Definition and purpose in chemical equilibrium

Summary

In chemical equilibrium, Le Chatelier’s principle states that when a system at equilibrium is disturbed by changing a determining factor (such as temperature, pressure, or concentration), the system will respond in a way that counteracts the disturbance as far as possible, shifting to a new equilibrium position. The purpose of the principle is to provide a qualitative prediction of how equilibrium composition will change after such external changes. Thermodynamically, the principle is tied to the stability of equilibrium under the second law: after an imposed “shock,” the system redistributes its energy and/or other state variables to reach a new equilibrium. In chemistry, this translates into practical rules—for example, increasing reactant concentration shifts equilibrium toward products, increasing temperature shifts equilibrium depending on whether the reaction is exothermic or endothermic, and changing pressure or volume affects equilibria involving gases in proportion to changes in partial pressures and the number of moles of gas. Catalysts, however, change only reaction rates and do not alter the equilibrium position.

Key Points

• 1

  Le Chatelier’s principle predicts the direction of equilibrium shift after changes in temperature, pressure, or concentration.

• 2

  The principle is grounded in thermodynamic stability: disturbances are counteracted as the system moves to a new equilibrium state.

• 3

  In gas-phase equilibria, pressure/volume effects depend on partial pressures and the difference in moles of gas between reactants and products.

• 4

  Catalysts speed up both forward and reverse reactions equally, so they do not change equilibrium composition.