Question

In an electrolytic tank, aluminium metal is being extracted by the electrolysis of molten aluminium oxide using carbon electrodes. It is observed that one of the carbon electrodes is gradually burnt away and has to be replaced.

1. Which carbon electrode (cathode or anode) is burnt away?
2. Why is this carbon electrode burnt away?

Answer 1

1. The carbon electrode acting as an anode (positively charged) is burnt away.
2. In the electrolysis of molten aluminium oxide using carbon electrodes, the pure metal is formed at the cathode (negatively charged electrode), and oxygen is formed at the anode (positively charged electrode). This oxygen reacts with positively-charged carbon electrode and produces carbon dioxide gas. Due to this reaction between oxygen and the carbon electrode (anode), the electrode gradually burns away and must be replaced.
   \[\ce{C_{(s)} + O2_{(g)} -> CO2_{(g)}}\]