DISSOLVING PRECIPITATES
A precipitate dissolves when the concentrations of its ions are reduced so that Kspis no
longer exceeded, that is, when QspKsp. The precipitate then dissolves until QspKsp.
The precipitates can be dissolved by several types of reactions. All involve removing ions
from solution.Converting an Ion to a Weak Electrolyte
Three specific illustrations follow.1.Converting OHto H 2 O.Insoluble Al(OH) 3 dissolves in acids. Hions react with
OHions [from the saturated Al(OH) 3 solution] to form the weak electrolyte H 2 O.
This makes [Al^3 ][OH]^3 Ksp, so that the dissolution equilibrium shifts to the
right and Al(OH) 3 dissolves.Al(OH) 3 (s) 34 Al^3 (aq)3OH(aq)
3H(aq)3OH(aq)88n3H 2 O()
overall rxn: Al(OH) 3 (s)3H(aq)88nAl^3 (aq)3H 2 O()2.Converting NH 4 to NH 3 .Ammonium ions, from a salt such as NH 4 Cl, dissolve
insoluble Mg(OH) 2. The NH 4 ions combine with OHions in the saturated
Mg(OH) 2 solution. This forms the weak electrolytes NH 3 and H 2 O. The result is
[Mg^2 ][OH]^2 Ksp, and so the Mg(OH) 2 dissolves.Mg(OH) 2 (s) 34 Mg^2 (aq)2OH(aq)
2NH 4 (aq)2OH(aq) 88n2NH 3 (aq)2H 2 O()
overall rxn: Mg(OH) 2 (s)2NH 4 (aq) 88nMg^2 (aq)2NH 3 (aq)2H 2 O()This process, dissolution of Mg(OH) 2 in an NH 4 Cl solution, is the reverse of the
reaction we considered in Example 20-10. There, Mg(OH) 2 precipitated from a
solution of aqueous NH 3.
3.Converting S^2 to H 2 S.Nonoxidizing acids dissolve most insoluble metal sulfides.
For example, 6 MHCl dissolves MnS. The Hions combine with S^2 ions to form
H 2 S, a gas that bubbles out of the solution. The result is [Mn^2 ][S^2 ]Ksp, and
so the MnS dissolves.MnS(s) 34 Mn^2 (aq)S^2 (aq)
S^2 (aq)H 2 O()88nHS(aq)OH(aq)
2H(aq)HS(aq)OH(aq)88nH 2 S(g)H 2 O()
overall rxn: MnS(s)2H(aq)88nMn^2 (aq)H 2 S(g)Converting an Ion to Another Species by a Redox Reaction
Most insoluble metal sulfides dissolve in hot dilute HNO 3 because NO 3 ions oxidize
S^2 ions, or actually their hydrolysis product (HS), to elemental sulfur. This removes
HS(and thus S^2 ) ions from the solution and promotes the dissolving of more of the
metal sulfide.3HS(aq)2NO 3 (aq)5H(aq)88n3S(s)2NO(g)4H 2 O()20-6
Solubility products, like other
equilibrium constants, are
thermodynamic quantities. They tell
us nothing about how fast a given
reaction occurs, only whether it can
occur under specified conditions.
840 CHAPTER 20: Ionic Equilibria III: The Solubility Product Principle
S^2 like O^2 does not exist in
appreciable amounts in aqueous
solutions.