19.3. Factors Affecting Chemical Equilibria http://www.ck12.org
FIGURE 19.5
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.Temperature
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, or 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, or the
exothermic direction. For the Haber-Bosch process, a decrease in temperature favors the forward reaction. Therefore
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 towards the reactants means that the Keqvalue
decreases. When the temperature is decreased, the shift in equilibrium towards 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
changing the temperature, the equilibrium between colorless N 2 O 4 and brown NO 2 can be manipulated, resulting in
a visible color change.