15.9: The Effect of Temperature Changes on Equilibrium

Learning Objectives
• Explain the following laws within the Ideal Gas Law

When temperature is the stress that affects a system at equilibrium, there are two important consequences:

• an increase in temperature will favor that reaction direction that absorbs heat (i.e. the endothermic reaction)
• the value of Keq will change

Consider the following equilibrium system

$\ce{N_2O_4(g) \leftrightarrow 2NO_2(g)}$

with $$\ce{\Delta H^{\circ}={58.0}\:kJ}$$

We see by the sign of ΔH° that the forward reaction is endothermic. Heat is absorbed (required as a reactant) when the reaction proceeds as

$\ce{N_2O_4(g) \rightarrow 2NO_2 (g)}$

By adding more heat, equilibrium will shift to use up the additional heat, thus favoring this forward direction.

Why will Keq change, when it did not change when concentration, pressure, and volume were the applied stresses?

When temperature changes cause an equilibrium to shift, one entire side of the reaction equation is favored over the other side. Mathematically, this will alter the value of Keq as follows:

$\ce{K_{eq}=\dfrac{[products]}{[reactants]}}$

 if the forward reaction is favored more products are produced; fewer reactants Keq will increase if the reverse reaction is favored fewer products; more reactants Keq will decrease

So in our example given above, increasing the temperature will favor the forward direction. The value of Keq will increase. Removing heat (making the system colder) will favor the exothermic reaction - the exothermic reaction releases heat to the surroundings, thus "replacing" the heat that has been removed.