Chem. 31 – 3/4 Lecture
Announcements I
• Exam 1
– Still Grading
– Key Posted
• Next Lab Report Due: Cl lab report
– Due next Wednesday
– Must turn in in Excel format
• Homework Set 2
– See website (sorry, but I did not make print outs)
– Still need to add additional problem 2.2
– Additional Problem 2.1 due next Wed. (+ quiz)
Announcements II
• Today’s Lecture
– Solubility Product Problems (precipitation and
selective precipitations)
– Complex Ions
– Acid Base Chemistry
Solubility Product Problems
Precipitation Problems
What occurs if we mix 50 mL of 0.020 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?
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+.
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
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)
O
O
Na+
O
Crown ether
added
• Complexometric titrations
(e.g. water hardness titration)
• Some Complexes are Colored
O
Diethyl ether
(use as indicators or for spectroscopic
measurements)
Sodium conc. given by
gray shading
water
Complex Ions
Step-wise vs. full reactions:
Example: addition of NH3 to Ag+
Reaction occurs in steps:
1) Ag+ + NH3(aq) ↔ AgNH3+
K1 (= β1)
2) AgNH3+ + NH3(aq) ↔ Ag(NH3)2+ K2
Net) Ag+ + 2NH3(aq) ↔ Ag(NH3)2+ β2 = K1·K2
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
0.004
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)
Complex Ions – “U” Shaped
Solubility Curves
Calcium Oxalate
1.00E+00
Solubility in
water
Complex ion
Common effect
ion effect
1.00E-01
1.00E-02
Conc. (M)
1.00E-03
1.00E-04
CaC2O4(aq)
Ca2+
1.00E-05
Ca(C2O4)22-
1.00E-06
Total Ca
1.00E-07
1.00E-08
1.00E-09
1.00E-10
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
[C2 O42-] (M)
Note: looks “U” shaped if not on log scale (otherwise “V” shaped)
Some Questions
1.
2.
3.
4.
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.
One More Question
1.
Cu2+ reacts with thiosulfate (S2O32-) to form a complex
which is most stable when two moles of thiosulfate to
one mole of Cu2+ are present. The b2 value is found to
be 2.00 x 106. If a solution containing both Cu2+ and
S2O32- is prepared and found to contain 1.7 x 10-3 M
free (uncomplexed) S2O32- at equilibrium, what is the
ratio of complexed to free Cu? Assume that little
CuS2O3 forms.
Acids, Bases and Salts
Definitions of Acids and Bases
- Lewis Acids/Bases (defined before, most
general category)
- Brønsted-Lowry Acids/Bases:
acid = proton donor
base = proton acceptor (must have free
electron pair so also is a Lewis base)
- definitions are relative
Brønsted-Lowry Acids - examples
HCO2H(aq) + H2O(l) ↔ HCO2- +
H3O+
acid
base
conjugate
conjugate
base
acid
CH3NH2(aq) + H2O(l) ↔ CH3NH3+ +
OHbase
acid
conjugate
conjugate
acid
base
H2SO4 + CH3CO2H(l) ↔ HSO4- + CH3CO2H2+
acid
base
conjugate
conjugate
base
acid