1.1 What is Chemistry?

19.3. Factors Affecting Chemical Equilibria http://www.ck12.org

of gas becomes favored. This is summarized in theTable19.8.

TABLE19.8:Stresses and Responses to Pressure Changes

Stress Response

pressure increase reaction produces fewer gas molecules

pressure decrease reaction produces more gas molecules

Like changes in concentration, the Keqvalue for a given reaction is unchanged by a change in pressure.

It is important to remember when analyzing the effect of a pressure change on equilibrium that only gases are
affected. If a certain reaction involves liquids or solids, they should be ignored. Calcium carbonate decomposes
according to the equilibrium reaction:

CaCO 3 (s)⇀↽CaO(s)+O 2 (g)

Oxygen is the only gas in the system. An increase in the pressure of the system has no effect on the rate of
decomposition of CaCO 3 , but it speeds the reverse reaction by forcing the oxygen molecules closer together, causing
a net shift to the left. When a system contains equal moles of gas on both sides of the equation, pressure has no
effect on the equilibrium position, as in the formation of HCl from H 2 and Cl 2.

H 2 (g)+Cl 2 (g)⇀↽2HCl(g)

It should also be noted that increasing the pressure by the addition of an inert gas (nonreactive gas) hasno effecton
the equilibrium. This can be thought of in terms of the pressure-dependent equilibrium constant (Kp). Unlike an
increase or decrease in the volume of the container, the addition of an inert gas does not affect the partial pressures of
any reaction components. Because the ratio of partial pressures is still equal to the equilibrium constant, no change
is necessary to maintain equilibrium.

Use of a Catalyst

Since a catalyst speeds up the rate of a reaction, you might think that it would have an effect on the equilibrium
position. However, catalysts have equal effects on the forward and reverse rates, so for a system that is already
at equilibrium, these two rates remain equal. A system will reach equilibrium more quickly in the presence of a
catalyst, but the equilibrium position itself is unaffected.

Lesson Summary

• Le Chatelier’s principle describes how a chemical system responds to stresses in order to reestablish equilib-
  rium.


• Changes in the concentrations of reactants or products will cause a reaction to shift left or right in order to
  reestablish an equilibrium position. These changes do not change the value of Keq.


• Changes in temperature will change the value of Keq. The direction of the change depends on whether the
  reaction is endothermic or exothermic.


• When the pressure of a system is changed by changing the volume of the container, reactions in which some
  components are in the gas phase may shift to reestablish equilibrium. Changing the pressure by adding an
  inert gas does not affect a system at equilibrium.


• Adding a catalyst speeds up the progression of the reaction towards equilibrium, but does not change the
  equilibrium position itself.