Question

Give the reasons for the following:
(i) Zinc is used in galvanisation and dry cells.
(ii) Nitric acid can be stored in aluminium containers.
(iii) Aluminium oxide cannot be reduced by carbon.
(iv) A neutral gas other than oxygen is formed at the anode during electrolysis of fused alumina.
(v) Extraction of aluminium was very difficult in the beginning.
(vi) Carbon anodes are used in the electrolytic extraction of aluminium.
(vii) Galvanized metal ions should not be used for storing food.

Answer 1

(i) Zinc ​is a highly reactive metal as it can lose electrons easily. Hence, it is used to manufacture containers of dry cells, which act as cathodes. It is also used in galvanisation, in which iron materials are coated with molten zinc. Because zinc is more reactive than iron, it reacts first and prevents iron from rusting.

(ii) Nitric acid makes metallic aluminium inert by forming an oxide layer on its surface and does not allow further reactions to take place. Thus, nitric acid can be stored in aluminium containers.

(iii) ​Aluminium is a reactive metal and placed in the top section of the reactivity series. Its affinity towards oxygen is more as compared to carbon. Because aluminium oxide is very stable and cannot be reduced by carbon, it is reduced by electrolysis.

(iv) During the electrolysis of fused alumina, oxygen is liberated at the anode, which reacts with the carbon of the anode, and oxidises it, forming neutral carbon dioxide gas.

(v) Even though aluminium is a highly abundant metal than iron, it was unknown to man for many years due to its high reactivity. Because of which it was not obatined by conventional methods of obtaining metals from their ores.

(vi) During the extraction of aluminium by electrolysis, molten alumina and cryolite is used as electrolyte. The electrolyte is present at a temperature of around 950^oC. Electrodes made from metals might melt at this high temperature. Thus, carbon electrodes are used in the process. Also, the oxygen formed during the electrolysis combines with carbon anode to form carbon dioxide gas.

(vii) Acids present in food may react with galvanised iron, resulting in the poisoning of food. Thus, galvanised metal iron should not be used for storing food.