Equilibrium Constants: The Principle of Le Chatelier

James Richard Fromm

The state of dynamic equilibrium, in which a chemical system is reacting to form and to destroy products, is a stable state. If a system is not in equilibrium, the driving force toward equilibrium is greater than the driving force away from equilibrium and the system has a net driving force toward equilibrium. Alternatively, if a system is not in equilibrium the rate of the reaction toward equilibrium is greater than the rate of the opposite reaction so there is a net movement always toward the equilibrium state. In other words, a system at equilibrium will tend to stay at equilibrium and a system not at equilibrium will tend to more toward equilibrium.

This principle was first clearly enunciated byHenri Le Chatelier (1850 - 1936) and bears his name. Its elegant formulation is:

If, to a system at equilibrium, a stress be applied, the system will react so as to relieve the stress.

A less elegant formulation used, perhaps, by a military drill sergeant would be "If you kick its tail hard enough, it moves where you boot it". This more-or-less common sense principle can be used to show what the effects of several variables are on the position of a chemical equilibrium.

As we will see in a later section, the qualitative principle of Le Chatelier is explained quantitatively by the equilibrium constant of a reaction.

Copyright 1997 James R. Fromm