Temperature & Le Châtelier’s Principle
Chemistry: IB Diploma Higher Level, topic 5.4.3
Temperature & Le Châtelier’s Principle
When an equilibrium shifts in response to a temperature change, it is re-established with a different relative composition that exhibits a different value for the equilibrium constant.
Temperature changes &
- Changes in the concentration of reactants or products change the value of , whereas changes in temperature affect .
- Consistent with the law of mass action, an equilibrium stressed by a change in concentration will shift to re-establish equilibrium without any change in .
- When an equilibrium shifts in response to a temperature change, however, it is re-established with a different relative composition that exhibits a different value for the .
Endothermic vs exothermic
Predicting the shift an equilibrium will experience in response to a change in temperature is most conveniently accomplished by considering the enthalpy change of the reaction.
For example, the decomposition of dinitrogen tetroxide is an endothermic (heat-consuming) process:
N2O4(g)⇌2NO2(g) Δ𝐻=+57.20 kJ
Heat as a reactant
- For purposes of applying Le Chatelier’s principle, heat (q) may be viewed as a reactant:
- Heat + N2O4(g)⇌2NO2(g)
- Raising the temperature of the system is akin to increasing the amount of a reactant, and so the equilibrium will shift to the right.
- Lowering the system temperature will likewise cause the equilibrium to shift left.
- For exothermic processes, heat is viewed as a product of the reaction and so the opposite temperature dependence is observed.
Catalysts
- Catalysts increase the rate of forward and reverse reactions equally, so have no impact on equilibrium position.
Test yourself on Temperature & Le Châtelier’s Principle
If the forward reaction of a system in dynamic equilibrium is endothermic, what would be the impact on the equilibrium position if the temperature was decreased?
If the forward reaction of a system in dynamic equilibrium is exothermic, what would be the impact on the equilibrium position if the temperature was decreased?