14.9
Principe de le Châtelier : modification de la température
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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.