The Solubilities of Salts of Weak Acids

Submitted by jwmoore on Mon, 03/28/2011 - 10:15

In many chemical operations it is an advantage not only to be able to form a precipitate but to be able to redissolve it. Fortunately, there is a wide class of sparingly solubleAble to dissolve in a solvent to a significant extent. salts which can almost always be redissolved by adding acidIn Arrhenius theory, a substance that produces hydrogen ions (hydronium ions) in aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond.. These are precipitates in which the anionA negatively charged ion. An ion that is attracted toward the anode in an electrolytic cell. is basic; i.e., they are the salts of weak acids. An example of such a precipitate is calcium carbonate, whose solubilityThe extent to which a solute dissolves in a solvent; often expressed as the mass of a substance that will dissolve in 100 mL of solvent. equilibriumA state in which no net change is occurring, that is, in which the concentrations of reactants and products remain constant; chemical equilibrium is characterized by forward and reverse reactions occurring at the same rate. is

\text{CaCO}_{3}\text{ }({s}) \rightleftharpoons  \text{Ca}^{2+}  ({aq}) + \text{CO}_{3}^{2-} ({aq})      (1)

If acid is now added to this solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture., some of the carbonate ions become protonated and transformed into HCO3 ions. As a result, the concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of the carbonate ion is reduced. In accord with Le Chatelier’s principle, the system will respond to this reductionThat part of a chemical reaction in which a reactant gains electrons; simultaneous oxidation of a reactant must occur. by trying to produce more carbonate ions. Some solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. CaCO3 will dissolve, and the equilibrium will be shifted to the right. If enough acid is added, the carbonate-ion concentration in the solution can be reduced so as to make the ion productAn equilibrium constant expression for a reaction in which the only products are ions and the reactants are such that their concentrations do not appear in the expression; applied to dissolution and autoionization reactions. (Q = cCa2+ × cCO32–) smaller than the solubility productThe equilibrium constant expression for the dissolution of an electrolyte; the reactant is a solid and its concentration does not appear in the expression, which is a product of the concentrations of the products (raised the to appropriate powers). Ksp so that the precipitate dissolves.

A similar behavior is shown by other precipitates involving basic anions. Virtually all the carbonates, sulfides, hydroxides, and phosphates which are sparingly soluble in water can be dissolved in acid. Thus, for instance, we can dissolve precipitates like ZnS, Mg(OH)2, and Ca2(PO4)3 because all the following equilibria

\text{ZnS}\text{ }({s})\rightleftharpoons \text{Zn}^{2+} ({aq}) + \text{S}^{2-} ({aq})

\text{Mg(OH)}_{2}\text{ }({s})\rightleftharpoons \text{Mg}^{2+} ({aq}) + \text{2OH}^{-} ({aq})

\text{Ca}_{3}(\text{PO}_{4})_{2}\text{ }({s})\rightleftharpoons \text{3Ca}^{2+} ({aq}) + \text{2PO}_{4}^{3-} ({aq})

can be shifted to the right by attacking the basic species S2–, OH, and PO43– with hydronium ions. Very occasionally we find an exception to this rule. Mercury(II) sulfide, HgS, is notorious for being insolubleUnable to dissolve appreciably in a solvent.. The solubility product for the equilibrium

\text{HgS}\text{ }({s})\rightleftharpoons \text{Hg}^{2+} ({aq}) + \text{S}^{2-} ({aq})

is so minute that not even concentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). acid will reduce the sulfide ion sufficiently to make Q smaller than Ksp.

Occasionally the shift in a solubility-product equilibrium caused by a decrease in pH may be undesirable. One example of this was mentioned in the section on chalcogens. Acid rainfall can occur when oxides of sulfur and other acidic air pollutants are removed from the atmosphereA unit of pressure equal to 101.325 kPa or 760 mmHg; abbreviated atm. Also, the mixture of gases surrounding the earth.. In some parts of the United States pH values as low as 4.0 have been observed. These acid solutions dissolve marble and limestone (CaCO3) causing considerable property damage. This is especially true in Europe, where some statues and other works of art have been almost completely destroyed over the last half century.