Ionic Equilibria
In chapter 9 , we discussed the concepts of solubility and solvation. The notion of the equilibrium
constant gives us a way of describing the process of dissolution more quantitatively. The process of
solvation, like other reversible chemical and physical changes, tends toward an equilibrium.
Immediately after solute has been introduced into a solvent, most of the change taking place is
dissociation, because no dissolved solute is initially present. However, as solute dissociates, the
reverse reaction (precipitation of the solute) also begins to occur. Eventually an equilibrium is
reached, with the rate of solute dissociation equal to the rate of precipitation.
THE SOLUBILITY PRODUCT CONSTANT
An ionic solid introduced into a polar solvent (for example, water) dissociates into its component
ions. The dissociation of such a solute in these solvents may be represented by:
AmBn (s) → mAn+ (aq) + nBm– (aq)The equilibrium constant for this reaction is [An+]m[Bm–]n, where the concentrations are those of the
ions in the saturated solution. (Recall that the solid does not appear in the mass action expression.)
To specify that this constant is for a dissolution process, it is also referred to as the solubility product
constant and given the symbol Ksp. Each salt has its own distinct Ksp at a given temperature.
Just as in the case of the more general law of mass action, it is important to realize that one can
always write an expression of the form [An+]m[Bm−]n; it is only when the solution is saturated that the
concentrations are related in such a way that [An+]m[Bm−]n = Ksp. An unsaturated solution will have
[An+]m[Bm−]n < Ksp, while a supersaturated solution will have [An+]m[Bm−]n > Ksp. If the supersaturated
solution is disturbed by adding more salt or other solid particles, or jarring the solution by a sudden
decrease in temperature, the solid salt will precipitate until the equality holds.