gas. Therefore, in the above reaction, equilibrium would shift to the right. If the
reaction system was stressed by an increase in its volume (which would decrease
the pressure of the system), then equilibrium would shift to the left. The
equilibrium of a reaction that involved equal moles of gaseous reactants and
products would not be affected by either a change in volume or a change in
pressure.
The Effects of Catalysts on Equilibrium
So how does the presence of a catalyst affect equilibrium? The answer is simple:
It doesn’t. Catalysts change reaction rates, but they do not affect equilibrium. A
catalyst can aid the reaction in achieving equilibrium more quickly, but it won’t
affect the concentration of product at equilibrium. This emphasizes what we said
at the start of this chapter: Kinetics and equilibrium address quite different
aspects of a chemical reaction.
Equilibrium in Precipitation Reactions
As you learned in the last chapter, some combinations of cation and anion form
ionic solids with very low water solubilities or precipitates. However, even the
most insoluble ionic precipitate dissolves into ions in water to a certain degree. If
pressure and temperature are maintained for a precipitate in water, equilibrium
exists between the precipitate and its dissolved ions. For instance, consider the
equilibrium between the precipitate lead chloride (PbCl 2 ) and its dissolved ions.
PbCl 2 (s) Pb2+(aq) + 2Cl−(aq)The equilibrium expression for the above equilibrium is
Ksys = [Pb2+][Cl−]^2Remember that solids are not included in equilibrium expressions! When we
consider equilibrium between a so-called insoluble ionic solid and its dissolved
ions, we call the equilibrium constant a solubility product constant and
symbolize it as Ksp. As you might expect, since the forward reaction is so
insignificant for these precipitates, Ksp values are typically very small. For PbCl 2
at 25°C, Ksp = 1.6 × 10−5 (or 0.000016). The smaller Ksp is for a given ionic