AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Aluminium is manufactured by the electrolysis of a molten mixture of…
Full specification wording
Using electrolysis to extract metals: Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode (anode).
Aluminium is extracted from its ore by electrolysis because carbon reduction is ineffective: aluminium is more reactive than carbon, so displacement from aluminium oxide will not occur.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Aluminium is extracted from its ore by electrolysis because carbon reduction is ineffective: aluminium is more reactive than carbon, so displacement from aluminium oxide will not occur. The oxide is dissolved in molten cryolite, which lowers the working temperature and lets the mixture conduct. In the cell, aluminium oxide dissociates into Al³⁺ and O²⁻ ions. At the carbon anode, oxide ions lose electrons to form oxygen gas: 2O²⁻ → O₂ + 4e⁻. At the cathode, Al³⁺ ions gain electrons: Al³⁺ + 3e⁻ → Al. The molten aluminium is denser than the electrolyte and is tapped off. The anode burns away because hot oxygen reacts with carbon to form carbon dioxide, so it is replaced periodically.
What earns the marks
- Aluminium is more reactive than carbon, so electrolysis rather than reduction by carbon is needed to extract it from aluminium oxide.
- The electrolyte is a molten mixture of aluminium oxide and cryolite; cryolite dissolves the oxide and lowers the melting point, reducing energy cost.
- At the positive electrode (anode), oxide ions are oxidised: 2O²⁻ → O₂ + 4e⁻.
- At the negative electrode (cathode), aluminium ions are reduced: Al³⁺ + 3e⁻ → Al.
- The carbon anode reacts with the oxygen produced, forming carbon dioxide, so it is gradually used up and must be replaced.
- Molten aluminium collects at the cathode and is tapped off because it is denser than the electrolyte.
Common mistakes and how to put them right
Writing the anode product as hydrogen or the cathode product as oxygen.
Put it right by linking oxide ions to the anode and aluminium ions to the cathode.
Stating that cryolite is added to increase the melting point.
Put it right by saying it lowers the melting point and saves energy.
Describing the anode as inert.
Put it right by explaining that carbon reacts with oxygen to form carbon dioxide and is consumed.
Exam tips
- Name both electrodes and give the ionic half-equations with state symbols where possible.
- Explain the role of cryolite in terms of melting point and energy cost, not just 'it helps'.
- Link anode replacement directly to the reaction of carbon with oxygen, forming carbon dioxide.
Try the lesson
The start of the Using electrolysis to extract metals: Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode (anode). lesson, made from checked teaching notes for this specification point.
Have a go first
In the industrial extraction of aluminium, what is the name of the positive electrode made of carbon?
Listen
Aluminium is extracted from its ore by electrolysis because carbon reduction is ineffective: aluminium is more reactive than carbon, so displacement from aluminium oxide will not occur. The oxide is dissolved in molten cryolite, which lowers the working temperature and lets the mixture conduct. In the cell, aluminium oxide dissociates into Al³⁺ and O²⁻ ions. At the carbon anode, oxide ions lose electrons to form oxygen gas: 2O²⁻ → O₂ + 4e⁻. At the cathode, Al³⁺ ions gain electrons: Al³⁺ + 3e⁻ → Al. The molten aluminium is denser than the electrolyte and is tapped off. The anode burns away because hot oxygen reacts with carbon to form carbon dioxide, so it is replaced periodically. A strong answer earns marks for these points. Aluminium is more reactive than carbon, so electrolysis rather than reduction by carbon is needed to extract it from aluminium oxide. The electrolyte is a molten mixture of aluminium oxide and cryolite; cryolite dissolves the oxide and lowers the melting point, reducing energy cost. At the positive electrode (anode), oxide ions are oxidised: 2O²⁻ → O₂ + 4e⁻. At the negative electrode (cathode), aluminium ions are reduced: Al³⁺ + 3e⁻ → Al. A common slip: Writing the anode product as hydrogen or the cathode product as oxygen. Instead, linking oxide ions to the anode and aluminium ions to the cathode. Exam tip: Name both electrodes and give the ionic half-equations with state symbols where possible.
Key words
Say what each word means before you turn the card.
Electrolysis of molten compounds
Try answering aloud or in your head before turning the card.
Extraction of reactive metals
Try answering aloud or in your head before turning the card.
Oxidation and reduction at electrodes
Try answering aloud or in your head before turning the card.
Industrial energy and cost considerations
Try answering aloud or in your head before turning the card.
See every definition
- Electrolysis of molten compounds
- Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode.
- Extraction of reactive metals
- Aluminium is extracted from its ore by electrolysis because it is more reactive than carbon, so carbon reduction is ineffective.
- Oxidation and reduction at electrodes
- At the anode oxide ions lose electrons to form oxygen gas, and at the cathode aluminium ions gain electrons to form aluminium.
- Industrial energy and cost considerations
- Cryolite dissolves the oxide and lowers the melting point, reducing energy cost, while the carbon anode is gradually used up and must be replaced.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Using electrolysis to extract metals: Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode (anode).
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge