Question

In electrolysis of Al2O3 by Hall-Heroult process :

• A. Cryolite Na3[AlF6] lowers the melting point of Al2O3 and increases its electrical conductivity.
• B. Al is obtained at cathode and probably CO2 at anode
• C. Both (1) and (2) are correct.
• D. None of the above is correct

Answer 1

Answer: (C)

Both (1) and (2) are correct.

Answer 2

Na3[AlF6] 3NaF + AlF3

NaF and AlF3 both are ionic compounds and so ionise to give ions. This increases the electrical conductivity and lowers the melting point of Al2O3 .

At cathode : Al3+ (melt) + 3e– Al.

At anode : C(s) + O2– (melt) $\longrightarrow$ CO (g) + 2e– ; C(s) + 2O2– (melt) $\longrightarrow$ CO2 (g) + 4e–.