Summarizing Precipitation and Dissolution

Key Concepts and Summary

The equilibrium constant for an equilibrium involving the precipitation or dissolution of a slightly soluble ionic solid is called the solubility product, K_sp, of the solid. When we have a heterogeneous equilibrium involving the slightly soluble solid M_pX_q and its ions M^m+ and X^n–:

\({\text{M}}_{p}{\text{X}}_{q}(s)⇌p{\text{M}}^{\text{m+}}(aq)+q{\text{X}}^{\text{n−}}(aq)\)

We write the solubility product expression as:

\({K}_{\text{sp}}={{\text{[M}}^{\text{m+}}]}^{p}{{\text{[X}}^{\text{n−}}]}^{q}\)

The solubility product of a slightly soluble electrolyte can be calculated from its solubility; conversely, its solubility can be calculated from its K_sp, provided the only significant reaction that occurs when the solid dissolves is the formation of its ions.

A slightly soluble electrolyte begins to precipitate when the magnitude of the reaction quotient for the dissolution reaction exceeds the magnitude of the solubility product. Precipitation continues until the reaction quotient equals the solubility product.

A reagent can be added to a solution of ions to allow one ion to selectively precipitate out of solution. The common ion effect can also play a role in precipitation reactions. In the presence of an ion in common with one of the ions in the solution, Le Châtelier’s principle applies and more precipitate comes out of solution so that the molar solubility is reduced.

Key Equations

• \({\text{M}}_{p}{\text{X}}_{q}(s)⇌p{\text{M}}^{\text{m+}}(aq)+q{\text{X}}^{\text{n−}}(aq)\phantom{\rule{4em}{0ex}}{K}_{\text{sp}}={[\text{M}}^{\text{m+}}{]}^{p}{{[\text{X}}^{\text{n−}}]}^{q}\)

Glossary

common ion effect

effect on equilibrium when a substance with an ion in common with the dissolved species is added to the solution; causes a decrease in the solubility of an ionic species, or a decrease in the ionization of a weak acid or base

molar solubility

solubility of a compound expressed in units of moles per liter (mol/L)

selective precipitation

process in which ions are separated using differences in their solubility with a given precipitating reagent

solubility product (K_sp)

equilibrium constant for the dissolution of a slightly soluble electrolyte