Question

write the formula of cryolite what is the purpose of adding it in the half process of extraction of aluminium

Answer 1

Electrolysis of the alumina/cryolite solution gives aluminium at the cathode and oxygen at the anode.

Al3+  +  3e-       Al (aluminium metal at the (-)cathode)
2O2-  -  4e-       O2 (oxygen gas at the (+)anode)

Aluminium is denser than the alumina/cryolite solution, and so falls to the bottom of the cell where it can be tapped off as pure liquid metal.

The overall reaction is

aluminium oxide       aluminium + oxygen
2Al23+O32-(l)                 4Al(l)   +     3O2(g)

Oxygen is discharged at the positive carbon (graphite) anode. Oxygen reacts with the carbon anode to form carbon dioxide gas.

carbon + oxygen    carbon dioxide
C(s)  +    O2(g)             CO2(g)

The carbon anode slowly reacts to make carbon dioxide and needs to be replaced regularly.