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In the extraction of aluminium ;
(i) name the process of concentration of bauxite.
(ii) write the cathode reaction in electrolytic reduction of alumina.
(iii) write the function and chemical formula of cryolite.
(iv) write a chemical equation for the action of heat on aluminium hydroxide.
(v) why is it necessary to replace anodes time to time?

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Solution

(i) The concentration of ore is done by Baeyer's process using chemical method.

(ii) The electrolytic reduction of alumina is done by Hall Heroult's process. The reaction at cathode is : Al3++3eAl
Thus, pure aluminium metal is formed at the cathode.

(iii) Cryolite is sodium aluminium fluoride (Na3AlF6). It is an ore of aluminium. Cryolite and fluorspar are added to the molten mixture of alumina to reduce the melting point and increase the electrical conductivity.

(iv) 2Al(OH)3Al2O3+3H2O

(v) The oxygen evolved at the anode escapes as gas or reacts with the carbon anode. The anode is thus oxidized to carbon monoxide which either burns giving CO2 or escapes. Thus, anode has to be replaced as it is consumed periodically.

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