Section 1 of 7Why electrolysis is needed
The ore
- The ore The main ore of aluminium is bauxite, which is purified to give aluminium oxide, Al2O3.
Too reactive for carbon
- Too reactive for carbon Aluminium is above carbon in the reactivity series, so carbon cannot reduce its oxide.
- The metal must be extracted by electrolysis.
Why use cryolite
- Why use cryolite Aluminium oxide melts at over 2000 °C, which would waste huge amounts of energy.
- Dissolving it in molten cryolite lowers the temperature needed, which saves energy and cost.
Section 2 of 7The electrolytic cell
Steel cell lined with graphite
Electrolyte
- Electrolyte Aluminium oxide dissolved in molten cryolite, which contains Al3+ and O2− ions.
Cathode (negative)
- Cathode (negative) The graphite lining of the cell.
Anodes (positive)
- Anodes (positive) Several large blocks of graphite lowered into the mixture.
Collection
- Collection Molten aluminium is denser than the electrolyte, sinks to the bottom and is tapped off.
Section 3 of 7Reactions at the electrodes
| Electrode | Half-equation | Type of change |
|---|---|---|
| Cathode | Al3+ + 3e− → Al | Reduction: aluminium ions gain electrons and molten aluminium forms. |
| Anode | 2O2− → O2 + 4e− | Oxidation: oxide ions lose electrons and oxygen gas forms. |
| Anode side reaction | C(s) + O2(g) → CO2(g) | The hot oxygen reacts with the carbon of the anode. |
Section 4 of 7Why the anodes are replaced
1Oxygen forms
- Oxygen forms Oxide ions are discharged at the positive electrodes.
2Carbon reacts
- Carbon reacts The hot oxygen reacts with the graphite to make carbon dioxide.
3Anode burns away
- Anode burns away The anodes gradually wear away and get smaller.
4Regular replacement
- Regular replacement Worn blocks must be replaced regularly, adding to the cost.
Section 5 of 7Environmental and economic points
Huge electricity demand
- Huge electricity demand Large currents are needed to keep the electrolyte molten and to drive the reactions,
- so aluminium plants are often sited near cheap sources of electricity,
- such as hydroelectric power.
Recycling
- Recycling Recycling aluminium needs only a small fraction of the energy used to extract it from the ore,
- so it saves money and reduces carbon dioxide emissions.
Mining impact
- Mining impact Bauxite is mined from open pits,
- which removes vegetation and can damage local habitats,
- and the purification step produces waste that must be managed.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Bauxite | The main ore of aluminium, containing aluminium oxide. |
| Cryolite | A compound that dissolves aluminium oxide and lowers the melting temperature of the mixture. |
| Electrolysis | Breaking down an ionic compound using electricity. |
| Reduction | Gain of electrons. |
| Oxidation | Loss of electrons. |
Section 7 of 7Exam technique
1
Saying that cryolite is the ore. Bauxite is the ore; cryolite is the solvent.
2
Explaining that carbon cannot be used because it is too expensive. The reason is that aluminium is more reactive than carbon.
3
Writing that the cathode wears away. It is the anodes that are burned away.
4
Forgetting to explain that electricity is a major expense of the process.
5
Saying aluminium oxide is melted on its own. It is dissolved in molten cryolite so that a lower temperature is enough.