.
Check 9
What forms at the anodes in the aluminium cell?
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Oxygen, which reacts with the carbon to make carbon dioxide.
Check 10
Why do the carbon anodes in the aluminium cell have to be replaced?
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The oxygen burns them away as carbon dioxide.
Check 11
Why can aluminium not be extracted from a solution of its salt?
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Aluminium is more reactive than hydrogen, so hydrogen would form at the cathode.
Check 12
Why is the aluminium oxide molten during electrolysis?
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So that its ions are free to move and carry the current.
Check 13
Why is aluminium expensive to extract?
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It needs a great deal of electricity to keep the mixture molten and to run the cell.
Check 14
How much energy does recycling aluminium save?
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About 95% of the energy needed to make it from the ore.
Check 15
What is the half equation for aluminium ions forming aluminium?
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Al³⁺ + 3e⁻ → Al.
Check 16
What is the half equation for oxide ions forming oxygen?
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2O²⁻ → O₂ + 4e⁻.
Check 17
What is the overall equation for the electrolysis of aluminium oxide?
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2Al₂O₃ → 4Al + 3O₂.
Check 18
What is the ratio of Al₂O₃ to Al in the overall equation?
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2 : 4, which is 1 : 2.
Check 19
What happens at the cathode in terms of electrons?
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Ions gain electrons. This is reduction.
Check 20
What happens at the anode in terms of electrons?
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Ions lose electrons. This is oxidation.
Check 21
Which metal is found uncombined in the ground and needs no extraction?
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Gold.
Check 22
Why are aluminium plants often built near hydroelectric power?
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Electricity is a very large part of the cost, so cheap power saves money.
Check 23
What is the half equation for sodium ions forming sodium?
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Na⁺ + e⁻ → Na.
Check 24
How do you check a reacting mass answer is sensible?
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A metal is lighter than its oxide, and the masses of the products add up to the mass of the reactant.
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