Question

The following is a sketch of an electrolytic cell used in the extraction of aluminium :
electrolytic-cell-aluminium-extraction-simplified-chemistry-icse-class-10-749x603
(i) What is the substance of which the electrodes A and B are made?
(ii) At which electrode (A or B) is the aluminium formed ?
(iii) What are the two aluminium compounds in the electrolyte C ?
(iv) Why is it necessary for electrode B to be continuously replaced ?

  • Metallurgy
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Answer

(i) Electrodes A and B are made of graphite (carbon).

(ii) Aluminium is formed at cathode (i.e., electrode A).

(iii) Fused alumina (Al2O3) and Cryolite (Na3AlF6) are the compounds in the electrolyte C.

(iv) The graphite [carbon] anodes (i.e electrode B) are continuously replaced during the electrolysis because :

  1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
  2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. 2C + O2 ⟶ 2CO [2CO + O2 ⟶ 2CO2]
  3. The carbon anode is hence consumed and renewed periodically after a certain period of usage.

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Give reasons for the following pertaining to Hall Herault’s process.
(i) The fusion temperature of the electrolyte has to be lowered before conducting the electrolytic reduction.
(ii) The constituents of the electrolyte in addition to one part of fused alumina contains three parts of cryolite and one part of fluorspar.
(iii) A layer of powdered coke sprinkled over the electrolytic mixture, protects the carbon electrodes.
(iv) It is preferred to use a number of graphite electrodes as anode, instead of a single graphite electrode.

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