Question

The purpose of adding $N{a}_{3}Al{F}_6$ to $A{l}_2{O}_3$ during electrolysis is :

• A. to decrease melting point of Al${}_2$O${}_3$
• B. to increase conductivity of electrolyte
• C. to provide reducing conditions in both
• D. Both A and B

Answer 1

Answer: (D)

Both A and B

The electrolysis of pure alumina faces some difficulties. Pure alumina is a bad conductor of electricity. The fusion temperature of pure alumina is about 2000${}^{\circ }$C and at this temperature when electrolysis is carried out on the fused mass, the metal formed vaporizes, as the boiling point of aluminium is 1800${}^{\circ }$C. These difficulties are overcome by using a mixture containing alumina, cryolite $\left(N{a}_{3}Al{F}_6\right)$, and fluorspar $\left(Ca{F}_2\right)$.

So in the electrolytic reduction of alumina, cryolite is added along with fluorspar to–

– decrease melting point of alumina

– decrease viscosity of electrolyte ($Ca{F}_2$ is used)

– increase conductivity