Question

Describe the principle involved in each of the following processes:
i. Mond process for refining of nickel.
ii. Column chromatography for purification of rare elements.

Answer 1

The process in which a substance is purified from its impure form is known as refining. The technique used for the purification and separation of elements that are available in minute quantities (rare elements) is known as column chromatography.

Complete step by step solution:
i. Nickel metal is purified using the Mond’s process. The principle of Mond’s process is as follows:
Nickel is heated in the presence of carbon monoxide at a temperature of $330 - 350{\text{ K}}$. During this heating, nickel tetracarbonyl $\left( {{\text{Ni}}{{\left( {{\text{CO}}} \right)}_{\text{4}}}} \right)$ is formed. Nickel carbonyl is a volatile compound.
Nickel tetracarbonyl is further heated at a temperature of $450 - 470{\text{ K}}$. At this temperature nickel tetracarbonyl decomposes and pure nickel metal is obtained.
The reactions involved in Mond’s process for refining of nickel are as follows:
${\text{Ni}} + {\text{4CO}}\xrightarrow{{{\text{330 - 350 K}}}}{\text{Ni(CO}}{{\text{)}}_{\text{4}}}$
${\text{Ni(CO}}{{\text{)}}_{\text{4}}}\xrightarrow{{{\text{450 - 470 K}}}}{\text{Ni}} + {\text{4CO}}$

ii. Column chromatography is based on the principle that the components of a mixture are adsorbed differently on the surface of the adsorbent.
There are two phases in column chromatography: Stationary phase and mobile phase.
The stationary phase is immiscible with the mixture and non-mobile. The stationary phase used in column chromatography is aluminium oxide $\left( {{\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}} \right)$.
The mobile phase is miscible with the mixture and is mobile. The mobile phase used is generally a liquid, gas or a supercritical liquid.

Note:
The impure nickel metal is in the form of nickel oxide which is initially reacted with syngas (mixture of carbon monoxide and hydrogen gas) to form impure nickel metal.
The advantage of column chromatography is that it has low cost and the stationary phase can be reused.