9701 MCQ Solution 49

Question 49: [Physical > Electrochemistry]

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, Al2O3, and cryolite, Na3AlF6.

What is the main purpose of the cryolite?

A Al2O3 is covalent, and AlF63- ions interact with it to produce Al3+ ions which can be discharged at the cathode.

B Cryolite is a base, forming NaAlO2 with bauxite, enabling aluminium to be discharged at the anode.

C Cryolite reduces the melting point of the bauxite.

D Cryolite minimises the release of O2- ions at the graphite anodes, which are otherwise burnt away to CO.

Reference: Past exam paper -June 2003 Paper 1 Q9

Solution 49:

Answer: C.

Cryolite is used to reduces the melting point of the bauxite because without the cryolite the melting point for the extraction of aluminium would be very high.