19.2. Le Châtelier’s Principle http://www.ck12.org
will cause either the forward or reverse reaction to be favored over the opposite process. When the forward reaction
is favored, the concentrations of the products increase, and the concentrations of the reactants decrease. When
the reverse reaction is favored, the concentrations of the products decrease, and the concentrations of the reactants
increase.
TABLE19.1:Reaction to Stresses
Original Equilibrium Favored Reaction Result
A⇀↽B forward: A→B [A] decreases; [B] increases
A⇀↽B reverse: A←B [A] increases; [B] decreasesConcentration
A change in the concentration of one of the substances in an equilibrium system typically involves either the addition
or the removal of one of the reactants or products. Consider the Haber-Bosch process for the industrial production
of ammonia from nitrogen and hydrogen gases:
N 2 (g)+3H 2 (g)⇀↽2NH 3 (g)If the concentration of one substance in a system is increased, the system will respond by favoring the reaction that
removes that substance. When more N 2 is added, the forward reaction will be favored because the forward reaction
uses up N 2 and converts it to NH 3. Initially, the forward reaction speeds up because one of the reactants is present at
a higher concentration, but the rate of the reverse reaction is unaffected. Since the two rates are no longer equal, the
system is no longer at equilibrium, and there will be a net shift to the right (producing more NH 3 ) until the two rates
are once again balanced. The concentration of NH 3 increases, while the concentrations of N 2 and H 2 decrease. After
some time passes, equilibrium is reestablished with new concentrations of all three substances. As illustrated in the
Figure19.3, the new concentration of NH 3 is higher than it was originally, because the forward reaction became
temporarily favored due to the stress. The new concentration of H 2 is lower. The final concentration of N 2 is higher
than it was in the original equilibrium, but lower than it was immediately after the addition of N 2 that disturbed the
original equilibrium. By responding in this way, the value of the equilibrium constant for the reaction, Keq, does not
change as a result of the stress to the system.
FIGURE 19.3
The Haber-Bosch process is an equilib-
rium between the reactants (N 2 and H 2 )
and the product (NH 3 ). When more N 2 is
added, the system favors the forward re-
action until equilibrium is reestablished.Conversely, if more NH 3 were added, the reverse reaction would be favored. This “favoring” of a reaction means
temporarily speeding up the reaction in that direction until equilibrium is reestablished. Recall that once equilibrium