D25.3 Le Châtelier’s Principle: Change in Temperature
Using an equilibrium constant and ICE table, we can calculate equilibrium concentrations. Using Q and K we can analyze which direction a system that is not in equilibrium reacts to establish equilibrium. Often, however, we do not know exact concentrations or partial pressures of reactants and products. In such a case Le Châtelier’s principle enables prediction of which direction an equilibrium will shift when conditions change.
Le Châtelier’s principle states that when a chemical system is at equilibrium and conditions are changed so that the reaction is no longer at equilibrium, the chemical system reacts to achieve new equilibrium concentrations or partial pressures in a way that partially counteracts the change in conditions.
When a chemical reaction is at equilibrium and the temperature changes, the reaction’s equilibrium constant is different at the new temperature. Here is an example to illustrate how Le Chatelier’s principle qualitatively predicts whether K would increase or decrease upon a temperature change. Consider the reaction:
This reaction is endothermic: when the reaction takes place in the forward direction, the temperature is lowered. Because we assume that ΔrH° does not vary with temperature, 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 would proceed towards products (the endothermic direction), which would lower the temperature. 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 K 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 proceeds to partially raise the temperature—in the exothermic direction. In this case, the result is the formation of more reactants. Thus, at a lower temperature the concentrations of reactants are higher, the concentrations of products are lower, and K is smaller.
Summarizing,
- An increase in temperature shifts a reaction in the endothermic direction (the direction with positive ΔrH°) because the endothermic reaction partially counteracts the increase in temperature
- A decrease in temperature shifts a reaction 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, K.
- The larger the magnitude of ΔrH° is, the larger the shift in the equilibrium is, and the greater the change in K is.
Exercise: Applying Le Chatelier’s Principle to Temperature Change
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