Changing the temperature

In general, heating a reaction causes it to shift to the endothermic (heat-absorbing) side. (If you have an exothermic reaction where heat is produced on the right side, then the left side is the endothermic side.) Cooling a reaction mixture causes the equilibrium to shift to the exothermic (heat-releasing) side. Suppose that you heat the reaction mixture of nitrogen and hydrogen. You know that the reaction is exothermic heat is given off, showing up on the right-hand side of the equation.

So if you heat the reaction mixture, the reaction shifts to the left to use up the extra heat and reestablish the equilibrium.  This shift uses up ammonia and produces more nitrogen and hydrogen. And as the reaction shifts, the amount of heat also decreases, lowering the temperature of the reaction mixture.