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Aluminium is more reactive than carbon so it must be extracted from its compounds using electrolysis. Even though aluminium is more abundant than iron in the Earth's crust, aluminium is more expensive than iron. This is mainly because of the large amounts of electrical energy used in the extraction process.

Electrolysis of aluminium oxide

The electrolyte

Aluminium ore is treated to produce pure aluminium oxide. The electrolytes used in electrolysis are ionic compounds:

• in the molten state, or
• dissolved in water

Aluminium oxide is insoluble in water, so it must be molten to act as an electrolyte. However, the melting point of aluminium oxide is high. A lot of energy must be transferred to break its strong ionic bonds, and this is expensive. To reduce costs, powdered aluminium oxide is dissolved in molten cryolite. This ionic compound melts at a lower temperature than aluminium oxide, reducing costs. However, significant amounts of energy are required to melt the cryolite.

The electrolysis process

The diagram shows an electrolysis cell used to extract aluminium. Both electrodes are made of graphite, a form of carbon with a high melting point and which conducts electricity.

During electrolysis:

• at the cathode, aluminium ions gain electrons and form aluminium atoms
• at the anode, oxide ions lose electrons and form oxygen gas

The oxygen reacts with the carbon anodes, forming carbon dioxide. So the anodes are gradually oxidised. They must be replaced frequently, adding to the cost of producing aluminium.

Worked example - Higher

Explain, with the help of a half equation, how oxide ions are oxidised during the electrolysis of aluminium oxide.

The half equation is: 2O^2- → O₂ + 4e^-. It shows that oxide ions lose electrons, and oxidation is loss of electrons.