http://www.ck12.org Chapter 19. 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 removal of one of the substances. 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 below (Table19.2).
TABLE19.2:Stresses and Responses
Stress Response
addition of reactant forward reaction favored
addition of product reverse reaction favored
removal of reactant reverse reaction favored
removal of product forward reaction favoredTemperature
Increasing or decreasing the temperature of a system at equilibrium is also a stress to the system. The equation for
the Haber-Bosch process is written again below, this time as a thermochemical equation:
N 2 (g)+3H 2 (g)⇀↽2NH 3 (g)+91 kJThe forward reaction is the exothermic direction: the formation of NH 3 releases heat. The reverse reaction is the
endothermic direction: as NH 3 decomposes to N 2 and H 2 , heat is absorbed. An increase in the temperature of a
system favors the direction of the reaction that absorbs heat, the endothermic direction. Absorption of heat in this
case is a relief of the stress provided by the temperature increase. For the Haber-Bosch process, an increase in
temperature favors the reverse reaction. The concentration of NH 3 in the system decreases, while the concentrations
of N 2 and H 2 increase.
Conversely, a decrease in the temperature of a system favors the direction of the reaction that releases heat: the
exothermic direction. For the Haber-Bosch process, a decrease in temperature favors the forward reaction. The
concentration of NH 3 in the system increases, while the concentrations of N 2 and H 2 decrease.
For changes in concentration, the system responds in such a way that the value of the equilibrium constant, Keq, is
unchanged. However, a change in temperature shifts the equilibrium and changes the value of Keq. As discussed in
the previous section, values of Keqare dependent on the temperature. When the temperature of the system for the
Haber-Bosch process is increased, the resultant shift in equilibrium toward the reactants means that the Keqvalue
decreases. When the temperature is decreased, the shift in equilibrium toward the products means that the Keqvalue
increases.
Le Châtelier’s principle, as related to temperature changes, can be illustrated easily by the equilibrium between
dinitrogen tetroxide and nitrogen dioxide.
N 2 O 4 (g)+heat⇀↽2NO 2 (g)Dinitrogen tetroxide (N 2 O 4 ) is colorless, while nitrogen dioxide (NO 2 ) is dark brown in color. When N 2 O 4 breaks
down into NO 2 , heat is absorbed according to the forward reaction above. Therefore, an increase in the temperature
of the system will favor the forward reaction, while a decrease in temperature will favor the reverse reaction. By