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

Question

Identify the correct order of solubility of \(\mathrm{Na}_{2} \mathrm{~S}, \mathrm{CuS}\) and \(\mathrm{ZnS}\) in aqueous medium. (a) \(\mathrm{Na}_{2} \mathrm{~S}>\mathrm{CuS}>\mathrm{ZnS}\) (b) \(\mathrm{Na}_{2} \mathrm{~S}>\mathrm{ZnS}>\mathrm{CuS}\) (c) \(\mathrm{CuS}>\mathrm{ZnS}>\mathrm{Na}_{2} \mathrm{~S}\) (d) \(\mathrm{ZnS}>\mathrm{Na}_{2} \mathrm{~S}>\mathrm{CuS}\)

Step-by-Step Solution

Answer

The correct order of solubility is (b) \( \text{Na}_2\text{S} > \text{ZnS} > \text{CuS} \).

Step 1: Understand Solubility Rules

Identify that sodium salts like \( \text{Na}_2\text{S} \) are highly soluble in water. In contrast, sulfides of transition metals such as \( \text{CuS} \) and \( \text{ZnS} \) are generally insoluble.

Step 2: Compare Solubilities of Transition Metal Sulfides

Consider the general trend for sulfides of transition metals: \( \text{CuS} \) has very low solubility, whereas \( \text{ZnS} \) is slightly more soluble than \( \text{CuS} \).

Key Concepts

Sodium Salts

Sodium salts are compounds formed by the combination of sodium (Na) with various anions. One of the key features of sodium salts is their high solubility in water. This is because sodium ion, \( \text{Na}^+ \), is a highly soluble cation due to its small ionic size and high hydration energy. For example, sodium sulfide, \( \text{Na}_2\text{S} \), readily dissolves in water, making solutions containing sodium salts conduct electricity well.

• Sodium salts dissolve due to the strong interactions between water molecules and sodium ions.
• The widespread solubility of sodium salts is crucial in various chemical processes and biological functions.

In many chemical reactions, sodium salts are utilized for their ability to dissociate completely in solution, allowing for optimal reactivity and interaction with other chemical species. Therefore, understanding the solubility of sodium salts helps predict their behavior in aqueous solutions.

Transition Metal Sulfides

Transition metal sulfides include compounds like \( \text{CuS} \) (copper sulfide) and \( \text{ZnS} \) (zinc sulfide), both of which feature a transition metal bond with sulfur. Generally, these compounds have low solubility in water due to the strong bonds between the metal ions and the sulfur atoms. However, there are subtle differences between them.

• \( \text{CuS} \) is an almost insoluble black compound, making it less reactive with water.
• \( \text{ZnS} \) is slightly more soluble than \( \text{CuS} \) and appears as a white solid.

The insolubility of these compounds is significant in processes like mineral extraction and purification. Understanding these properties is essential for industries focused on metal recovery and environmental regulation of metals.

Solubility Rules

Solubility rules are guidelines that help predict whether a compound will dissolve in a solvent, such as water. These rules are determined based on empirical observations of various salts and involve generalizations about ionic compounds. Here are a few key points:

• Sodium salts are generally soluble in water, reflecting their easy dissociation and hydration.
• Sulfides of transition metals like copper and zinc are typically insoluble, with minor exceptions.
• The rules also highlight that compounds like nitrates and ammonium salts are usually soluble.

By understanding the solubility rules, chemists can anticipate the outcomes of chemical reactions, decide on the appropriate methods for compound purification, and predict environmental impact assessments. This knowledge is foundational for laboratory work and chemical engineering practices.