5.4
The Response of Equilibria to the Conditions
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.
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 (like pressure, temperature, or concentration), the composition of the system adjusts in a way that counteracts the effect of this change, thereby attempting to restore the equilibrium.
According to Le Chatelier's principle, for exothermic reactions, when the system's temperature is increased, the system will try to reduce the temperature. This means that equilibrium composition typically shifts towards the reactants. Conversely, when endothermic reactions are exposed to an increased temperature, the equilibrium composition shifts towards the products to consume the excess heat. The relationship between the temperature and the equilibrium constant (K) is given by the van't Hoff equation.
By solving the van't Hoff equation, we can infer that with a temperature rise, the equilibrium constant of an endothermic reaction (where heat is a reactant) increases as the system tries to consume more heat. Alternatively, for exothermic reactions (where heat is a product), increasing temperature leads to a decrease in the equilibrium constant as the system tries to consume the excess product.
A gas phase equilibrium system reacts to pressure changes by reducing the number of gaseous molecules. It can be achieved by shifting the equilibrium towards the side of the reaction with fewer gas molecules. However, if the number of gaseous molecules in reactants and products is equal, pressure changes do not impact the equilibrium composition.
According to Le Châtelier’s principle, increasing the concentration of a reactant drives the reaction forward, while increasing the concentration of a product drives it backward. Removing a species causes the equilibrium to shift in the direction that replaces what was removed. At a fixed temperature, changing concentrations alters the reaction quotient Q, and the system shifts until Q returns to the equilibrium constant K.
A catalyst typically speeds up the reaction rate by lowering the activation energy, but does not affect the equilibrium itself. This means that the equilibrium composition and equilibrium constant are independent of the presence of a catalyst. A catalyst helps the system reach equilibrium faster.
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.
From Chapter 5:
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