At constant temperature, a decrease in volume (increase in pressure) increases the concen-
trations of both A and D. In the expression for Q,the concentration of D is squared and
the concentration of A is raised to the first power. As a result, the numerator of Qincreases
more than the denominator as pressure increases. Thus, Q Kc, and this equilibrium
shifts to the left. Conversely, an increase in volume (decrease in pressure) shifts this reac-
tion to the right until equilibrium is reestablished, because Q Kc. We can summarize
the effect of pressure (volume) changes on thisgas-phase system at equilibrium.Stress Q* Direction of Shift of A(g) 34 2D(g)Volume decrease, Q Kc Toward smaller number of moles of gas (left for
pressure increase thisreaction)
Volume increase, Q Kc Toward larger number of moles of gas (right for
pressure decrease thisreaction)*In Q for thisreaction, there are more moles of gaseous product than gaseous reactant.In general, for reactions that involve gaseous reactants or products, LeChatelier’s Prin-
ciple allows us to predict the following results.1.If there is no change in the total number of moles of gases in a reaction,a volume
(pressure) change does not affect the position of equilibrium; Qis unchanged by
the volume (or pressure) change.
2.If a reaction involves a change in the total number of moles of gases,changing the
volume (or pressure) of a reaction mixture changes the value of Q; itdoes not
changethe value of Kc. For such a reaction:
(a) A decrease in volume (increase in pressure) shifts a reaction in the direction
that produces the smaller total number of moles of gas,until Qagain equals Kc.
(b) An increase in volume (decrease in pressure) shifts a reaction in the direction
that produces the larger total number of moles of gas,until Qagain equals Kc.The foregoing argument applies only when pressure changes are due to volume
changes. It does not applyif the total pressure of a gaseous system is raised by merely
pumping in an inert gas, for example, He. If the gas that is introduced is not involved in
the reaction, the partial pressureof each reacting gas remains constant, so the system
remains at equilibrium.EXAMPLE 17-9 Applying a Stress to a System at Equilibrium
(a) Given the following reaction at equilibrium in a closed container at 500°C, predict the effect
of increasing the pressure by decreasing the volume.N 2 (g)3H 2 (g) 34 2NH 3 (g)(b) Make the same prediction for the following reaction at high temperature.H 2 (g)I 2 (g) 34 2HI(g)Plan
We apply LeChatelier’s Principle to predict the effect on each reaction.Study this tabulation carefully. How
would these conclusions change for
a reaction in which there are more
moles of gaseous reactants than moles
of gaseous products?
One practical application of these ideas
is illustrated in the next section by the
Haber process.
724 CHAPTER 17: Chemical Equilibrium