Question

Which of the following reactions exhibits decrease in entropy?

• A. $2\text{H}_2\text{O}_{2(l)} \longrightarrow 2\text{H}_2\text{O}_{(l)} + \text{O}_{2(g)}$
• B. $\text{H}_{2(g)} \longrightarrow 2\text{H}_{(g)}$
• C. $\text{CaCO}_{3(s)} \xrightarrow{\Delta} \text{CaO}_{(s)} + \text{CO}_{2(g)}$
• D. $2\text{H}_{2(g)} + \text{O}_{2(g)} \longrightarrow 2\text{H}_2\text{O}_{(l)}$

Hint:

Count gas moles: Fewer gas moles in products $\rightarrow$ Decrease in Entropy.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Entropy ($\Delta\text{S}$) decreases when randomness decreases, typically when moles of gas decrease or gas turns to liquid/solid.
Step 2: Key Formula or Approach:
$\Delta\text{S}<0$ if $\Delta\text{n}_{\text{gas}}<0$.
Step 3: Detailed Explanation:
- (A) Liquid $\rightarrow$ Gas: $\Delta\text{S}>0$.
- (B) 1 mol gas $\rightarrow$ 2 mol gas: $\Delta\text{S}>0$.
- (C) Solid $\rightarrow$ Gas: $\Delta\text{S}>0$.
- (D) 3 moles of gas reactant $\rightarrow$ 0 moles of gas product (liquid). Randomness significantly decreases. Hence, $\Delta\text{S}<0$.
Step 4: Final Answer:
Reaction (D) shows a decrease in entropy.