Question

Give the ionization reactions of electrolyte used in Hall’s process. write the reaction at the cathode and the anode. Why the anode has to be replaced in this process?

Answer 1

The ionization reactions of electrolyte in Hall's process.

`Cryolite:  Na_3AlF_6 ⇌  3Na^+ + Al^3+ + 6F^-`

`Fluorspar:  CaF_2 ⇌ Ca^(2+) + 2F^-`

`Alumina:  Al_2O_3 2Al^(3+) + 3O^(2-)`

The reaction at the cathode and anode are:
Cathode:

`4Al^(3+) + 12e^- →  4Al`

Anode:

`6O^(2-) -12e^- → 6[O]`

`3O + 3O → 3O_2`

The anode has to be replaced from time to time as it gets oxidized by the oxygen evolved at the anode.