19.3. Factors Affecting Chemical Equilibria http://www.ck12.org
changes in the concentrations of reactants or products, changes in the temperature of the system, or changes in the
pressure of the system. We will discuss each of these stresses separately. In each case, the change to the equilibrium
position 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.
TABLE19.6: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
consumes 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 can be seen
inFigure19.5, 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.
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
is reestablished, the rates of the forward and reverse reactions are again equal. The addition of NH 3 would result in
a net increase in the formation of the reactants, N 2 and H 2.
An equilibrium can also be disrupted by the full or partial removal of a reactant or product. If the concentration
of a substance is decreased, the system will respond by favoring the reaction that replaces that substance. In the
industrial Haber-Bosch process, NH 3 is removed from the equilibrium system as the reaction proceeds. As a result,
the forward reaction is favored so that more NH 3 will be produced. The concentrations of N 2 and H 2 decrease.
Continued removal of NH 3 will eventually force the reaction to go to completion until all of the reactants are used
up. If either N 2 or H 2 were removed from the equilibrium system, the reverse reaction would be favored, and the
concentration of NH 3 would decrease.
The effects of changes in concentration on a system at equilibrium are summarized inTable19.7.
TABLE19.7:Stresses and Responses
Stress Response
addition of reactant forward reaction favored