14.9
Le Chatelier's Principle: Changing Temperature
Consistent with the law of mass action, an equilibrium stressed by a change in concentration will shift to re-establish equilibrium without any change…
Temperature affects the rate of a chemical reaction; therefore, a change in temperature for a reaction at equilibrium acts as a stress on the system. Le Châtelier's principle predicts how the system will respond to minimize such disturbances.
A change in the temperature changes the value of the equilibrium constant, unlike a change in concentration or volume, which shifts the equilibrium without changing the value of K.
Consider the decomposition of gaseous phosphorus pentachloride into phosphorus trichloride and chlorine gas. For this endothermic reaction, the heat absorbed can be thought of as a reactant.
An increase in temperature adds heat to the system, similar to adding more of a reactant. Thus, the equilibrium position shifts towards the products and produces more phosphorus trichloride and chlorine to consume the extra heat because the value of the equilibrium constant, K, has increased.
On the other hand, a decrease in temperature removes heat from the system, similar to removing a reactant. The equilibrium position shifts towards the reactants and produces more phosphorus pentachloride to release heat because the value of K has decreased.
For an exothermic reaction, such as the gaseous reaction between sulfur dioxide and oxygen to produce sulfur trioxide, the heat released can be thought of as a product.
An increase in temperature is similar to adding more of a product. This causes the equilibrium position to shift toward the reactants, producing more sulfur dioxide and oxygen to absorb some of the added heat because the value of K has decreased.
Conversely, decreasing the temperature of this exothermic reaction removes heat, like removing a product. The equilibrium position shifts towards the products and produces more sulfur trioxide to release heat as K has increased.
Thus, an increase in temperature favors the products in an endothermic reaction, whereas a decrease in temperature favors the products in an exothermic reaction.
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Q1: Why does temperature change affect the equilibrium constant differently than concentration changes?
Temperature changes alter the equilibrium constant value itself, whereas concentration changes shift equilibrium without changing K. This occurs because temperature affects the rate constants for both forward and reverse reactions through the Arrhenius equation. Since the rate constants vary with temperature, the equilibrium constant—which depends mathematically on these rate constants—also changes with temperature.
Q2: How does increasing temperature affect an endothermic reaction at equilibrium?
For endothermic reactions, heat is treated as a reactant. Increasing temperature adds heat to the system, similar to adding more reactant. The equilibrium shifts toward products to consume the extra heat, and the equilibrium constant K increases. This produces more products like phosphorus trichloride and chlorine in the decomposition of phosphorus pentachloride.
Q3: What happens to an exothermic reaction when temperature decreases?
For exothermic reactions, heat functions as a product. Decreasing temperature removes heat, similar to removing a product. The equilibrium shifts toward products to release heat and restore the removed energy, and K increases. This produces more sulfur trioxide in the reaction between sulfur dioxide and oxygen.
Q4: How can you predict which direction an equilibrium will shift with a temperature change?
Consider the reaction's enthalpy change. For endothermic reactions, increasing temperature favors products; decreasing temperature favors reactants. For exothermic reactions, the opposite occurs: increasing temperature favors reactants, while decreasing temperature favors products. This prediction follows from treating heat as either a reactant or product depending on reaction type.
Q5: Why does decreasing temperature shift an exothermic equilibrium toward products?
In exothermic reactions, heat is a product. Removing heat by decreasing temperature is like removing a product, causing the equilibrium to shift right toward products. This shift produces more products to release additional heat and counteract the temperature decrease, while the equilibrium constant K increases.
Q6: How does temperature change differ from volume or pressure changes in affecting equilibrium?
Temperature changes alter the equilibrium constant value, while volume or pressure changes shift equilibrium without changing K. This fundamental difference occurs because temperature affects the rate constants underlying K through the Arrhenius equation. Understanding this distinction is essential for predicting how equilibrium systems respond to different stresses.
Q7: What role does the equilibrium constant play when temperature changes?
The equilibrium constant K is a mathematical function of the forward and reverse rate constants. Since temperature changes affect these rate constants differently through the Arrhenius equation, K itself changes. A new equilibrium is established with a different K value and different relative composition of reactants and products compared to the original equilibrium.