1. Chem. 31 – 10/11 Lecture

2. Announcements Statistical Calculations Lab AA Lab Today’s Lecture
Due today
AA Lab
Tap water data posted online
Today’s Lecture
Chapter 6
Parts missed last time
Sparingly Soluble Salts
Solubility in water
Solubility in common ion
Precipitation + Selective Precipitation
Complex Ions

3. Example question
The reaction N2(g) + O2 (g) ↔ 2NO(g) has a positive DH. Under what conditions is this process spontaneous?
- all temperatures
- low temperatures
- high temperatures
- never

4. Thermodynamics ΔG and Equilibrium
ΔG = ΔG° + RTlnQ Q = Reaction Quotient
(for A ↔ B, Q = [B]/[A])
At equilibrium, ΔG = 0 and Q = K
ΔG° = -RTlnK

5. Solubility Product Problems - Solubility in Water
Example: solubility of Mg(OH)2 in water
Solubility defined as mol Mg(OH)2 dissolved/L sol’n or g Mg(OH)2 dissolved/L sol’n or other units
Use ICE approach:
Mg(OH)2(s) ↔ Mg2+ + 2OH-
Initial
Change x +2x
Equilibrium x 2x
Note: x = [Mg2+] = solubility

6. Solubility Product Problems - Solubility of Mg(OH)2 in water
Equilibrium Equation: Ksp = [Mg2+][OH-]2
Ksp = 7.1 x = x(2x)2 = 4x3 (see Appendix F for Ksp)
x = (7.1 x 10-12/4)1/3 = 1.2 x 10-4 M
Solubility = 1.2 x 10-4 M = [Mg2+]
Conc. [OH-] = 2x = 2.4 x 10-4 M

7. Solubility Product Problems - Solubility of Mg(OH)2 in Common Ion
If we dissolve Mg(OH)2 in a common ion (OH- or Mg2+), from Le Châtelier’s principle, we know the solubility will be reduced
Example 1) What is the solubility of Mg(OH)2 in a pH = 11.0 buffer?
No ICE table needed because, from pH, we know [OH-]eq and buffer means dissolution of Mg(OH)2 doesn’t affect pH.

8. Solubility Product Problems - Solubility of Mg(OH)2 at pH 11 – cont.
[H+] = 10-pH = M
and [OH-] = Kw/[H+] = 10-3 M
Ksp = [Mg2+][OH-]2
Moles Mg(OH)2 dissolved = moles Mg2+
[Mg2+] = Ksp/[OH-]2 = 7.1 x 10-12/(10-3)2
[Mg2+] = 7 x 10-6 M

9. Solubility Product Problems - Solubility of Mg(OH)2 in Common Ion
Example 2) Solubility of Mg(OH)2 in 5.0 x 10-3 M MgCl2.

10. Precipitations Used for Separations
Example: If we wanted to know the concentrations of Ca2+ and Mg2+ in a water sample. EDTA titration gives [Ca2+] + [Mg2+]. However, if we could selectively remove Ca2+ or Mg2+ (e.g. through titration) and re-titrate, we could determine the concentrations of each ion.
Determine if it is possible to remove 99% of Mg2+ through precipitation as Mg(OH)2 without precipitating out any Ca(OH)2 if a tap water solution initially has 1.0 x 10-3 M Mg2+ and 1.0 x 10-3 M Ca2+. 10

11. Solubility Product Problems Precipitation Problems
What occurs if we mix 50 mL of M BaCl2 with 50 mL of 3.0 x 10-4 M (NH4)2SO4?
Does any solid form from the mixing of ions?
What are the concentrations of ions remaining? 11

12. Complex Ions Example Reaction: Ag+ + 2NH3(aq) ↔ Ag(NH3)2+
Metal Ligand Complex Ion
Why does reaction occur?
Metal is a Lewis acid (electron pair acceptor)
NH3 is a Lewis base (electron pair donator)
Metal-ligand bonds are intermediate strength

13. Complex Ions – Why Study?
Crown ether (12-crown-4)
Useful in separations
Complexed metals become more organic soluble
Effects on metal solubility
(e.g. addition of NH3 on AgCl solubility)
Complexometric titrations
(e.g. water hardness titration)
Some Complexes are Colored
(use as indicators or for spectroscopic measurements)
Na+
Crown ether added
Diethyl ether
Sodium conc. given by gray shading
water

14. Complex Ions Step-wise vs. full reactions:
Example: addition of NH3 to Ag+
Reaction occurs in steps:
1) Ag+ + NH3(aq) ↔ AgNH K1 (= β1)
2) AgNH3+ + NH3(aq) ↔ Ag(NH3) K2
Net) Ag NH3(aq) ↔ Ag(NH3)2+ β2 = K1·K2

15. Complex Ions
Due to large exponents on ligand concentration, a small change in ligand concentration has a big effect on how metal exists
Example:
Al3+ + 3C2O42- ↔ Al(C2O4)33- β3 = 4.0 x 1015
[C2O42-] [Al(C2O4)33-]/[Al3+]
10-4 M 4000
10-5 M 4
10-6 M

16. Complex Ions – “U” Shaped Solubility Curves
Many sparingly soluble salts release cations and anions that form complexes with each other
Example: calcium oxalate (CaC2O4)
CaC2O4(s) ↔ Ca2+ + C2O42- (Ksp = 1.3 x 10-8M)
increased [C2O42-] decreases Ca2+ solubility
for above reaction only, but ...
Ca2+ + C2O42- ↔ CaC2O4(aq) K1 = 46
CaC2O4(aq) + C2O42- ↔ Ca(C2O4)22- K2 = 490
β2 = K1·K2 = 2.3 x 104 = [Ca(C2O4)22-]/([Ca2+][C2O42-]2)

17. Complex Ions – “U” Shaped Solubility Curves
Solubility in water
Complex ion effect
Common ion effect
Note: looks “U” shaped if not on log scale (otherwise “V” shaped)

18. Some Questions
In the reaction: Ca2+ + Y4- ↔ CaY2- (where Y4- = EDTA), which species is the Lewis acid?
List two applications in which the formation of a complex ion would be useful for analytical chemists.
List two applications in the lab in which you used or are using complex ions.
AgCN is a sparingly soluble salt. However, a student observed that adding a little of a NaCN solution to a saturated solution of AgCN did not result in more precipitation of solid. Addition of more NaCN solution resulted in total dissolution of the AgCN. Explain what is happening.