When a chemical reaction is at equilibrium and the temperature changes, the reaction’s equilibrium constant is different at the new temperature. Le Chatelier’s principle can be used to predict which direction an equilibrium shifts and hence whether increasing temperature increases or decreases K. Remember that, according to Le Chatelier’s principle, an equilibrium shifts in a direction that partially counteracts the change in conditions.
Consider the reaction
N2(g) + O2(g) ⇌ 2 NO(g) ΔrH° = 180.5 kJ/mol at 25 °C
As shown by the enthalpy change, this reaction is endothermic: when the reaction takes place in the forward direction, the temperature is lowered. Because enthalpy change does not vary significantly with temperature, we can assume that the forward reaction is endothermic at all temperatures where all reactants and products are in the gas phase, and the reverse reaction is exothermic.
Suppose that the reaction is at equilibrium at a particular temperature and the temperature is suddenly increased. To partially compensate for the temperature increase, the reaction shifts toward products (the endothermic direction), which lowers the temperature a bit. Thus, when equilibrium is reached at the higher temperature, the concentration of NO is higher and the concentrations of N2 and O2 are lower. This results in a larger value for Kc at the higher temperature, in agreement with a van’t Hoff plot for an endothermic reaction.
If the temperature of the reaction is suddenly lowered, the reaction shifts to partially raise the temperature—in the exothermic direction. In this case, the shift is to form more reactants. Thus, at a lower temperature the concentrations of reactants are higher, the concentrations of products are lower, and the equilibrium constant is smaller.
- An increase in temperature shifts an equilibrium in the endothermic direction (the direction with positive ΔrH°) because the endothermic reaction partially counteracts the increase in temperature
- A decrease in temperature shifts an equilibrium in the exothermic direction (the direction with negative ΔrH°) because the exothermic reaction partially counteracts the decrease in temperature.
- The different concentrations in the new equilibrium system (after the shift resulting from the temperature change) correspond to a different value for the equilibrium constant.
- The larger the magnitude of ΔrH° is, the larger the shift in the equilibrium is, and the greater the change in the equilibrium constant is.