Chemistry 9701/12 — October/November 2013

At the age of 17, in a woodshed in Ohio, Charles Martin Hall discovered the commercial process for the production of aluminium metal by the electrolysis of a mixture of bauxite, $\text{Al}_2\text{O}_3$, and cryolite, $\text{Na}_3\text{AlF}_6$.

What is the main purpose of the cryolite?

Options

A   $\text{Al}_2\text{O}_3$ is covalent, and $\text{AlF}_6^{3-}$ ions interact with it to produce $\text{Al}^{3+}$ ions which can be discharged at the cathode.
B   Cryolite is a base, forming $\text{NaAlO}_2$ with bauxite, enabling aluminium to be discharged at the anode.
C   Cryolite minimises the release of $\text{O}^{2-}$ ions at the graphite anodes, which are otherwise burnt away to CO.
D   Cryolite reduces the melting point of the bauxite.