1. Solubility & Simultaneous Equilibria Part II: Effect of pH, Complex Ion Formation & Selective Precipitation
Jespersen Chap. 18 Sec 3, 4 & 5
Dr. C. Yau
Spring 2013 1

2. Effect of pH on Solubility
Consider the following equilibrium:
CaCO3 (s) Ca2+ (aq) + CO32- (aq)
Why would the pH affect the solubility?
Could addition of H3O+ affect the solubility?
Would you expect it to affect Ca2+ or CO32-?
CO32- (aq) + H3O+ (aq) HCO3- (aq) + H2O (l)
HCO3- (aq) + H3O+ (aq) H2CO3 (aq) + H2O (l)
H2O (l) + CO2 (g)
If the cmpd contains the anion of a weak acid, addition of H3O+ (from a strong acid) increases its solubility. 2

3. Addition of acid to a mineral sample is a field test to identify presence of carbonates.
What would one look for as a positive test for carbonates?
CO32- (aq) + H3O+ (aq) HCO3- (aq) + H2O (l)
HCO3- (aq) + H3O+ (aq) H2CO3 (aq) + H2O (l)
H2O (l) + CO2 (g)
If the addition of acid to a mineral sample produces effervescence, the sample contains carbonates.

4. Effect of Adding Strong Acid
Write balanced eqns to explain whether addition of a strong acid would affect the solubility of the following compounds:
a) lead(II) bromide
b) copper(II) hydroxide
c) iron(II) sulfide
Write equations to explain the effect.

5. Example 18.8 p. 848
If a solution contains 0.10 M Cl- and 0.10 M I-, then what conc of AgNO3 will allow the separation of these halides by selective precipitation?
Ksp of AgCl = 1.8x10-10
Ksp of AgI = 8.5x10-17
Pract Exer 14 & 15 p.849

6. Quantitative Treatment of Effect of pH on Solubility
#18.52 p. 864 What is the molar solubility of Fe(OH)2 in a buffer that has a pH of 9.50?
Why might pH affect the solubility? Would you expect it to be more or less soluble in pH of 9.50?
Revised Ans. 4.8x10-8 M
Practice Exercise 9 p. 840 and p. 864 #18.53

7. Selective Precipitation
Consider the Ksp of AgCl and Ag2CrO4:
Ksp Solubility
AgCl 1.8x x10-5 M
Ag2CrO4 2.6x x10-5 M
If we add Ag+ to a soln containing
0.10 M Cl- and 0.10 M CrO42-, what will happen?
Ans. Ag2CrO4 is more soluble. AgCl will precipitate out first, leaving CrO42- in solution.

8. Ag+
CrO42-(aq)
Cl- (aq)
CrO42-(aq)
AgCl(s)
CrO42- and Cl- can be separated by adding Ag+.
Ag+ will precipitate out the Cl- as AgCl (s).
The mixture is centrifuged to spin down the white AgCl ppt.
The supernatant soln containing the CrO42- is then pipetted out.

9. Complex Ion Formation
Metal ions can combine with anions or neutral molecules to form complex ions:
Cu H2O Cu(H2O)42+
Fe CN Fe(CN)64
The H2O and CN in the above reactions are called ligands. They are acting as Lewis bases. The metal cations are acting as Lewis acids.
(Metal cations are e- pair acceptors.)

10. Aqueous Metal Ions Are Complex Ions
Water acts as a ligand, the Lewis base that forms a coordinate covalent bond with the metal.
In the solvation of ionic compounds, ions are dissolved in water through ion-dipole interactions
Complex ions are soluble, hence complexation is a means of dissolving some solids.
Figure 17.7 The complex ion of Cu2+ and ammonia. Ammonia molecules displace water molecules from Cu(H2O)42+ (left) to give the deep blue Cu(NH3)42+ ion (right).

11. Writing Complex Ion Formation Eqns
Write the equation for the complex ion formation of the aluminum ion with 4 hydroxide as ligands.
Al3+ (aq) + 4 OH- (aq)
Write the equation for the complex ion formation of the tin(IV) ion with 6 fluoride ions as ligands.
Write the equation for the complex ion formation of the mercury(II) ion with 4 chloride ions as ligands.
Practice: p. 865 #18.68, #18.72

12. Complexation & Kinst
Complexes are governed by the instability constant, Kinst
Ag(NH3) Ag+ + 2NH3
When we reverse an equation, we invert K, thus Kform=1/Kinst.
Ag+ + 2NH Ag(NH3)2+
Practice: p. 865 #74
ChemFAQ: Estimate the solubility of a metal salt in the presence of a ligand.

13. Complex Ion Formation Increases Solubility of a Salt
Consider the dissolution of AgBr(s) and what happens when NH3 is added:
AgBr (s) Ag+ (aq) + Br- (aq)
Ag(NH3)2+
+ 2 NH3
AgBr becomes more soluble.

14. Classical Scheme for Identifying "Group I Cations"
Ksp Solubility
Hg2Cl2 1.2x x10-7 M
AgCl 1.8x x10-5 M
PbCl2 1.7x x10-2 M
Given a soln possibly containing some combination of Hg22+, Ag+ and Pb2+ ions, you are to determine which one or ones are present.

15. Solubility Hg2Cl2. 6. 7x10-7 M AgCl. 1. 3x10-5 M PbCl2. 1
Solubility Hg2Cl2 6.7x10-7 M AgCl 1.3x10-5 M PbCl2 1.6x10-2 M (soluble when hot)
Write a flowchart for this analysis.
To unknown soln, add excess amt of 6MHCl.
If a ppt forms, one or more of the cations are present.
Heat the mixture to boiling and remove the supernatant. To the supernatant, add K2CrO4. If a yellow ppt forms, Pb2+ is present.
To the ppt, add 6 M NH3. If a ppt remains, Hg22+ is present.
To the supernatant, neutralize the NH3 with HCl. If a white ppt forms, Ag+ is present.