Extracting aluminium
Aluminium is more than carbon so it must be extracted from its using . Even though aluminium is more than iron in the Earth's , aluminium is more expensive than iron. This is mainly because of the large amounts of used in the process.
Electrolysis of aluminium oxide
The electrolyte
Aluminium is treated to produce pure aluminium oxide. The used in electrolysis are :
- in the state, or
- in water
Aluminium oxide is in water, so it must be molten to act as an electrolyte. However, the of aluminium oxide is high. A lot of energy must be transferred to break its strong , 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 are made of graphite, a form of carbon with a high melting point and which conducts electricity.
During electrolysis:
- at the , aluminium gain and form aluminium
- at the , oxide ions lose electrons and form oxygen gas
The oxygen reacts with the carbon anodes, forming carbon dioxide. So the anodes are gradually . They must be replaced frequently, adding to the cost of producing aluminium.
Worked example - Higher
Explain, with the help of a , 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.
Question
Explain, with the help of a half equation, how aluminium ions are during the electrolysis of aluminium oxide.
