In a reversible reaction $K_c>K_p$ and  $\mathrm{\Delta H}=+40$   K. Cal / mole. Backward reaction is favoured by

$$\begin{gathered} {\mathrm{K}}_{\mathrm{c}}>{\mathrm{K}}_{\mathrm{p}} \mathrm{is} \mathrm{applicable} \mathrm{when} , ,\mathrm{i}.\mathrm{e}.. \mathrm{backward} \mathrm{reaction} \mathrm{is} \mathrm{accompanied} \mathrm{by} \mathrm{increase} \mathrm{in} \mathrm{number} \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{gaseous} \mathrm{reactants}. \\ \mathrm{Hence} \mathrm{backward} \mathrm{reaction} \mathrm{is} \mathrm{favoured} \mathrm{by} \mathrm{decreasing} \mathrm{pressure} \mathrm{and} \mathrm{also} \mathrm{decrease} \mathrm{in} \mathrm{temperature} \mathrm{because} \mathrm{backward} \mathrm{reaction} \mathrm{is} \mathrm{exothermic}. \end{gathered}$$