Describe the industrial extraction of aluminium by electrolysis, including the role of cryolite and the reactions at the electrodes.

Here are the solutions to the questions:

Question 9:

a) Name:

i) Electrolyte A: Molten aluminium oxide (alumina) dissolved in cryolite.

ii) Substance D: Carbon lining (cathode).

iii) The electrode E: Carbon anode.

iv) The material that electrodes B and E are made of: Carbon (graphite).

b)

i) Write the equation for the reaction that produces aluminium at electrode B.
Electrode B is the cathode, where reduction occurs.

$$\boxed{\text{Al}^{3+}\text{(l)} + \text{3e}^- \longrightarrow \text{Al(l)}}$$

ii) Explain why electrode E has to be replaced periodically.
Electrode E is the carbon anode. During electrolysis, oxide ions ($\text{O}^{2-}$) are oxidized to oxygen gas at the anode. This oxygen then reacts with the hot carbon anode to form carbon dioxide gas, consuming the anode material.

$$\text{C(s)} + \text{O}_2\text{(g)} \longrightarrow \text{CO}_2\text{(g)}$$

iii) Explain why cryolite is added to aluminium oxide before electrolysis.
Cryolite ($\text{Na}_3\text{AlF}_6$) is added to lower the melting point of aluminium oxide from over $2000^\circ\text{C}$ to approximately