The following reaction is endothermic.
\mathrm{CaCO_{3}(s)}\rightleftarrows \mathrm{CaO}(s)\,+\,\mathrm{CO}_{2}(g)What is the effect of increasing the temperature of the reaction mixture? Decreasing the temperature?
Since the reaction is endothermic, we can think of heat as a reactant.
\mathrm{Heat} + \mathrm{CaCO_{3}(s)}\underset{\longleftarrow}{\longrightarrow}\mathrm{CaO}(s)\,+\,\mathrm{CO}_{2}(g)Raising the temperature is adding heat, causing the reaction to shift to the right. Lowering the temperature is removing heat, causing the reaction to shift to the left.