AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to: explain why a mixture is used as the…
Full specification wording
Using electrolysis to extract metals: Students should be able to: explain why a mixture is used as the electrolyte explain why the positive electrode must be continually replaced.
In aluminium extraction, pure aluminium oxide has a very high melting point, so using it alone would demand excessive energy and cost.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 5 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In aluminium extraction, pure aluminium oxide has a very high melting point, so using it alone would demand excessive energy and cost. Dissolving the oxide in molten cryolite produces a mixture that melts at a much lower temperature, so the cell operates at a lower temperature and uses less energy. The mixture also contains mobile ions, allowing it to conduct electricity. At the positive carbon anode, oxide ions are discharged as oxygen: 2O²⁻ → O₂ + 4e⁻. This hot oxygen reacts with the carbon of the anode to form carbon dioxide, so the anode gradually burns away. It must therefore be replaced periodically, which is a recognised operating cost of the process.
What earns the marks
- The electrolyte is a mixture of aluminium oxide and cryolite because pure aluminium oxide has a very high melting point.
- Cryolite lowers the melting point of the mixture, so less energy is needed to keep the electrolyte molten.
- The molten mixture contains mobile ions, so it conducts electricity and electrolysis can occur.
- At the positive electrode, oxide ions are oxidised to oxygen: 2O²⁻ → O₂ + 4e⁻.
- The oxygen produced reacts with the carbon anode to form carbon dioxide, so the anode is gradually used up.
- The anode must be continually replaced because it is consumed by reaction with oxygen, adding to the cost of the process.
Common mistakes and how to put them right
Saying cryolite is added to increase conductivity only.
Put it right by explaining that its main role is to lower the melting point and reduce energy cost.
Claiming the anode is replaced because it melts.
Put it right by stating that it reacts with oxygen to form carbon dioxide and is chemically consumed.
Writing that oxygen is produced at the cathode.
Put it right by placing oxygen production at the positive electrode (anode).
Exam tips
- Use the phrase 'lowers the melting point' and link it to reduced energy demand.
- For anode replacement, name the reaction product carbon dioxide and state that carbon is used up.
- Keep electrode names and charges consistent: positive electrode (anode), negative electrode (cathode).
Try the lesson
The start of the Using electrolysis to extract metals: Students should be able to: explain why a mixture is used as the electrolyte explain why the positive electrode must be continually replaced. lesson, made from checked teaching notes for this specification point.
Have a go first
In the extraction of aluminium, why is a mixture of aluminium oxide and cryolite used as the electrolyte?
Listen
In aluminium extraction, pure aluminium oxide has a very high melting point, so using it alone would demand excessive energy and cost. Dissolving the oxide in molten cryolite produces a mixture that melts at a much lower temperature, so the cell operates at a lower temperature and uses less energy. The mixture also contains mobile ions, allowing it to conduct electricity. At the positive carbon anode, oxide ions are discharged as oxygen: 2O²⁻ → O₂ + 4e⁻. This hot oxygen reacts with the carbon of the anode to form carbon dioxide, so the anode gradually burns away. It must therefore be replaced periodically, which is a recognised operating cost of the process. A strong answer earns marks for these points. The electrolyte is a mixture of aluminium oxide and cryolite because pure aluminium oxide has a very high melting point. Cryolite lowers the melting point of the mixture, so less energy is needed to keep the electrolyte molten. The molten mixture contains mobile ions, so it conducts electricity and electrolysis can occur. At the positive electrode, oxide ions are oxidised to oxygen: 2O²⁻ → O₂ + 4e⁻. A common slip: Saying cryolite is added to increase conductivity only. Instead, explaining that its main role is to lower the melting point and reduce energy cost. Exam tip: Use the phrase 'lowers the melting point' and link it to reduced energy demand.
Key words
Say what each word means before you turn the card.
Electrode reactions in molten electrolysis
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Industrial costs and maintenance
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Oxidation at the anode
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See every definition
- Electrode reactions in molten electrolysis
- At the positive electrode, oxide ions are oxidised to oxygen: 2O²⁻ → O₂ + 4e⁻.
- Industrial costs and maintenance
- The anode must be continually replaced because it is consumed by reaction with oxygen, adding to the cost of the process.
- Oxidation at the anode
- At the positive electrode, oxide ions are oxidised to oxygen: 2O²⁻ → O₂ + 4e⁻.
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