Le Chatelier's Principle - Effect of Changing Temperature

Greetings,

This post will focus on the effect on equilibrium position from changing temperature.  Equilibrium position will change at a new temperature and tend not to return to the original position as long as the new temperature is held.  This also means that a change in equilibrium system temperature results in a new Keq value: In other words, Keq is temperature dependent!  The direction of the equilibrium shift depends on the thermodynamic properties of the reaction and whether the temperature is raised or lowered.

Explanation of the Temperature Change Effect

A temperature change of an equilibrium reaction effects the entire system and so is different then simply changing concentrations of reactant or product in solutions.  The temperature effect can be considered an exception to the Le Chatelier Principle because the changed position does not tend to return to its initial relative reactant/product concentrations.  New equilibrium concentrations at the new temperature result in a new Keq value.  If we consider the equilibrium reaction, prior to the point at which equilibrium has actually occurred, we find that heat energy is either consumed or produced.  When the  pre-equilibrium reaction consumes energy, the thermodynamic change is endothermic and we can think of heat as a reactant.  Conversely, when the pre-equilibrium reaction produces energy, the thermodynamic change is exothermic and we can think of heat as a product.  Then, thinking of heat as a reactant or product, an increase in temperature increases the heat of reaction which causes the equilibrium position to shift away from the heat location (reactant or product).  Dropping the temperature causes the equilibrium position to shift toward the heat location in the equation.  The following diagram demonstrates the temperature effect for an exothermic equilibrium.

 A diagram could be made for the endothermic equilibrium, but the only difference would be heat appearing on the left side of the equation with position changes opposite to the exothermic reaction for temperature changes in the same direction.  It is important to understand that a change in temperature causes an equilibrium shift in order to keep the heat of reaction constant, not to keep the position constant!

That's all for now.  My next post will feature pressure change effects on gaseous equilibrium systems.

Thanks for Reading!

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