Explanation

(a)
$2HI(g) \rightleftharpoons H_{2}(g) + I_{2}(g)\mathcal{; - Q}kJ$
$n_{g}$ No. of moles of gaseous products – No. of moles of gaseous reactants
$2 - 2\ \ \ \ \ 0$
As the number of moles of products and reactants are equal, so equilibrium constant is not affected by any change in pressure and volume. Catalyst also does not affect the equilibrium constant. It is an endothermic reaction, hence equilibrium constant depends only upon the temperature.