A precipitation reaction occurs when two or more soluble species combine to form an insoluble product that we call a precipitate. The most common precipitation re- action is a metathesis reaction, in which two soluble ionic compounds exchange parts. When a solution of lead nitrate is added to a solution of potassium chloride, for example, a precipitate of lead chloride forms. We usually write the balanced re- action as a net ionic equation, in which only the precipitate and those ions involved in the reaction are included. Thus, the precipitation of PbCl2 is written as
Pb2+(aq) + 2Cl–(aq) < = = = = > PbCl2(s)
In the equilibrium treatment of precipitation, however, the reverse reaction de- scribing the dissolution of the precipitate is more frequently encountered.
PbCl2(s) < = = = = > Pb2+(aq) + 2Cl–(aq)
The equilibrium constant for this reaction is called the solubility product, Ksp, and is given as
Ksp = [Pb2+][Cl–]2 = 1.7 x 10–5
Note that the precipitate, which is a solid, does not appear in the Ksp expression. It is important to remember, however, that equation 6.6 is valid only if PbCl2(s) is present and in equilibrium with the dissolved Pb2+ and Cl–. Values for selected solu- bility products can be found in Appendix 3A.