The simplified diagram below shows an electrolytic cell used in the industrial extraction of aluminium (Al) from aluminium oxide at temperatures as high as 1000 °C - NSC Physical Sciences - Question 9 - 2017 - Paper 2

The half-reaction that takes place at electrode X is oxidation.

The function of the cryolite in the electrolytic process is to lower the melting point of aluminium oxide and allow it to dissolve more easily, improving the efficiency of the electrolysis.

The reduction half-reaction can be represented as:

$Al^{3+}(aq) + 3e^- \rightarrow Al(s)$

The balanced equation that illustrates why the carbon rod must be replaced is:

$2Al_2O_3(l) + 3C \rightarrow 4Al(l) + 3CO_2(g)$

This reaction indicates that carbon is consumed during the process, leading to the necessity for regular replacement of the carbon rod.