IGCSE Chemistry 0620: Extraction of Aluminium by Electrolysis

An aluminium smelting plant extracts aluminium from purified aluminium oxide (obtained from bauxite) by electrolysis. The aluminium oxide is dissolved in molten cryolite and electrolysed using carbon electrodes, at a temperature far below the melting point of pure aluminium oxide.

(a) State the role of cryolite in this process. [1]

(b) Write ionic half-equations for the reactions at: (i) the cathode [1] (ii) the anode [1]

(c) Explain why the carbon anodes in this cell need to be replaced at regular intervals, while the cathode does not need to be replaced in the same way. [2]

(d) Plant records show that, at a steady rate of operation, $300$ tonnes of anode carbon are consumed for every $900$ tonnes of aluminium produced. Assuming the plant maintains this same ratio, calculate the mass of anode carbon that would be consumed while producing $1500$ tonnes of aluminium. [2]

Part (a): The role of cryolite

Pure aluminium oxide has an extremely high melting point, so melting it alone would need huge amounts of energy. Cryolite dissolves the aluminium oxide and, in doing so, lowers the temperature at which the mixture will conduct electricity and undergo electrolysis, making the whole process far more economical.

Part (b): Half-equations at each electrode

At the cathode (negative electrode), aluminium ions gain electrons (reduction):

$\text{Al}^{3+} + 3e^- \rightarrow \text{Al}$

At the anode (positive electrode), oxide ions lose electrons (oxidation):

$2\text{O}^{2-} \rightarrow \text{O}_2 + 4e^-$

Part (c): Why the anodes need replacing

The oxygen gas produced at the anode does not simply escape, at the high operating temperature of the cell, it reacts with the hot carbon anode itself, forming carbon dioxide gas. This slowly burns away (erodes) the anode material, so the anodes must be replaced at regular intervals.

The cathode does not suffer this problem: it simply has aluminium metal deposited onto it, which does not consume the electrode itself.

Part (d): Scaling the anode-carbon ratio

The plant’s ratio of anode carbon consumed to aluminium produced is:

$300 \text{ tonnes carbon} : 900 \text{ tonnes aluminium}$

Keeping this ratio constant for $1500$ tonnes of aluminium:

$300 \times \frac{1500}{900} = 300 \times \frac{5}{3} = 500$

So $\boxed{500}$ tonnes of anode carbon would be consumed.

Final answers

(a) Cryolite dissolves the aluminium oxide and lowers the temperature needed for electrolysis.
(b) Cathode: $\text{Al}^{3+} + 3e^- \rightarrow \text{Al}$ ; Anode: $2\text{O}^{2-} \rightarrow \text{O}_2 + 4e^-$
(c) Oxygen from the anode reacts with the hot carbon, forming $\text{CO}_2$ and burning the anode away; the cathode is not consumed this way.
(d) $500$ tonnes