Q. 85.0( 2 Votes )

Explain the method of obtaining pure aluminium from alumina by electrolysis. In this process the carbon rods are replaced from time to time. Why?

Answer :

Alumina produced by concentration of bauxite is mixed with cryolite and subjected to electrolysis. Electricity is passed through this mixture, it gets heated, cryolite melts and alumina dissolves in it. Electricity acts as reducing agent here. The alumina gets dissociated into aluminium cations and oxygen anions. The following reactions occur at cathode and anode. Aluminium is produced at cathode.

Al3+ + 3e- Al at cathode


2O2- O2 + 4e- at anode



The carbon rods which are used as anodes are periodically


replaced because oxygen liberated at anodes as a result of electrolysis reacts with carbon to produce carbon dioxide. So anodes are consumed slowly. Hence, they are replaced.


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