Electrolysis of Aluminium Oxide Half Equations

During electrolysis aluminium oxide ions (Al3+) and oxygen ions (O2-) are separated in an electrolytic cell. This is done by passing electricity between two electrodes – a negative cathode and a positive anode. The anode is made from graphite which is a form of carbon and is a good conductor of electricity. The cathode is made from steel which has a low melting point. The aluminium ions are discharged at the cathode to become molten aluminium metal, which is then siphoned off. Oxide ions are also discharged at the anode to become oxygen gas which reacts with the carbon on the graphite anodes to give off some carbon dioxide CO2.

The half equation below shows the balanced chemical equation for the electrolytic extraction of aluminium from its oxide (Al2O3). This is an oxidation/reduction reaction and it is important to remember that electrons must be transferred from the cathode to the anode during electrolysis if the process is to continue.

The main expense associated with this process is the cost of the anodes, which are made from graphite – a form of carbon. The anodes need to be replaced frequently as they erode, but this can be minimised by dissolving the aluminium oxide in molten cryolite, which lowers the melting point of the aluminium oxide and makes it easier to extract the aluminium. This also reduces the energy required to produce aluminium because the ions are much less likely to break away from the anode material.