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

(b) \( \mathrm{HgCl}_{2} \)

Step 1: Identify Components of Aqua Regia

Aqua regia is a mixture of concentrated nitric acid (HNO₃) and hydrochloric acid (HCl), typically in a 1:3 ratio. Its powerful oxidizing and chlorinating ability dissolves metals like gold, platinum, and others.

Step 2: Understand Reaction of Mercuric Sulphide

Mercuric sulphide (HgS) is an insoluble compound. When dissolved in aqua regia, it reacts with HCl to form a soluble complex. It doesn't react with nitric acid directly, but nitric acid provides an oxidizing environment.

Step 3: Recognize Product Formation

In the presence of aqua regia, HgS reacts with HCl. The chlorine from HCl can chlorinate mercury leading to the formation of either mercurous or mercuric chloride. Given the higher oxidation state stabilization in aqua regia, mercury will likely form a chloride in the +2 oxidation state.

Step 4: Identify the Probable Compound

Considering the reaction environment and the oxidation states involved, the most likely chloride produced from mercuric sulphide in aqua regia is mercuric chloride. Hence, the reaction produces \ \( \mathrm{HgCl}_{2} \ \).

Key Concepts

Mercuric Sulphide

Mercuric sulphide, represented as HgS, is a significant chemical compound known for its vibrant red color in its cinnabar form. This compound is insoluble in water which means it does not dissolve easily, making it quite stable.
When mercury and sulfur combine, they form mercuric sulphide, which is also used as a pigment. This compound is stable under normal conditions but can react with strong acids such as aqua regia.
Aqua regia, a mixture of nitric acid and hydrochloric acid, can break down HgS by providing a strong oxidizing as well as chlorinating environment. This powerful mixture allows the breakdown of this stable compound into a soluble form, which is essential when dealing with reactions involving mercuric sulphide.

Oxidation State

Understanding the oxidation state is crucial when dealing with chemical reactions. Essentially, the oxidation state indicates the degree of oxidation of an atom and can help us predict the outcome of chemical reactions.
Mercuric sulphide involves mercury in a +2 oxidation state. When aqua regia is used, it provides a highly oxidative environment, encouraging chemical transformations. Here, the oxidation state of mercury tends to remain stable at +2, as aqua regia encourages formations of compounds with higher states rather than reducing them.
This tendency, along with the presence of chlorine from HCl, results in the transformation of HgS into a more reactive form, typically leading to the formation of mercuric chloride ( HgCl_2 ), rather than other stable or inert forms.

Mercuric Chloride

Mercuric chloride, or HgCl_2 , is the likely product formed when mercuric sulphide reacts with aqua regia. This compound involves mercury bonded in its +2 oxidation state, reacting predominantly with chlorine in the environment provided by aqua regia.
As a corrosive substance, mercuric chloride is soluble in water, unlike mercuric sulphide. This transformation from HgS to HgCl_2 indicates a significant change in the properties, including solubility. This is essential in chemical processes where soluble forms are necessary for further reactions or extractions.
In aqua regia, mercury's inclination towards a higher oxidation state ensures that HgCl_2 remains the most probable compound, reflecting both its chemical stability and reactivity in such an oxidizing medium.