Question

Assertion: In the extraction of aluminium by electrolysis it is necessary to renew the anode periodically.
Reason: Oxygen gas is formed and at high temperature of the electrolysis cell it burns and oxidises away the carbon electrodes to form toxic carbon monoxide or carbon dioxide.
A.Both assertion and reason are correct and the reason is the correct explanation for assertion.
B.Both assertion and reason are correct and the reason is not the correct explanation for assertion.
C.Assertion is correct but reason is incorrect.
D.Both assertion and reason are incorrect.

Answer 1

Hint: Aluminium is extracted by two different methods, Bayer’s Process and Hall-Heroult Process. The hall-Heroult process involves the extraction of aluminium by electrolysis. The hall-Heroult process is widely used for the extraction of aluminium.

Complete answer:
In the Hall-Heroult process, pure aluminium oxide is mixed with Calcium fluoride or cryolite to lower the melting point of the mixture and increase the conductivity of the mixture. In this process, a steel vessel is used which contains a lining of carbon and graphite rods. Carbon lining acts as the cathode and graphite act as the anode. On passing electricity to the electrolytic cell oxygen is formed at the anode which reacts with carbon of the anode to form carbon monoxide and carbon dioxide. For getting one kilogram of Aluminium approximately a half kilogram of carbon is consumed. Aluminium ions are created at the cathode from aluminium oxide and aluminium ions sink as they are heavier than the cryolite solution and molten aluminium is obtained.
From the above information, we can say that the first statement is correct as the anode is renewed periodically in the Hall-Heroult process. The second statement correctly explains the reason behind the first statement because oxygen formed at the anode burns and oxidises the carbon of the anode.
Hence option (A) is the correct answer.

Note:
Another method for the extraction of aluminium is Bayer’s process. In Bayer’s process, ore of aluminium is treated with concentrated sodium hydroxide, which forms sodium aluminate which is filtered off. The filtrate is heated with water to form aluminium hydroxide. Aluminium Hydroxide is strongly heated which gives alumina.