Which is the correct order of K for Cus, ZnS and CdS?

Options

A. CuS gt ZnS gt Cds
B. COS gt Cus gt ZnS
C. ZnS gt CDS gt Cus
D. ZnS gt Cus gt Cds

Answer

To determine the correct order of the solubility product constant (K) for copper sulfide (CuS), zinc sulfide (ZnS), and cadmium sulfide (CdS), we can follow these steps:

### Step 1: Understand the Concept of Solubility Product Constant (K)
The solubility product constant (K) is a measure of the solubility of a compound in water. It is defined as the product of the concentrations of the ions in a saturated solution of the compound, raised to the power of their coefficients in the balanced equation.

### Step 2: Write the Dissociation Reactions
For each sulfide:
- CuS(s) ⇌ Cu²⁺(aq) + S²⁻(aq)
- ZnS(s) ⇌ Zn²⁺(aq) + S²⁻(aq)
- CdS(s) ⇌ Cd²⁺(aq) + S²⁻(aq)

### Step 3: Identify the Cations and Anions
In all three compounds, the anion (S²⁻) is the same. The cations differ:
- Cu²⁺ for CuS
- Zn²⁺ for ZnS
- Cd²⁺ for CdS

### Step 4: Analyze the Polarizability and Ionic Character
The polarizability of cations affects the covalent character of the sulfides:
- Cu²⁺ has the highest polarizing power, making CuS more covalent.
- Cd²⁺ has moderate polarizing power, making CdS less covalent than CuS but more than ZnS.
- Zn²⁺ has the lowest polarizing power, making ZnS the most ionic.

### Step 5: Determine the Relationship Between Covalent Character and K
More covalent character in a compound typically results in lower solubility, hence a lower K value. Conversely, more ionic character results in higher solubility and a higher K value.

### Step 6: Establish the Order of K
Based on the analysis:
- CuS (most covalent, lowest K)
- CdS (moderate covalent character, intermediate K)
- ZnS (most ionic, highest K)

Thus, the order of K for the compounds is:
**K(CuS) < K(CdS) < K(ZnS)**

### Final Answer
The correct order of K for CuS, ZnS, and CdS is:
**K(CuS) < K(CdS) < K(ZnS)**