Section 1 of 5Why Electrolysis Is Used
Aluminium is too reactive to be extracted by reduction with carbon, so it is obtained by electrolysis.
High reactivity
- Aluminium is above carbon in the reactivity series, so carbon cannot reduce its oxide.
Ore purification
- Bauxite is purified to produce aluminium oxide, Al₂O₃, which is the compound that is electrolysed.
Melting point
- Aluminium oxide melts above 2000 °C, which would require a very large amount of energy.
Cryolite
- Dissolving aluminium oxide in molten cryolite lowers the melting point of the mixture, which saves energy.
Section 2 of 5Reactions in the Cell
The cell is made of steel with a graphite lining, which acts as the negative electrode.
1
Large graphite blocks act as the positive electrodes, dipping into the molten mixture of aluminium oxide and cryolite.
2
At the cathode, aluminium ions gain electrons: Al³⁺ + 3e⁻ → Al, and molten aluminium collects at the bottom of the cell.
3
At the anode, oxide ions lose electrons: 2O²⁻ → O₂ + 4e⁻, so oxygen gas is released at the positive electrodes.
4
The oxygen reacts with the hot carbon anodes to form carbon dioxide, C(s) + O₂(g) → CO₂(g), so the anodes wear away.
Section 3 of 5Costs and Control
Costs and Control
Anode replacement
- The graphite anodes are used up by reacting with oxygen and must be replaced regularly.
Electricity
- Large amounts of electricity are needed, which is a major expense in aluminium production.
Fresh oxide
- Molten aluminium is siphoned off from time to time, and fresh aluminium oxide is added to the cell.
Recycling
- Recycling aluminium needs far less energy than electrolysis of the ore, so it is economically attractive.
Section 4 of 5Worked Example
Question How many moles of electrons are needed to produce 2.0 mol of aluminium at the cathode?
- Cathode equationAl³⁺ + 3e⁻ → Al, so 3 mol of electrons are needed for every 1 mol of aluminium.
- For 2.0 mol of aluminium: 2.0 × 3 = 6.0 mol of electrons.
Answer 6.0 mol of electrons are required.
Section 5 of 5Exam Checklist & Tips
3.4.6 – Key Points for the Exam
- Cryolite A compound that dissolves aluminium oxide and lowers its melting point.
- Bauxite The main ore of aluminium, which contains aluminium oxide.
- Siphoning Removing a liquid from a container by drawing it off through a tube.
- Oxide ion An ion of oxygen with a charge of 2−, written O²⁻.
Common Mistakes
- Stating that carbon reduces aluminium oxide in this process; electrolysis is required instead.
- Saying aluminium forms at the anode; it forms at the cathode.
- Forgetting that the carbon anodes react with oxygen and wear away.
- Describing cryolite as a reactant; it dissolves the oxide and is not used up.