Question

Which of the following are the main requirements for vapour phase refining?
(i) Metal should form a volatile compound with the reagent
(ii) The volatile compound should be easily decomposable to give back pure metal.
(iii) Metal should be very reactive and form a stable compound with the reagent.

A ) (i), (ii) and (iii)
B ) (i) and (ii)
C ) (ii) and (iii)
D ) (i) and (iii)

Answer 1

Hint: The requirements for the vapour phase refining will be met when the metal can be easily converted into a volatile compound, and then can be easily regenerated to the metallic state. If the metal to be purified is very reactive and forms a stable compound with the reagent, then it will be very difficult to get back the metal in elemental (uncombined) state.

Complete step by step answer:
In the vapour phase refining technique, metal to be purified, is converted into a volatile compound by reaction with suitable reagent. In the next step, this volatile compound of metal is decomposed to obtain pure metal. The vapour phase refining technique is used for the refining of nickel, titanium and zirconium metals.

Impure nickel metal is obtained from the reaction of nickel oxide with syngas at \[473{\text{ }}K\]. Nickel metal thus formed has some impurities of iron and cobalt.
\[{\text{NiO}}({\text{s}}){\text{ + }}{{\text{H}}_{\text{2}}}({\text{g}}) \to {\text{Ni}}({\text{s}}){\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}}({\text{g}}).\]

Nickel is refined by mond’s process. Impure nickel reacts with carbon monoxide at around \[330 - 350{\text{ }}K\] to form nickel tetracarbonyl. Nickel tetracarbonyl is a volatile compound and is obtained as a vapour. The impurities are left behind.
At a high temperature of around \[450 - 470{\text{ }}K\], nickel tetracarbonyl is decomposed to regenerate pure nickel.
Write the balanced chemical equations for the process.
\[
  {\text{Ni + 4 CO }} \to {\text{ Ni}}{\left( {{\text{CO}}} \right)_4} \\
  {\text{Ni}}{\left( {{\text{CO}}} \right)_4} \to {\text{ Ni + 4 CO}} \\
\]
In the first reaction,one nickel atom combines with four carbon monoxide molecules to form nickel tetracarbonyl. In the next step, one molecule of nickel tetrachloride decomposes to give one nickel atom and four carbon monoxide ligands.
The diagram for the Mond process is as follows:

At the end of the process, it is necessary that the metal obtained is in uncombined, elemental state. For this, the metal to be purified should not be very reactive and it should not form a stable compound with the reagent. If the metal to be purified, does not form a stable compound with the reagent then it will be easy to get back the metal in elemental (uncombined) state.

Hence, the option B ) (i) and (ii) is the correct option.

Note:
Some of the methods that can be used for the refining of metals are distillation, liquation, poling, electrolysis, zone refining and vapour phase refining.