The Effect of Temperature on Keq
The equilibrium constant, Keq is temperature dependent and this is one of the reasons why temperature conditions are often stated in equilibrium problems. If the concentration of a reactant or product is changed, then the system will adjust so that the ratio of reactants to products is in the same proportion needed to achieve the same Keq.
We have previously seen that when the temperature of an equilibrium system is changed, the equilibrium will shift, resulting in an increase in the production of reactants or products, whilst the other is decreased. This means that changes in temperature result in a change to the ratio of reactants to products. This in turn changes the equilibrium constant, Keq.
When analysing problems relating to temperature changes and equilibrium systems, there are four factors that should be linked and understood:
- The enthalpy of the reaction – are the forward and reverse reactions exothermic or endothermic?
- The temperature changed imposed on the system
- Which way the equilibrium shifts as a result of the change in temperature – is there an increase in the concentration of reactants or products?
- Will Keq increase or decrease as a result of this change in temperature?
Impacts of Temperature on Exo/Endo Reactions:
Recall the following:
A ⇌ B (ΔH is negative and −ΔH)
A ⇌ B + heat energy
A ⇌ B (ΔH is positive and +ΔH)
A + heat energy ⇌ B
- An increase in temperature will favour the endothermic direction
- A decrease in temperature will favour the exothermic direction
Another way to visualise energy changes to equilibrium systems:
- If the temperature of the system is increased, the equilibrium will shift away from the heat that may be stated in the equation.
- If the temperature of the system is decreased, the equilibrium will shift toward the heat that may be stated in the equation.
This analogy illustrated below:
Understanding the shift in equilibrium makes it easier to predict the change to Keq:
- If a system shifts right, Keq increases
- If a system shifts right, Keq decreases
For the following equilibrium:
N2(g) + 3H2(g) ⇌ 2NH3(g) (-∆H)
Does Keq increase or decrease when the temperature of the system is increased?
Keq will decrease – an increase in temperature will shift the equilibrium in the endothermic direction (left). This will favour the formation of reactants and a decrease in Keq.
Keq for a particular reaction is 2.5 × 102 at 500K. At 600K the value of Keq is 3.2 × 102.
Justify whether this is an exothermic or endothermic reaction in the forward direction:
The increased temperature will favour the endothermic direction. An increase in temperature has caused an increase in Keq, indicating a shift to the right to favour products. Therefore, in the forward direction this reaction is endothermic.