Chemistry
Aluminium is obtained from alumina:
Answer the following :
(i) Electrolyte used
(ii) Write cathode reaction
(iii) Why anode is replaced
Metallurgy
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Answer
(i) Mixture of fused alumina (Al2O3), cryolite (Na3AlF6) and fluorspar [CaF2]
(ii) At cathode : 2Al3+ + 6e- ⟶ 2Al
(iii) The anodes are continuously replaced during the electrolysis because :
- The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
- 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] - The carbon anode is hence consumed and renewed periodically after a certain period of usage.
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