Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following on reaction with \(\mathrm{H}_{2} \mathrm{~S}\) does not produce metallic sulphide? (a) \(\mathrm{CdCl}_{2}\) (b) \(\mathrm{ZnCl}_{2}\) (c) \(\mathrm{CoCl}_{2}^{2}\) (d) \(\mathrm{CuCl}_{2}^{-}\)

Step-by-Step Solution

Answer

CuCl\(_2\) does not produce metallic sulfide with H\(_2\)S due to its behavior in acidic solutions.

Step 1: Understanding the Concept

When metal chloride reacts with hydrogen sulfide (H\(_2\)S), a metal sulfide and hydrogen chloride (HCl) are produced. The ability of metals to form sulfides depends on their chemical properties. Thus, in this question, we need to determine which compound will not produce a metallic sulfide when reacted with H\(_2\)S.

Step 2: Evaluate Each Option

Let's examine the possible reactions:(a) CdCl\(_2\) + H\(_2\)S will produce CdS + 2HCl.(b) ZnCl\(_2\) + H\(_2\)S will produce ZnS + 2HCl.(c) CoCl\(_2\) + H\(_2\)S should produce CoS + 2HCl.(d) CuCl\(_2\) may not form CuS with H\(_2\)S since Copper (II) ions do not readily precipitate as sulfides in acidic solutions.

Step 3: Analyze the Acidic Solution Context

In an acidic solution, the solubility of the sulfides increases. Copper (II) chloride is in such a form that it does not precipitate easily as a sulfide in a strong acidic environment, unlike cadmium and zinc, which form more stable sulfides.

Step 4: Final Decision

Given the conditions of the reaction and their behavior in acidic solutions, the compound that will not produce metallic sulfide is (d) CuCl\(_2\). It is less likely to precipitate as CuS compared to the others under acidic conditions.

Key Concepts

Hydrogen Sulfide Reactions

Hydrogen sulfide (H\(_2\)S) is a colorless gas recognized for its distinct "rotten egg" smell. It is important in chemistry due to its ability to react with various metal salts to form solid metal sulfides. This reaction is commonly represented as: Metal chloride (MCl) + H\(_2\)S → Metal sulfide (MS) + Hydrochloric acid (HCl).
For metal sulfide formation, hydrogen sulfide provides the sulfide ion (S\(^{2-}\)), which combines with metal ions to form insoluble metal sulfides. The resulting sulfide precipitates out of the solution, which accounts for the characteristic appearance of the product of these reactions. However, not all metals form sulfides equally well. Understanding the behavior of H\(_2\)S with different metals helps predict whether a reaction will successfully produce a metallic sulfide or not.

Metal Chloride Reactions

Metal chlorides are compounds containing metal cations (M\(^{n+}\)) and chloride anions (Cl\(^-\)). They can react with hydrogen sulfide to form metal sulfides, which usually precipitate out due to their low solubility. Here is a closer look at the chemical reactions for some typical metal chlorides:

• Cadmium chloride (CdCl\(_2\)) reacts readily to form cadmium sulfide (CdS), with a yellow precipitate.
• Zinc chloride (ZnCl\(_2\)) converts to zinc sulfide (ZnS), which is also poorly soluble and precipitates as a white solid.
• On the other hand, copper (II) chloride (CuCl\(_2\)) is less likely to precipitate as copper sulfide (CuS) under acidic conditions.

The varying abilities of metal ions to form sulfides with hydrogen sulfide depend on the stability of the resulting compound. Cadmium and zinc ions tend to form reasonably stable sulfides in these reactions, whereas copper ions require specific conditions to do the same.

Acidic Solution Behavior

In an acidic solution, the solubility of many metal sulfides increases. This is critical to understand, as it influences whether a metal chloride will form a stable sulfide with hydrogen sulfide gas.
The acidity of a solution lowers the chance of sulfide ion (S\(^{2-}\)) being available to form a precipitate because the excess hydrogen ions (H\(^+\)) can react with sulfide ions to form hydrogen sulfide gas. This reaction decreases the concentration of free sulfide ions that are needed to react with metal ions.
For example, copper ions (Cu\(^{2+}\)) are particularly susceptible to this effect. In acidic solutions, copper sulfide is more soluble and thus does not precipitate as readily as sulfides from metals like cadmium and zinc, which form more robustly. Hence, the presence of acid in the solution is a key factor that affects the likelihood of copper ions forming sulfides.