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) Na₂S > ZnS > CuS.

Step 1: Determine Types of Compounds

Identify the types of compounds. Na₂S is a salt that is highly soluble in water. CuS and ZnS are sulfides of transition metals, which are typically less soluble.

Step 2: Analyze Solubility Rules

Recall the general solubility rules: most sodium salts like Na₂S are highly soluble in water. However, sulfides of transition metals like CuS and ZnS are generally insoluble.

Step 3: Compare Transition Metal Sulfides

Among transition metal sulfides, ZnS is typically more soluble than CuS. This is due to ZnS having a higher solubility product constant (Ksp) compared to CuS.

Step 4: Order the Solubility

From the above analysis, Na₂S is the most soluble, followed by ZnS, and then by CuS. Thus, the solubility order is Na₂S > ZnS > CuS.

Key Concepts

Understanding Sodium Salts

Sodium salts are compounds that contain sodium ions and an accompanying anion. One common example is sodium sulfide, represented as Na₂S. A notable feature of sodium salts is their high solubility in water. This means they dissolve easily, allowing sodium ions to separate out freely.
Water molecules surround these ions, breaking the solid structure of the salt, which is why sodium salts are often considered among the most soluble of compounds. This high solubility of sodium salts is a common theme in chemistry, as most sodium-containing compounds will readily dissolve in water.

• For instance, when you dissolve table salt, which is a sodium chloride (NaCl), you can observe this solubility behavior firsthand.
• The overall solubility of sodium salts makes them versatile in various chemical reactions, especially in aqueous solutions, where reactions can occur swiftly due to the free movement of ions.

Properties of Transition Metal Sulfides

Transition metal sulfides, such as CuS (copper sulfide) and ZnS (zinc sulfide), are known for their lower solubility in water. These compounds are formed from transition metals paired with sulfur, and their solubility is influenced significantly by the nature of the metal involved.
Transition metals have unique electron configurations and can form complex structures with sulfide ions that are quite stable. This stability is a key reason for their low solubility.

• Copper sulfide, CuS, is one of the least soluble transition metal sulfides because of its strong lattice structure.
• Zinc sulfide, ZnS, while still generally insoluble, has a slightly higher solubility than CuS, attributed to the differences in their lattice energies.

It's important to note that while these compounds are not water-friendly, they have significant applications in fields such as electronics and pigments, where their unique properties are advantageous.

The Solubility Product Constant (Ksp) Explained

The solubility product constant, often abbreviated as Ksp, is a useful measure to understand the solubility of sparingly soluble compounds. For compounds like ZnS and CuS, this constant helps in determining how much of the compound can dissolve in water before the solution becomes saturated.
Lower Ksp values indicate lower solubility. For instance, CuS has a very low Ksp, meaning it is quite insoluble in water. ZnS, on the other hand, has a slightly higher Ksp than CuS, allowing it to dissolve a bit more readily.

• The Ksp is calculated using the concentrations of the ions formed when a compound dissolves. For ZnS, we express this as: \[ K_{sp} = [Zn^{2+}][S^{2-}] \].
• Knowing the Ksp values of different compounds is crucial for predicting precipitation reactions and understanding equilibrium in saturated solutions.

This concept is widely applied across different areas of chemistry, especially in analytical processes and during the selective precipitation of compounds in synthesizing reactions.