Question

In which case does the change in entropy (\( \Delta S \)) become negative?

• A. Evaporation of water
• B. Expansion of a gas at constant temperature
• C. Sublimation of solid to gas
• D. \( 2H(g) \rightarrow H_2(g) \)

Hint:

Entropy decreases when multiple molecules combine into fewer molecules, reducing randomness.

Answer 1

The Correct Option is D

Step 1: Understanding entropy (\( \Delta S \)). 
- Entropy represents the disorder or randomness of a system. 
- A positive \( \Delta S \) means increased randomness, while a negative \( \Delta S \) means decreased randomness. 
Step 2: Analyzing the given processes. 
- (A) Evaporation of water: Liquid to gas transition increases entropy (\( \Delta S>0 \)). 
- (B) Gas expansion: Increase in volume increases entropy (\( \Delta S>0 \)). 
- (C) Sublimation: Solid to gas transition increases entropy (\( \Delta S>0 \)). 
- (D) Formation of \( H_2 \) from \( H \) atoms: Two gas molecules combine into one, decreasing entropy (\( \Delta S<0 \)). 
Step 3: Identifying the correct answer. 
- Since option (D) represents a decrease in entropy, it is the correct answer.