Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Which compound does not dissolve in hot dilute \(\mathrm{HNO}_{3} ?\) (a) \(\mathrm{PbS}\) (b) \(\mathrm{HgS}\) (c) CuS (d) \(\mathrm{CdS}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{HgS}\) does not dissolve in hot dilute \(\mathrm{HNO}_{3}\).

Step 1: Understanding Solubility in HNO3

Determine if these metal sulfides can dissolve in hot, dilute \(\mathrm{HNO}_{3}\). Most metal sulfides dissolve due to oxidation of the sulfide ion by the nitric acid, but this can depend on the metal involved.

Step 2: Analyzing Metal Sulfide Properties

Consider the unique properties of each metal sulfide: - **PbS** (Lead sulfide) is relatively insoluble, but can react with oxidizing agents. - **HgS** (Mercury sulfide) is also known as cinnabar and is notable for its extreme insolubility in both dilute and concentrated nitric acid. - **CuS** (Copper sulfide) can generally dissolve in hot, concentrated nitric acid due to formation of copper nitrate. - **CdS** (Cadmium sulfide) can be oxidized and dissolved in hot, dilute nitric acid.

Step 3: Identifying the Exception

From step 2, it becomes evident that \(\mathrm{HgS}\) is the compound that does not dissolve in hot dilute \(\mathrm{HNO}_{3}\), owing to its stability and insolubility in nitric acid.

Key Concepts

Mercury sulfide properties

Mercury sulfide (HgS) is primarily known for its remarkable insolubility in both dilute and concentrated nitric acid (HNO₃). This distinctive characteristic makes it an outlier among metal sulfides. Often occurring in nature as the mineral cinnabar, HgS possesses a bright red appearance. The insolubility is largely due to the fact that mercury forms very stable sulfides that are not easily broken down by oxidation or dissolution processes.

Some notable properties of mercury sulfide include:

• Non-reactivity with oxidizing acids like \\( \mathrm{HNO}_{3} \)
• High density and relative stability
• Uses in pigment production, especially in traditional red vermilion pigments

The stability of HgS even in reactive environments makes it a unique compound amongst other sulfides.

Behavior of sulfides in acids

The behavior of metal sulfides when introduced to acids varies significantly, often depending on the specific metal present. Typically, when metal sulfides come into contact with acids like hydrochloric acid (HCl) or nitric acid (HNO₃), they undergo oxidation or dissolution. However, the extent to which they react can vary based on the sulfide's properties:

• Sulfides like \\( \mathrm{CdS} \) and \\( \mathrm{CuS} \) tend to react more readily with acids, especially if the acid is hot or concentrated. This is due to their ability to form soluble metal nitrates.
• In contrast, \\( \mathrm{PbS} \) is somewhat resistant and requires stronger acidic conditions to dissolve due to its relatively higher stability compared to other sulfides. However, it still can react with oxidizing agents.

Sulfides in contact with nitric acid can oxidize the sulfide ions, where sulfur is transformed into sulfur dioxide or elemental sulfur depending on the conditions. As such, mercury sulfide (HgS) remains an exception due to its unique inability to dissolve even in oxidizing environments.

Oxidation reactions with nitric acid

Oxidation reactions involving nitric acid are significant when examining the behavior of metal sulfides. Nitric acid is a powerful oxidizing agent that can dissolve many metal compounds by providing the necessary oxidative potential.

During the reaction, nitric acid oxidizes sulfide ions (S²⁻) to either sulfur dioxide (SO₂) gas or elemental sulfur (S), which can further react and form sulfates. This process often accompanies the formation of nitrogen oxides as by-products. The reactions are usually temperature dependent, being more efficient at elevated temperatures:

• The oxidation of \\( \mathrm{CuS} \) in hot, concentrated nitric acid involves the formation of soluble copper nitrate and releases nitrogen oxides.
• Similarly, \\( \mathrm{CdS} \) also undergoes oxidation and forms cadmium nitrate.

However, mercury sulfide (HgS) stands out by not reacting or oxidizing in the presence of nitric acid, distinctively resisting dissolution. This specific behavior underscores the unique chemical stability and robustness of HgS compared to other sulfides.