Le Châtelier’s Principle

Henri Louis Le Châtelier was a professor of Chemistry and Metallurgy at the École des Mines (1877), the Collège de France (1898) and finally at the Sorbonne (1907). He is best remembered for his ideas related to systems at equilibrium and their reactions to various changes.

Image from the Edgar Fahs Smith Collection
at Michigan State University

Le Châtelier’s Principle is an application of equilibrium that requires us to think about what changes the rates of reactions and then to see how that will affect a system at equilibrium.

Let’s start with the English definition of the principle. Fair warning - this English sentence is much worse than the idea actually is.

When a system is at equilibrium and a stress is applied to the system, the system will shift in the direction that relieves the stress.

Let’s pull that apart:

When a system is at equilibrium - Le Châtelier’s Principle always starts with the assumption that the system is already at equilibrium. Remember that this means that the rates of the forward and backward reactions are the same.

a stress is applied to the system - this means that something is done that will change the rate of one (or both) reactions.

the system will shift - as a result of changing the rates, they will no longer be the same. Thus, the reaction will either be running forward faster than backward, or backward faster than forward. The result of this is that the amounts of the reactants and products (which had been constant) will now be changing. If the forward reaction runs faster than the backward reaction, the amount of product will increase and we say that the reaction is shifting forward (or shifting to the right). If the backward reaction runs faster, the amount of reactant will go up and we say that the reaction is shifting backward (or shifting to the left).

in the direction that relieves the stress - the result of this shifting will eventually rebalance the equilibrium. For instance, if our stress makes the backward reaction faster, the shift will decrease the amount of products and slow that reaction back down, thus “relieving the stress.”

Te really understand this, we need to look at the various "stresses" that a system can be put under and see how each of them affects the system.

It is also worth our time to look at some "real-life" examples of Le Châtelier’s Principle.