Question

The correct descending order of the heat liberated (in kJ) during the neutralization of the acids CH$_3$COOH (W), HF (X), HCOOH (Y) and HCN (Z) under identical conditions is \[ K_a(\text{CH}_3\text{COOH}) = 1.8 \times 10^{-5}, \quad K_a(\text{HCOOH}) = 1.8 \times 10^{-4}, \quad K_a(\text{HCN}) = 4.9 \times 10^{-10}, \quad K_a(\text{HF}) = 3.2 \times 10^{-4} \]

• A. Y > X > Z > W
• B. X > Y > W > Z
• C. W > X > Y > Z
• D. Z > W > Y > X
• E. Z > Y > X > W

Hint:

Higher $K_a$ ⇒ stronger acid ⇒ more heat of neutralization.

Answer 1

The Correct Option is B

Concept: Heat of neutralization vs strength of acid

• Strong acid → complete ionization → maximum heat
• Weak acid → partial ionization → less heat
• More $K_a$ ⇒ stronger acid ⇒ more heat released ---

Step 1: Compare $K_a$ values
\[ \text{HF} = 3.2 \times 10^{-4} \] \[ \text{HCOOH} = 1.8 \times 10^{-4} \] \[ \text{CH}_3\text{COOH} = 1.8 \times 10^{-5} \] \[ \text{HCN} = 4.9 \times 10^{-10} \] ---

Step 2: Arrange in decreasing strength
\[ \text{HF} > \text{HCOOH} > \text{CH}_3\text{COOH} > \text{HCN} \] ---

Step 3: Heat of neutralization order
\[ X > Y > W > Z \] --- Final Answer: \[ \boxed{\text{(B)}} \]