State why aluminium is extracted from its oxide by electrolysis while copper, lead, iron by reducing agents and mercury and silver by thermal decomposition.

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Solution

Aluminium has a great affinity towards oxygen and so cannot be reduced by carbon or carbon monoxide. So it is extracted from its oxide by electrolysis.
Metals like copper, lead and iron are placed in the middle of the activity series and re moderately reactive and their oxides can be reduced by carbon, CO and hydrogen.
Mercury and silver are less reactive and are placed lower in the reactivity series. The oxides of these metals are reduced to metals by heating their oxides.

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Give reasons for the following :

(a) Electrolysis of molten lead bromide is considered to be a reaction in which oxidation and reduction go side, i.e., a redox reaction.

(b) The blue colour of aqueous copper sulphate fades when it is electrolysed using platinum electrodes.

(d) Aluminium is extracted from its oxide by electrolytic reduction and not by conventional reducing agents.

(e) The ratio hydrogen and oxygen formed at the cathode and anode is 2: 1 by volume.

(f) In the electrolysis of acidified water, dilute sulphuric acid is preferred to dilute nitric acid for acidification.

(g) Ammonia is unionised in the gaseous state but in the aqueous solution, it is a weak electrolyte.

(h) A graphite anode is preferred to other inert electrodes during electrolysis of fused lead bromide.

(i) For electroplating with silver, silver nitrate is not used as the electrolyte.

(j) Carbon tetrachloride is a liquid but does not conduct an electrolyte.

(k) Potassium is not extracted by electrolysis of its aqueous salt solution.

M

H_{2}, C O

M

A l

A l

A l_{2} O_{3}

O_{2}

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