Question

For which reaction change of entropy will be positive?

• A. $H_2\left(g\right)+I_2\left(g\right)\rightleftharpoons 2HI\left(g\right)$
• B. $HCl\left(g\right)+N{H}_3\left(g\right)\rightleftharpoons N{H}_{4}Cl\left(g\right)$
• C. $N{H}_{4}N{O}_3\left(s\right)\rightleftharpoons N_{2}O\left(g\right)+2H_{2}O\left(g\right)$
• D. $MgO\left(s\right)+H_2\left(g\right)\rightleftharpoons Mg\left(s\right)+H_{2}O\left(l\right)$

Answer 1

The correct option is C $N{H}_{4}N{O}_3\left(s\right)\rightleftharpoons N_{2}O\left(g\right)+2H_{2}O\left(g\right)$

Entropy is a measure of randomness of the system, so entropy of a substance in different physical states is in the order:

Gas > liquid > solid

$\Delta S=S_{product}-S_{reactant}$

In option (C), solid is converted into a gaseous state, so $\Delta S$ will be positive.