Question

Which of the following reaction(s) has a positive entropy change?
$\left(I\right)A{g}^{+}\left(aq\right)+C{l}^{-}\left(aq\right)\to AgCl\left(s\right)\left(II\right)N{H}_{4}Cl\left(s\right)\to N{H}_3\left(g\right)+HCl\left(g\right)\left(III\right)2N{H}_3\left(g\right)\to N_2\left(g\right)+3H_2\left(g\right)$

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

Answer 1

The correct option is C II and III

Entropy change will be positive in the cases where the number of gaseous moles of the product is more than the number of gaseous moles of the reactants.
Which means, $△n_g>0$
where, $△n_g=n_p-n_R$
$n_P=$ total number of gaseous moles of the products
$n_R=$ total number of gaseous moles of the reactants

In the cases,

(II) $△n_g=2$

(III) $△n_g=2$

In both these cases, as
$△n_g>0$ $\Rightarrow △S>0$

For a given substance at a given temperature, $S_{gas}>S_{liquid}>S_{solid}$
In case (I), entropy decreases as the conversion is taking place from liquid to solid state.
So for case (I), $△S<0$

So, $△S>0$ in cases (II) and (III).