Question

In which of the following reactions does entropy decrease? (i) $2Pb(NO_{3})_{2(s)}\rightarrow 2PbO_{(s)}+4NO_{2(g)}+O_{2(g)}$ (ii) $H_{2}O_{(g)}\rightarrow H_{2}O_{(l)}$ (iii) $Br_{2(l)}\rightarrow 2Br_{(g)}$ (iv) $C_{6}H_{6(l)}\rightarrow C_{6}H_{6(s)}$

• A. (ii), (iii) and (iv)
• B. (i) and (iii)
• C. (i) and (iii)
• D. (i) and (iv)
• E. (ii) and (iv)

Hint:

Phase change toward a solid state always decreases entropy.

Answer 1

Answer: (E)

Step 1: Concept
Entropy ($S$) measures randomness. Gas $>$ Liquid $>$ Solid.

Step 2: Meaning
Entropy decreases when randomness reduces (e.g., gas to liquid).

Step 3: Analysis
(ii) gas to liquid = decrease; (iv) liquid to solid = decrease.

Step 4: Conclusion
Reactions (ii) and (iv) show a decrease in entropy. Final Answer: (E)