This depends in what type reaction is occurring. It may be an endothermic reaction or an exothermic reaction. If the reaction is endothermic or heat is being absorbed by the reaction, then the forward reaction is favored or the products are being formed by the addition of heat. If the reaction is exothermic or heat is released by the reaction, then the backward reaction is favored or the reactants are being formed by the addition of heat.
