|
Applications of Aqueous Equilibria
Interactive Student Tutorial |
|
15.12 Factors That Affect Solubility |
||
|
The Common-Ion Effect |
||
|
The common-ion effect (Interactive Student Tutorial section of Chapter 15.2) applies to solubility equilibria as well as acid–base equilibria. For example, let's consider the solubility of MgF2.
Magnesium fluoride is less soluble in a 0.10 M solution of sodium fluoride than in pure water. The presence of the common ion (fluoride) shifts the equilibrium to the left, resulting in less dissolution of the solid. Remember, regardless of the source of the fluoride ion, the product of the ion concentrations raised to their respective powers must equal the numerical value of Ksp.
|
||
|
Common-Ion Effect |
||
|
|
||
|
The pH of the Solution |
||
|
The solubility of ionic solids that contain ions that react with water is affected by pH. There is no effect on solubility of salts containing anions of strong acids (such as Cl– or NO3– ) because these ions are not protonated by water. An ionic compound that contains a basic anion becomes more soluble as the acidity of the solution increases.
Low pH will enhance the solubility of a salt that produces hydroxide ions.
|
||
|
Dissolution of Mg(OH)2 by Acid |
||
|
|
||
|
|
||
|
Formation of Complex Ions |
||
|
The solubility of an ionic compound increases dramatically if the solution contains a Lewis base than can form a coordinate covalent bond to the metal cation. Silver chloride is insoluble in water and in acid, but dissolves in aqueous ammonia to form the complex ion Ag(NH3)2+.
The equilibrium constant for this process is Kf, called the formation constant or stability constant. For Ag(NH3)2+, the value of Kf is 1.7
|
||
|
Amphoterism |
||
|
Some oxides are amphoteric (Interactive Student Tutorial section of Chapter 14.9), meaning they dissolve in both strongly acidic and strongly basic solutions. Aluminum oxide provides a good example of an amphoteric oxide.
The corresponding hydroxides behave similarly.
The dissolution of Al(OH)3 in excess base is also an example of the formation of a complex ion. Al(OH)4– is the aluminate ion. |
||
107. The large value indicates that this complex ion is very stable.
