For the reaction in equilibrium N_2(~g)+3 H_2(~g) leftharpoons 2...

$\mathrm{N}_2(\mathrm{~g})+3 \mathrm{H}_2(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_3(\mathrm{~g}), \Delta \mathrm{H}=-\mathrm{Q}$ According to Le Chatelier's principle. - Exothermic reactions are favoured at low temperature. - Increase in pressure shifts the reaction in direction having lesser number of moles. Hence, the given reaction shifts forward on increasing pressure. - Increasing the concentration of reactants shifts the reaction in forward direction. So high concentration of $\mathrm{H}_2$ shifts reaction in forward direction.