Question

Calomel $\left( {{{H}}{{{g}}_2}{{C}}{{{l}}_2}} \right)$ on reaction with ammonium hydroxide gives:
A. ${{HgO}}$
B. ${{H}}{{{g}}_2}{{O}}$
C. ${{N}}{{{H}}_2} - {{Hg}} - {{Hg}} - {{Cl}}$
D. ${{HgN}}{{{H}}_2}{{Cl}}$

Answer 1

Hint: Calomel is a common name used for mercurous chloride. Its chemical formula is ${{H}}{{{g}}_2}{{C}}{{{l}}_2}$. In this compound, mercury has $ + 1$ oxidation state. Generally, mercury has $ + 1, + 2$ oxidation states. Mercury is the only liquid metal in the periodic table.

Complete step by step answer:
Mercurous chloride is formed by the following reaction:
${{H}}{{{g}}_2}^{2 + }\left( {{{aq}}} \right) + 2{{C}}{{{l}}^ - }\left( {{{aq}}} \right) \rightleftharpoons {{H}}{{{g}}_2}{{C}}{{{l}}_2}\left( {{s}} \right)$
The product formed is termed as calomel. This can be precipitated by soluble chlorides like hydrochloric acid, ${{HCl}}$. When aqueous ammonia reacts with calomel, it undergoes a disproportionation reaction. We know that in a disproportionation reaction, the element or ion or compound will undergo both oxidation and reduction. The compound will act as both oxidizing agent and reducing agent.
The chemical reaction of ammonium hydroxide with calomel is given below:
 ${{H}}{{{g}}_2}{{C}}{{{l}}_2} + 2{{N}}{{{H}}_4}{{OH}} \to {{Hg + HgN}}{{{H}}_2}{{Cl}} + {{N}}{{{H}}_4}{{Cl}} + 2{{{H}}_2}{{O}}$
The products formed are elemental mercury, mercury amidochloride, ammonium chloride and water molecules. Mercurous chloride changed to elemental mercury and mercuric amidochloride. Generally, we tell amides are the compounds having a carbonyl group connected with an amino group. Here, instead of a carbonyl group, mercury is attached to the amino group. It can form a polymer with chloride counter ions. When we add base to it, it gets converted to a compound called Millon’s base. The chemical formula of Millon’s base is $\left[ {{{H}}{{{g}}_2}{{N}}} \right]{{OH}}{{.}}{\left( {{{{H}}_2}{{O}}} \right)_{{n}}}$.
So we can say that when calomel is reacted with ammonium hydroxide, we get ${{HgN}}{{{H}}_2}{{Cl}}$.
Hence, the correct option is D.

Note: It has been known that calomel is used for making calomel electrodes. These calomel electrodes are used for measuring potential between two electrodes in an electrolyte solution.
The chemical reaction between aqueous ammonia and calomel is given below:
${{H}}{{{g}}_2}{{C}}{{{l}}_2} + {{N}}{{{H}}_3} \to {{Hg}}{\left( {{{N}}{{{H}}_4}{{Cl}}} \right)_2} + {{Hg}}$
Here, mercury in mercurous chloride changes its oxidation state from $ + 2$ to zero.