Describe the principle behind each of the following processes :
(i) Vapour phase refining of a metal.
(ii) Electrolytic refining of a metal.
(iii) Recovery of silver after silver ore was leached with NaCN.

Explanation

(i) In this method crude metal is freed from impurities by first converting it into a suitable volatile compound by heating it with a specific reagent at a lower ten:perature a nd then decomposing the volatile compound at some higher temperature to give the pure metal. Thus, the two requirements are :
- the metal should from a volatile compound with a suitable reagent.
- the volatile compound should be easily decomposable so that the recovery is easy.
(ii) See Q. 21 (or part) (ii), Set-I, Outside Delhi, 2010.
(iii) Role of NaCN in the extraction of silver is to do the Leaching of silver ore in the presence of air from which the silver is obtained Later by replacement
`4Ag(s)+8CN^(-)(aq)+2H_2O(aq)+O_2(g) to4[Ag(CN)_2]^(-)(aq)+4OH^(-)(aq)`
`2[Ag(CN)_2]^(-)(aq)+Zn^(-) (s)to2Ag(s) [Zn(CN_4)]^(2-)(aq)`.