than the equilibrium constant. Equilibrium is reestablished only by having more reactants go on to
yield products: lowering [A] and [B] and raising [C] and [D] so that the value of the reaction quotient
would once again be equal to the equilibrium constant.
Note that this new equilibrium does not contain the same amounts of A, B, C, and D; in fact, we
know that [C] and [D] will be higher than before, and [B] will be lower (since it is depleted by reaction
with the excess A that the system was trying to get rid of). Yet the value of the equilibrium constant
is not changed: the concentrations adjust themselves to new values while maintaining the equality
stated in the law of mass action. A more concrete example may help: Suppose the concentrations of
A, B, C, and D are 3 M, 4 M, 2 M, and 2 M, respectively, at equilibrium. The injection of more A brings
its concentration up to 5 M, at which point the system is no longer at equilibrium. After we have
waited for the system to react to this stress and settle into the new equilibrium, we find that the
concentrations are now 4.6 M, 3.8 M, 2.4 M, and 2.4 M. They are all different from before, yet both
sets of values correspond to equilibrium, and both sets satisfy the law of mass action:
Graphically, a plot of the concentrations of the species as a function of time would look as follows:
The system has reached its initial equilibrium by time t 1 . At time t 2 , more A is injected, bringing its
concentration up to 5 M. This perturbs the system and it is no longer at equilibrium. The
concentrations of the species adjust themselves to new values and settle into a new equilibrium at
time t 3.
Taking advantage of this aspect of Le Châtelier’s principle is a common way in industry to increase
the yield of a useful product or drive a reaction to completion. If D were constantly removed from