10.7: The Effect of Temperature Changes on Equilibrium
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 K eq will change
Consider the following equilibrium system
\[\ce{N_2O_4(g) \leftrightarrow 2NO_2(g)}\]
with \(\ce{\Delta H^{\circ}={58.0}\:kJ}\)
Delta H is a measurement showing heat was gained by the reaction. We will cover heat and energy soon.
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 K eq 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 K eq as follows:
\[\ce{K_{eq}=\dfrac{[products]}{[reactants]}}\]
|
if the forward reaction is favored |
more products are produced; fewer reactants |
K eq will increase |
|
if the reverse reaction is favored |
fewer products; more reactants |
K eq will decrease |
So in our example given above, increasing the temperature will favor the forward direction. The value of K eq 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.