Problem 21 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

The correct order is (d) \(\mathrm{Na}_{2} \mathrm{~S} > \mathrm{ZnS} > \mathrm{CuS}\).

Step 1: Understand Solubility Rules

In aqueous solution, the ionic compounds that are highly soluble usually contain alkali metal ions or ammonium ions. Sodium sulfide (\(\mathrm{Na}_{2} \mathrm{~S}\)), containing sodium ions, generally has high solubility. Meanwhile, transition metal sulfides like \(\mathrm{CuS}\) and \(\mathrm{ZnS}\) tend to be much less soluble.

Step 2: Compare Solubility of Sodium Sulfide

\(\mathrm{Na}_{2} \mathrm{~S}\) is expected to be more soluble compared to \(\mathrm{CuS}\) and \(\mathrm{ZnS}\) because it contains sodium, which generally forms soluble compounds. Thus, \(\mathrm{Na}_{2} \mathrm{~S}\) should have the highest solubility.

Step 3: Comparing Copper Sulfide and Zinc Sulfide

Both \(\mathrm{CuS}\) and \(\mathrm{ZnS}\) are sulfides of transition metals. Generally, \(\mathrm{ZnS}\) is slightly more soluble than \(\mathrm{CuS}\) due to the lower lattice energy and greater solubility of zinc salts compared to copper.

Step 4: Determine the Order

Based on the solubility knowledge, the order from most to least soluble is \(\mathrm{Na}_{2} \mathrm{~S} > \mathrm{ZnS} > \mathrm{CuS}\). Thus, the correct order of solubility in an aqueous medium is choice (d).

Key Concepts

Ionic Compounds

Ionic compounds form when metals transfer electrons to nonmetals, resulting in positive and negative ions. These oppositely charged ions attract each other, creating a strong bond known as an ionic bond.
In an aqueous solution, the solubility of ionic compounds depends on various factors, primarily the ions involved.
Some key points to understand about ionic compounds include:

• High Solubility: Compounds with alkali metals like sodium, potassium, or ammonium ions usually dissolve well in water. For example, sodium sulfide (Na₂S) is highly soluble because it contains sodium ions.
• Low Solubility: Compounds with transition metals (such as copper and zinc) often have lower solubility due to their unique electronic configurations and bonding characteristics.
• Solubility Rules: These general rules help predict which compounds will dissolve in water, but exceptions can occur based on specific conditions and compounds.

Transition Metal Sulfides

Transition metal sulfides exhibit unique characteristics due to their composition. Transition metals, located in the central block of the periodic table, have distinct properties because of their partially filled d orbitals.
These metals form compounds with sulfur known as sulfides, such as copper sulfide (CuS) and zinc sulfide (ZnS).
Some aspects of transition metal sulfides include:

• Reduced Solubility: Transition metal sulfides are generally not very soluble in water, primarily because of their strong ionic bonds and lattice energies.
• Unique Conductivity: Some transition metal sulfides have metallic properties, contributing to their application in catalysts and electronic materials.
• Solubility Variations: Despite low solubility compared to alkali metal sulfides, differences still exist among transition metal sulfides, influenced by factors like lattice energy and stability under various conditions.

Lattice Energy

Lattice energy is an essential concept in understanding the solubility of ionic compounds such as transition metal sulfides. It refers to the energy released when ions in the gaseous state come together to form a solid ionic compound.
This energy is crucial because higher lattice energy means stronger ionic bonds, resulting in lower solubility.
Here are some key points about lattice energy:

• Influence on Solubility: Compounds with high lattice energies, like most transition metal sulfides, typically have decreased solubility in water.
• Factors Affecting Lattice Energy: Lattice energy depends on both the charge and size of the ions. Higher charges and smaller ionic sizes result in stronger interactions and higher lattice energy.
• Differentiating Compounds: Differences in lattice energy can help explain why zinc sulfide (ZnS) might be more soluble than copper sulfide (CuS), as ZnS usually has a slightly lower lattice energy.