Le Châtelier’s Principle

The French chemist Henry Louis Le Châtelier stated that a system at equilibrium to which a stress is
applied tends to change so as to relieve the applied stress. This rule, known as Le Châtelier’s
principle, is used to determine the direction in which a reaction at equilibrium will proceed when
subjected to a stress, such as a change in concentration, pressure, temperature, or volume.

CHANGES IN CONCENTRATION

Increasing the concentration of a species once the system has reached equilibrium will tend to shift
the equilibrium away from the species that is added, in order to reestablish its equilibrium
concentration, and vice versa. For example, in the reaction

A + B C + D,

BASIC CONCEPT

if the concentration of A and/or B is increased, the equilibrium will shift toward (or favor production
of) C and D. Conversely, if the concentration of C and/or D is increased, the equilibrium will shift
away from the production of C and D, favoring production of A and B. Similarly, decreasing the
concentration of a species will tend to shift the equilibrium toward the production of that species.
For example, if A and/or B is removed from the above reaction, the equilibrium will shift so as to
favor increasing concentration of A and B.

All of this could be read from the law of mass action. If, for example, the concentration of A is
increased by the injection of more A into the system, the reaction quotient would have a lower value

Increase    reactant    concentration:  favors  products

Increase    product concentration:  favors  reactants