Why electrolysis

Aluminium is above carbon, so aluminium oxide cannot be economically reduced by carbon. Electrolysis of molten ionic material supplies electrons directly to Al3+ ions.

Cryolite

Purified aluminium oxide is dissolved in molten cryolite. Cryolite lowers the mixture’s melting and operating temperature, reducing electrical energy cost and helping form a workable electrolyte.

Cell structure

Carbon blocks are positive anodes. The carbon-lined container acts as the cathode. Molten aluminium forms at the bottom and is removed.

Energy and environmental cost

The process consumes large amounts of electricity. Its environmental impact depends strongly on the source of electrical energy and on carbon-anode consumption.

High-value recall and connections

Essential recall: Why is electrolysis used for aluminium? Aluminium is too reactive for carbon reduction. What is the role of cryolite? It dissolves Al2O3 and lowers the operating/melting temperature. Where does molten aluminium collect? At the carbon-lined cathode at the bottom. Practical connection: Label a cross-section of the aluminium cell and explain why aqueous aluminium oxide cannot be used to deposit aluminium. Examination connection: State that cryolite lowers the operating or melting temperature; do not claim it is the catalyst.

Review questions and suggested answers