1885

Le Châtelier’s Principle

Henry-Louis Le Châtelier (1850–1936)

Chemists everywhere are familiar with French chemist Henry-Louis Le Châtelier’s principle: a chemical equilibrium changes to offset any disturbance that’s made to it. Le Châtelier made the chemistry of cement his choice of study after being told by a senior chemist that the available fields of research were so numerous that he could pick a topic at random and find something interesting. Le Châtelier’s cement studies led to the investigation of the laws of chemical equilibrium, and he first described his principle in an 1885 paper.

A good example of the principle is the Dean-Stark trap, an apparatus invented in 1920 by American chemists Ernest Dean and David Stark. In a condensation reaction, one equivalent of water is given off as the two reactants condense. Left undisturbed, a reaction like this will produce some product, but the water that’s formed can react with the product and send the reaction back in the other direction, essentially undoing the work originally done. Eventually, the system reaches equilibrium, where the rates of product formation and product reversion cancel each other out. The Dean-Stark trap disturbs this situation by taking the newly formed water out of the system, so the reaction continues until the starting materials are consumed.

The same technique can be used to drive many reactions, either by loading more starting material onto one side of the equilibrium or by draining one of the products off from the other. The second method is usually preferred, so as not to waste material. You can distill out a more volatile product, for example, or run the reaction in a solvent that allows the product to crystallize out. As long as the product can’t participate in the reaction system anymore, the equilibrium will shift to compensate. Changes in temperature, pressure, or other conditions can be applied as well.

Such a general idea has naturally diffused out to other fields. Its applications to biochemistry make it relevant to pharmacology and medicine, but it’s also been used as far afield as economics. Le Châtelier’s principle underlies every equilibrium reaction, and living creatures themselves are gigantic, complicated collections of equilibrating reactions.

The equilibrium between two types of nitrogen oxide as they are being shifted from dark nitrogen dioxide (in a hot water bath) to lighter-colored dinitrogen tetroxide in ice.