Metals more reactive than carbon are extracted by electrolysis

1. Metals more reactive than carbon, such as aluminium, cannot be extracted by reduction with carbon.
2. These metals are extracted by electrolysis of a molten compound instead.
3. The compound must be molten so its ions are free to move to the electrodes.

Aluminium is extracted from aluminium oxide, using cryolite to save energy

1. Aluminium oxide, ${Al}_2{O}_3$, has a very high melting point, so melting it alone would use huge amounts of energy.
2. It is dissolved in molten cryolite, which lowers the operating temperature and cuts the energy cost.
3. At the cathode, aluminium ions are reduced to molten aluminium.
   1. $${Al}^{3+}+3e^{-}\to Al$$
4. At the anode, oxide ions are oxidised to oxygen gas.
   1. $$2O^{2-}\to O_2+4e^{-}$$

Electrolysis is costly, and the carbon anodes burn away

1. Large amounts of energy are needed to melt the compound and to drive the current, so the process is expensive.
2. The oxygen made at the hot carbon anodes reacts with them to form carbon dioxide.
3. The anodes gradually burn away, so they must be replaced regularly, which adds to the cost.