Reduction of alumina that is obtained, either by Hall's proces or Bayer's process, to aluminium metal is carred out by electrolysis. During this process, cryolite and fluorspar are added to alumina, the entire mixture is covered with coke powder. (i) Why can alumina not be directly electrolysed? (ii) What is the role of addition of cryolite and fluorspar? (iii) What are the different cathodic and anodic reaction taking place during electrolysis?

Reduction of alumina that is obtained, either by Hall's proces or Baye

1.	Reduction of alumina that is obtained, either by Hall's proces or Bayer's process, to aluminium metal is carred out by electrolysis. During this process, cryolite and fluorspar are added to alumina, the entire mixture is covered with coke powder. (i) Why can alumina not be directly electrolysed? (ii) What is the role of addition of cryolite and fluorspar? (iii) What are the different cathodic and anodic reaction taking place during electrolysis?
Answer» Solution :(i) Alumina can be directly elecrtolysed because alumina has high thermal stability and hence, very high temperature is required for melting pure alumina. (ii) Cryolite and fluorspar are ADDED to alumina in ORDER to decrease the melting point of alumina and increase the electrical conductivity of the mixture. (iii) For the electrolysis of alumina, cryolite and fluorspar are added to it. The mixture undergoes electrolysis in the following way. `Al_(2)O_(3)to2Al^(+3)+3O^(-2)` `Na_(3)AlF_(6)to3Na^(+)+Al^(+3)+6F^(-)` `CaF_(2)toCa^(+2)+2F^(-2)` At anode : `O^(2-)-2e^(-)to[O]""C+2[O]toCO_(2)` At cathode : `Al^(+3)+3e^(-)toAl`