Equilibrium and Changes to Temperature

Le Châtelier’s Principle tells us that if you increase the temperature of a chemical reaction at equilibrium, the system will try to cool itself by shifting the equilibrium position in the endothermic (heat absorbing) direction. However, LCP does not provide an explanation for this prediction – for that, we must look to collision theory.

According to collision theory, an increase in the temperature of an equilibrium system will increase the frequency of collisions and the proportion of collisions which are successful, therefore increasing the rate of reaction in both directions. But, the rate of reaction for the endothermic direction will increase more. Consequently, the equilibrium position will shift in this endothermic direction, partially counteracting the increase in temperature.

The rate of endothermic reaction will increase more than the rate of exothermic reaction because the increase in the proportion of molecules with the kinetic energy needed to overcome the endothermic activation energy (Ea) barrier is larger than the increase in the proportion of molecules with the kinetic energy needed to overcome the exothermic activation energy barrier…

For an equilibrium system, the activation energy for the endothermic reaction will always be larger than the activation energy for the exothermic reaction. So, when an equilibrium is disturbed by an increase in temperature, and the distribution of molecular kinetic energies shifts from the blue curve to the red curve (as pictured above), the relative increase in the area of the curve that exists beyond the endothermic activation energy (B/D) is larger than the relative increase in the area of the curve that exists beyond the exothermic activation energy ((A+B)/(C+D)). Consequently, the rate of endothermic reaction will increase by more than the rate of exothermic reaction, therefore shifting the equilibrium position in this endothermic direction.