Equilibrium Concepts

Changing Equilibria and Le Chatelier's Principle

When a system in equilibrium undergoes stress, such as changes in concentration, pressure, and volume, the position of equilibrium moves in an opposing direction to regain equilibrium. This is called Le Chatelier's principle. Changing temperature will establish a new equilibrium with a different Kc or Kp value.

In a closed system, equilibrium constants are calculated using concentrations or partial pressures. Both Kc and Kp are calculated at constant temperature. Any change in temperature will change the equilibrium constant.

The effects of these changes can be understood by applying Le Chatelier's principle. Le Chatelier's principle states that a change in the temperature, pressure, or concentration of a component will cause the equilibrium condition of a chemical system to change in a way that reduces the change. The principle helps to predict the direction in which a reversible reaction moves when key factors are altered. The shift will be in the direction that creates the least amount of change. Some examples of stresses that can affect the equilibrium are changing the volume, which affects pressure of the system, and adding more of a product or a reactant, which affects the concentrations of products and reactants. Changing temperature will cause the system to arrive at a new equilibrium, with a new equilibrium constant. Changes in pressure ordinarily do not affect the concentration of reacting species in condensed phase. This is due to both liquids and solids being incompressible. However, gases are greatly affected by changes in pressure. A change in concentration, pressure, or volume may alter the equilibrium position, but this change does not alter the value of the equilibrium constant. The equilibrium constant is altered only by a change in temperature.