5.4
The Response of Equilibria to the Conditions
Named after the French chemist Henry Louis Le Chatelier, Le Chatelier's principle states that when a system at equilibrium is subjected to any change…
According to Le Chatelier's principle, when a system at equilibrium is subjected to a change, the composition of the system adjusts to counteract the effect of that change.
In an exothermic reaction, when the system's temperature increases, the equilibrium composition shifts toward the reactants, while in an endothermic reaction, it shifts toward the products.
The van’t Hoff equation shows how temperature changes affect the equilibrium constant.
For an endothermic reaction, the standard reaction enthalpy, ΔrH°, is positive. Increasing the temperature shifts the equilibrium toward the products, raising the equilibrium constant.
For an exothermic reaction, ΔrH° is negative; increasing the temperature decreases the equilibrium constant, producing fewer products.
In gas-phase equilibria, increasing pressure favors the side with fewer gas molecules, while decreasing pressure favors the side with more gas molecules.
If both sides have an equal number of gas molecules, pressure has no effect on the equilibrium.
A catalyst increases the rates of both forward and reverse reactions equally, so it does not alter the equilibrium position or the equilibrium constant.
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Q1: What is Le Chatelier's principle and how does it explain equilibrium shifts?
Le Chatelier's principle states that when a system at equilibrium experiences a change in pressure, temperature, or concentration, the composition adjusts to counteract that change. The system shifts in a direction that attempts to restore equilibrium by reducing the effect of the disturbance. This principle predicts how reactions at equilibrium and the equilibrium constant respond to external stresses.
Q2: How does temperature affect equilibrium in endothermic versus exothermic reactions?
In endothermic reactions, increasing temperature shifts equilibrium toward products because the system absorbs heat. The equilibrium constant increases. In exothermic reactions, increasing temperature shifts equilibrium toward reactants as the system opposes the added heat. The equilibrium constant decreases. The van't Hoff equation quantifies this temperature-dependent relationship mathematically.
Q3: Why does pressure change affect gas-phase equilibria differently depending on molecular composition?
In gas-phase equilibria, increasing pressure favors the side with fewer gas molecules, while decreasing pressure favors the side with more molecules. If both sides contain equal numbers of gas molecules, pressure changes have no effect on equilibrium composition. The system shifts to minimize the total number of gas molecules and reduce pressure stress.
Q4: What happens to equilibrium when a catalyst is added to a reaction?
A catalyst increases the rates of both forward and reverse reactions equally, so it does not alter the equilibrium position or the equilibrium constant. The catalyst helps the system reach equilibrium faster by lowering activation energy, but the final composition remains unchanged. Equilibrium composition and the equilibrium constant are independent of catalyst presence.
Q5: How do concentration changes affect a system at equilibrium?
According to Le Chatelier's principle, increasing a reactant's concentration drives the reaction forward, while increasing a product's concentration drives it backward. Removing a species causes equilibrium to shift in the direction that replaces what was removed. At fixed temperature, concentration changes alter the reaction quotient Q until it returns to the equilibrium constant K.
Q6: What role does the van't Hoff equation play in understanding temperature effects on equilibrium?
The van't Hoff equation shows the quantitative relationship between temperature changes and the equilibrium constant. For endothermic reactions with positive standard enthalpy, rising temperature increases the equilibrium constant. For exothermic reactions with negative standard enthalpy, rising temperature decreases the equilibrium constant, producing fewer products at the new equilibrium.
Q7: How does the reaction quotient Q determine the direction of equilibrium shift?
The reaction quotient Q compares current concentrations to equilibrium concentrations. When Q differs from the equilibrium constant K, the system shifts to restore balance. If Q is less than K, the reaction proceeds forward; if Q exceeds K, it proceeds backward. This mechanism allows systems to respond dynamically to concentration changes until equilibrium is reestablished.