Solubility and Precipitation Reactions | General Chemistry 3

The solubility and precipitation reactions are studied in this chapter: the solubility-product constants, the common ion effect, the formation of complexes, the precipitation criteria and the amphoteric hydroxides.

Solubility-Product Constants

Solubility (generally in g.L-1): quantity of a substance that is dissolved in a saturated solution
Solubility-product constant Ksp: equilibrium-constant of a dissolution reaction
The solid does not appear in the Ksp expression because it is a pure solid so equal to 1
 

AgBr (s)  Ag+ (aq) + Br- (aq)
Ksp = [Ag+][Br-]

Ag2CO3 (s)  2 Ag+ (aq) + CO32- (aq)
Ksp = [Ag+]2[CO32-]

 

 The solubility s of an ionic solid can be determined using Ksp
 

AgBr (s) Ag+ (aq) + Br- (aq)
According to the reaction stoichiometry: [Ag+] = [Br-]

Solubility s of AgBr (s) in mol.L-1 = [Ag+] = [Br-]
⇒ Ksp = [Ag+][Br-] = s2
⇒ s = Ksp

Solubility S of AgBr (s) in g.L-1 = s x MAgBr = Ksp x MAgBr

Common Ion Effect

Common ion: ion already contained in the solution
Common ion effect: the solubility of an ionic solid decreases when a common ion is present in solution

Formation of Complexes

Complex ion: metal ion with small molecules or ions attached to it
Complexation reaction: reaction between a metal ion and a molecular or ionic entity called ligand that forms a complex
Formation constant Kf: equilibrium-constant of the formation of a complex ion
 

Formation of [Ag(NH3)2]+, a complex ion:

Ag+ (aq) + 2 NH3 (aq)  [Ag(NH3)2]+ (aq)

Kf = Ag(NH3)2+Ag+ NH32

 

 The solubility of an ionic solid is increased by the formation of a soluble complex ion

Precipitation Criteria

Solubility-product concentration quotient Qsp: same form as Ksp but is expressed in terms of arbitrary concentrations

Qsp and Ksp can be used to predict whether an ionic solid can precipitate:
Qsp > Ksp: precipitate forms
Qsp < Ksp: no precipitate forms
At the equilibrium: Qsp = Ksp

When equilibrium is disturbed:
Qsp > Ksp: more precipitate forms until Qsp = Ksp
Qsp < Ksp: precipitate dissolves (until Qsp = Ksp or until the complete dissolution of precipitate)

Amphoteric Hydroxides

Amphoteric metal hydroxides: metal hydroxides insoluble in neutral aqueous solutions but soluble in both acidic and basic solutions

In neutral aqueous solutions: no reactions occur
In acidic solutions: reaction similar to an acid-base neutralization reaction
In basic solution: formation of a soluble hydroxy complex ion
 

Al(OH)3 is an amphoteric metal hydroxides:

Neutral solution: insoluble
Acidic solution: Al(OH)3 (s) + 3 H3O+ (aq)  Al3+ (aq) + 6 H2O (l)     [acid-base neutralization]
Basic solution: Al(OH)3 (s) + HO- (aq)  [Al(OH)4]- (aq)     [formation of a soluble hydroxy complex ion]