Problem 92 - A Complete Resource Book in Chemistry for JEE Main

Question

Mercuric sulphide dissolves in aqua regia to form (a) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\) (b) \(\mathrm{HgCl}_{2}\) (c) \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right) \mathrm{Cl}\) (d) \(\mathrm{Hg}_{2}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

Mercuric sulphide dissolves in aqua regia to form HgCl₂.

Step 1: Understanding Aqua Regia

Aqua regia is a mixture of concentrated nitric acid (HNO₃) and hydrochloric acid (HCl), usually in a molar ratio of 1:3. It is a powerful solvent capable of dissolving gold and platinum. For mercury compounds, it primarily helps in oxidation and chlorination processes.

Step 2: Analyzing Mercuric Sulphide (HgS)

Mercuric sulphide (HgS) does not dissolve in dilute acids. However, in aqua regia, HgS reacts due to the presence of both nitric acid and hydrochloric acid, which facilitate oxidation and complex formation.

Step 3: Reaction in Aqua Regia

In aqua regia, mercuric sulphide (HgS) reacts as follows: The nitric acid acts as an oxidizing agent, converting mercury in HgS to mercury ions (Hg²⁺). The hydrochloric acid then provides chloride ions (Cl⁻) which combine with mercury ions to form mercuric chloride (HgCl₂).

Step 4: Conclusion

Based on these reactions, when mercuric sulphide dissolves in aqua regia, the product formed is mercuric chloride. Therefore, the correct answer should have been after dissolving mercuric sulphide (HgS) in aqua regia, the compound formed is the one that contains Hg and Cl.

Key Concepts

Mercuric Sulphide

Mercuric sulphide (HgS), also known as cinnabar, is a naturally occurring mineral and the most common source of mercury. This compound is notorious for being quite insoluble in water, which makes it stable in various environments. Despite its insolubility in water and many solvents, it reacts with aqua regia, a unique chemical mixture.
Aqua regia is capable of dissolving HgS due to its powerful reactive properties. The interaction of mercuric sulphide with aqua regia exemplifies how even stable compounds can be transformed through appropriate chemical conditions.

Oxidation Process

The oxidation process in aqua regia begins with nitric acid ( HNO_3 ), which is a robust oxidizing agent. In the context of dissolving mercuric sulphide, the nitric acid specifically works by oxidizing mercury in the HgS . This shifts mercury from its sulfide form to a more reactive ionic form, specifically mercury ions ( Hg^{2+} ).
Oxidation is crucial here because it allows the mercury to break free from its bonds with sulphur, thus making the mercury ions available to react with other components in the aqua regia. This transformation is the first critical step in the complete chemical reaction with HgS .

Complex Formation

Once the oxidation process creates mercury ions, these ions readily interact with chlorine ions ( Cl^- ) supplied by the hydrochloric acid in the aqua regia. This step is crucial for complex formation. Chloride ions are essential because they stabilize the free mercury ions through chemical bonding.
This stabilization results from the formation of mercuric chloride ( HgCl_2 ), a complex compound that is soluble in the aqua regia solution. Hence, the initial reaction breaking bonds in HgS is completed through a secondary reaction forming a stable product.

Mercuric Chloride

The final and resultant compound of the reactions between mercuric sulphide and aqua regia is mercuric chloride ( HgCl_2 ). This substance is a white crystalline solid that is soluble in water. It signifies the completion of dissolution as the ionic products formed from oxidation and complexation reach a stable structure.
Understanding the result of this reaction is crucial as mercuric chloride is quite distinct chemically from mercuric sulphide, demonstrating the power of aqua regia in transforming substances. This capability of forming compounds like HgCl_2 is why aqua regia is such an invaluable tool in both chemical synthesis and analysis. Its effectiveness lies in the synergetic action of both its oxidizing and chlorinating components.