Question

When Alumina is electrolysed in presence of cryolite,the gas liberated at graphite anode is ____________.

• A. $F_2$
• B. $O_2$
• C. $C{F}_4$
• D. $C{l}_2$

Answer 1

The correct option is B $O_2$

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 $\left(A{l}_2{O}_3\right)$, cryolite $\left(N{a}_{3}Al{F}_6\right)$, and fluorspar $\left(Ca{F}_2\right)$. In the electrolytic reduction, $O_2$ is liberated at graphite anode. Also, graphite anode gets corroded due to reaction with $O_2$, hence it has to be changed periodically.