Question

In which of the following reactions \(\Delta S\) (entropy change) is positive?

• A. H_2O(l)arrow H_2O(s)
• B. 3O_2(g)arrow2O_3(g)
• C. H_2O(l)arrow H_2O(g)
• D. N_2(g)+3H_2(g)arrow2NH_3(g)

Hint:

Vaporization always increases entropy because gaseous molecules possess maximum freedom of motion.

Answer 1

The Correct Option is C

Concept: Entropy measures randomness or disorder. \[ \text{Gas}>\text{Liquid}>\text{Solid} \] Entropy increases when disorder increases.

Step 1: Examine each option.
(A) \[ Liquid \rightarrow Solid \] Randomness decreases. \[ \Delta S<0 \] (B) \[ 3\text{ moles gas}\rightarrow2\text{ moles gas} \] Number of gas molecules decreases. \[ \Delta S<0 \] (C) \[ Liquid \rightarrow Gas \] Randomness increases greatly. \[ \Delta S>0 \] (D) \[ 4\text{ moles gas}\rightarrow2\text{ moles gas} \] Entropy decreases. \[ \Delta S<0 \] Hence only option (C) has positive entropy change. \[ \boxed{H_2O(l)\rightarrow H_2O(g)} \]