Question

Predict which of the following reaction(s) has a positive entropy change?
$I. Ag^{+}\left(aq\right) + Cl^{-}\left(aq\right)\to AgCl\left(s\right)$
$II. NH_4Cl\left(s\right)\to NH_3\left(g\right)+ HCl\left(g\right)$
$III. 2NH_3\left(g\right)\to N_2\left(g\right)+3H_2\left(g\right)$

• A. $I$ and $II$
• B. $III$
• C. $II$ and $III$
• D. $II$

Answer 1

Answer: (C)

We know that
$\Delta S _{\text {reaction }}^{0}=\Sigma \Delta S ^{0}$ Product $-\Sigma \Delta S ^{0}$ Reactant
When we compare standard molar entropies for substance, we will find that standard molar entropy of Gas is grater than Liquids and Liquid's standard molar entropy is grater than Solids.
Using these information it is clear that reaction 2 and 3 has positive entropy change.