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.
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 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.
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.
Explain, with the help of a half equation, how oxide ions are oxidised during the electrolysis of aluminium oxide.
The half equation is: 2O2- → O2 + 4e-. It shows that oxide ions lose electrons, and oxidation is loss of electrons.
Explain, with the help of a half equation, how aluminium ions are reduced during the electrolysis of aluminium oxide.
The half equation is: Al3+ + 3e- → Al
It shows that aluminium ions gain electrons, and reduction is gain of electrons.