Question

In which of the following reactions, increase in pressure will favour the forward reaction?

• A. $P C l_{5}(g) \rightleftharpoons P C l_{3}(g)+C l_{2}(g)$
• B. $2 NO (g)+ O _{2}(g) \rightleftharpoons 2 NO _{2}(g)$
• C. $C ( s )+ H _{2} O (g) \rightleftharpoons CO (g)+ H _{2}(g)$
• D. $2 H I(g) \rightleftharpoons H_{2}(g)+I_{2}(g)$

Answer 1

Answer: (B)

$\underset{2 mol}{2 NO} +\underset{1 mol }{ O _{2}} \rightleftharpoons \underset{2 mol}{2 NO _{2}(g)}$
If $\Delta n=0$, no effect on equilibrium due to pressure.
$\Delta n>0$,
the increase in pressure favours backward reaction,
$\Delta n<0$,
the increase in pressure favours forward reaction for [a]
$\Delta n=n_{p}-n_{r}=2-1=1$,
$[ b ] \Delta n=2-3=-1$
$[ c ] \Delta n=2-1=1$
$[ d ] \Delta n=2-2=0$
Hence, in case of [b] An $<0$,
the increase in pressure will favour the forward reaction.