Question

When mercury (II) chloride is treated with excess of stannous chloride, the products obtained are ---- respectively.
A. liquid Hg and $SnC{l_4}$
B. $H{g_2}C{l_2}$ and $SnC{l_4}$
C. $H{g_2}C{l_2}$ and ${[SnC{l_4}]^{2 - }}$
D. liquid Hg and ${[SnC{l_4}]^{2 - }}$

Answer 1

Hint: Here we recall oxidation and reduction reaction. Reaction between mercury (II) chloride and stannous chloride will be a redox reaction. In redox reaction one reactant acts as oxidizing agent and another reactant acts as reducing agent.

 Complete step by step answer:
Oxidation reaction: loss of electrons is known as oxidation. A species undergoing oxidation is known as a reducing agent.
Reduction reaction: gain of electrons is known as reduction. A species undergoing reduction is known as oxidizing agent.
In this reaction, Mercuric chloride oxidises stannous chloride to stannic chloride and itself get reduced to mercurous chloride. Mercury has an oxidation state of +1 in mercurous chloride. $2HgC{l_2}{\text{ + SnC}}{{\text{l}}_2}{\text{ }} \to {\text{ H}}{{\text{g}}_2}C{l_2}{\text{ + SnC}}{{\text{l}}_4}$ . Mercurous chloride is further reduced to metallic mercury, when it reacts with excess of stannous chloride. Mercury chloride gets reduced to mercury and acts as an oxidising agent. Stannous chloride gets oxidized to stannic chloride and acts as a reducing agent. $H{g_2}C{l_2}{\text{ + SnC}}{{\text{l}}_2}{\text{ }} \to {\text{ 2H}}{{\text{g}}_{(l)}}{\text{ + SnC}}{{\text{l}}_4}$ .

Hence, correct answer is (A)

Additional information: Stannous chloride is used to reduce dissolved divalent mercury to elementary mercury. The dissolved elementary mercury then stripped from solution by air sparging. Reaction between Mercuric chloride and stannous chloride is a redox reaction and it is a spontaneous reaction. Spontaneous reaction proceeds by itself without any external support. Mercury has +2 oxidation state in mercuric chloride, +1 oxidation state in mercurous chloride and zero oxidation state in metallic mercury. Mercury is a liquid metal at room temperature.

Note:
Mercuric chloride is obtained by the action of chlorine on mercury or mercury chloride. It can also be produced by the addition of hydrochloric acid to a hot, concentrated solution of mercury(I) compounds such as the nitrate. \[{\text{H}}{{\text{g}}_{\text{2}}}{\left( {{\text{N}}{{\text{O}}_{\text{3}}}} \right)_{\text{2}}}{\text{ + 4HCl }} \to {\text{ 2HgC}}{{\text{l}}_{\text{2}}}{\text{\; + 2}}{{\text{H}}_{\text{2}}}{\text{O + 2N}}{{\text{O}}_{\text{2}}}\] .