Aluminium is extracted by electrolysis. Some information about the process is given.
- Pure aluminium oxide melts at 2072 °C.
- The aluminium oxide is dissolved in molten cryolite, and the electrolysis is carried out at about 950 °C.
- The electrodes are made of carbon. Aluminium forms at the cathode and oxygen forms at the anode.
(a) Suggest why the aluminium oxide is dissolved in molten cryolite. [2]
(b) Write the half-equation for the formation of aluminium at the cathode. [1]
(c) Suggest why the carbon anodes have to be replaced regularly. [2]
- (a) lowers the temperature needed / mixture is molten (melts) at a lower temperature (than 2072 °C) (allow lowers the melting point; allow so the ions can move / it conducts at a lower temperature)(1)
- (a) so less (heat) energy is needed / reduces (energy / fuel) costs / cheaper (ignore 'quicker' / 'easier')(1)
- (b) $\text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al}$ (allow e for e⁻; ignore state symbols)(1)
- (c) the (hot) carbon (anode) reacts with the oxygen (produced there) / the anode burns / is oxidised by the oxygen (merely restating that oxygen forms at the anode scores 0)(1)
- (c) forming carbon dioxide (allow carbon monoxide; allow $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$) / so the anode wears away / gets smaller / is used up (either for the mark)(1)
- Answer: (a) lowers melting point, saving energy (b) Al³⁺ + 3e⁻ → Al (c) O₂ reacts with carbon anodes forming CO₂
- Examiner tip: Use the data you are given — 'lowers the melting point' only scores fully when you link it to less energy or lower cost.