Lecture:
1) Last Solubility Products (Ksp) describe equilibrium
between a solid and its constituent ions.
2) Complexes form between some of the
released metals and strong ligands also in
solution.
3) These complexes cause more solid to
dissolve by sequestering the metal in a
new molecular form.
Complex Formation
Consider AgCl(s) and two equilibria:
#1 AgCl(s) = Ag+(aq) + Cl(aq)
#2 Ag+(aq) + 2NH3(aq) = [Ag(NH3)2]+
Kf =
+
[ Ag ( NH 3 )2 ]
[ Ag + ][ NH 3 ]2
= 1.6i10
7
Salt solubility increases due to complex
formation, but both equilibria are satisfied.
Equilibria Involving Complex Ions
AgCl(s)
Ksp = 1.810-10
Ag+(aq) + Cl(aq)
Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+
Kf = 1.6107
Now, a peptalk about complexation
to put your studies into context:
The surefire way to gain a Nobel Prize-1) Malaria kills 0.2 % of the population
in sub-Saharan Africa each year.
2) Work on a vaccine, cure or treatment.
Even a small advance saves many lives.
Parasite infecting red blood cells.
Heme is how you transport O2 in your body.
It is iron ligated to a porphyrin ligand.
Thallasemia is a disorder of sparse working heme. It
is an evolved strategy to survive malaria by hiding
the iron from the parasite.
Fe(OH)3 (goethite)
is fantastically insoluble
without a ligand to
bind the iron.
You all just did this
calculation in the last
lecture.
John Nielands at UCB wondered
how the organisms get iron. He found
that they make these special Fe-binding
ligands, called hydroxamates.
That's why you need to understand
complexation of metals in solution.
Complex Formation increases salt solubility!
Consider AgCl(s):
AgCl(s)
Ag+(aq) + 2NH3(aq)
Ag+(aq) + Cl(aq)
Ksp = [Ag+][Cl]
[Ag(NH3)2]+
Le Chatelier: Solubility equilibrium shifted to the right
because the complex sequesters the Ag+(aq) into a new form:
[[Ag(NH3)2]+]
Kf = [Ag+][NH ]2 = 1.6107
3
Complex Formation increases salt solubility!
I now must recast the equation to reflect this new complex
by adding the equilibria:
AgCl(s) Ag+(aq) + Cl(aq)
Ag+(aq) + 2NH3(aq) [Ag(NH3)2]+
Ksp = 1.810-10
Kf = 1.6107
AgCl(s) + 2 NH3(aq) [Ag(NH3)2]+(aq) + Cl-(aq)
K = Ksp Kf = 2.910-3
The total dissolved silver concentration increases considerably.
Example Solubility when Complex forms
A) What is the solubility of silver chloride? Ksp=1.810-10
B) What is the solubility in 0.10M ammonia solution?
Kf ([Ag(NH3)2]+) = 1.6107
Solution Solubility when Complex forms
A) What is the solubility of silver chloride?
AgCl(s) Ag+(aq) + Cl-(aq)
s2 = 1.810-10
s = 1.310-5 M
B) What is the solubility in 0.10M ammonia solution?
The net reaction and K are:
AgCl(s) + 2NH3(aq) [Ag(NH3)2]+(aq) + Cl-(aq); K=2.910-3
Init
Chg
Eqm
K=
0.10
-2s
0.10-2s
[[ Ag ( NH 3 )2 ]+ ][Cl ]
[ NH 3 ]2
+s
s
=
+s
s
s2
(0.01 2 s ) 2
= 2.9i103
s=0.0048M
(370 times greater)
Copper(II) Complexes
In copper sulfate solution
Cu2+(aq) + 4 H2O(aq) [Cu(H2O)4]2+(aq)
[Cu(H2O)4]2+(aq)
Add ammonia
[Cu(NH3)4]2+(aq)
Cu2+(aq) + 4 NH3(aq)
[Cu(NH3)4]2+ (aq)
[Cu(H2O)4]2+(aq)
Cu2+(aq) + 4 NH3(aq)
[Cu(H2O)4]2+(aq) + 4 NH3(aq)
Cu2+(aq) + 4 H2O(aq)
[Cu(NH3)4]2+ (aq)
[Cu(NH3)4]2+(aq) + 4 H2O(l)
Start with CuSO4 solution:
Cu(OH)2(s)
Cu2+(aq) + 2OH-(aq)
Ksp=4.810-20
What does LeChetelier say about adding NaOH?
How about adding NH3?
Cu2+(aq) + 4 NH3(aq)
Cu(OH)2(s) + 4 NH3(aq)
[Cu(NH3)4]2+ (aq)
Kf=1.11013
[Cu(NH3)4]2+ (aq) + 2OH-(aq)
K=510-7
Add strong acid (HNO3)?
NH3 + H3O+
[Cu(H2O)4]2+(aq) + 4 NH3(aq)
NH4+ + H2O
[Cu(NH3)4]2+(aq) + 4 H2O(l)
Example- predicting chemistry with complex ions
A 0.10 mol sample of AgNO3 is dissolved in 1.00 L of 1.00 M NH3.
If 0.010 mol NaCl is added to this solution, will AgCl(s) precipitate?
What kind of problem is this?
Solution:
AgCl(s) +
Ag+(aq) Cl-(aq)
Ksp = 1.810-10
Qsp = [Ag+][Cl-] = ( ? )(1.010-2) compare to Ksp
Solubility will be enhanced because of complex formation.
Ag+(aq)
Initial conc.
Change
Eqlbrm conc.
0.10 M
-x
0.10-x
[[Ag(NH3)2]+]
=
Kf =
+][NH ]2
[Ag
3
+ 2 NH3(aq) [Ag(NH3)2]+(aq)
1.00 M
-2x
1.00-2x
x
(0.10-x) (1.00-2x)2
Solve cubic equation! Ouch!
0M
+x
x
= 1.6107
There is a simpler way than solving the cubic
equation if you make some reasonable assumptions
about the strength of the complexation.
Assume Kf is large: Ag+(aq)
Initial conc.
0.10 M
Change
-0.10 M
Eqlbrm conc.
(0) M
Use Kf to calculate [Ag+]:
Initial concs.
0M
Changes
+x M
Eqlbrm conc.
xM
+ 2 NH3(aq) [Ag(NH3)2]+(aq)
1.00 M
0M
-0.20 M
+0.10 M
0.80 M
0.10 M
0.80 M
+2x M
0.80 + 2x M
0.10 M
-x M
0.10 - x M
[[Ag(NH3)2]+]
0.10-x
0.10
= 1.6107
=
Kf =
[Ag+][NH3]2 x(0.80 + 2x)2
x(0.80)2
0.10
+] =
x = [Ag
= 9.810-9 M
(1.6x107)(0.80)2
Qsp = [Ag+][Cl-] = (9.810-9)(1.010-2) = 9.810-11
< Ksp = 1.810-10
- not affected,
Cl
AgCl does not precipitate.
just what was added.
Slides for the next lecture---use only
if there is extra time.
Chapter 19: Spontaneous Change:
Entropy and Free Energy
Courtesy: drippingblood.com
Spontaneous Change
A
B
A spontaneous process is one that occurs in a system left to itself no external action is needed for the process continue.
A non-spontaneous process will not occur without external action.
e.g.
Melting ice at room T (endothermic and spontaneous)
Dissolving NH4NO3(s) ice packs ( )
enthalpy change is not the answer
Nature maximizes disorder
Order and Disorder
Deck of 52 cards
Shuffle the deck
how many different arrangements of cards are possible?
Answer: 52! = 1068
Only 1 way or arranging the cards to have a perfectly ordered deck.
Q: What is the probability of dealing a perfectly ordered deck?
A: 1/52! = 10-68, VERY UNLIKELY
Due to the enormous odds against having an ordered system,
nature tends to maximize disorder because it is more likely!
Order and Disorder
Deck of 52 cards
What is the probability of dealing a perfectly ordered deck?
A: 1/52! = 10-68, VERY UNLIKELY
Due to the enormous odds against having an ordered system,
nature tends to maximize disorder because it is more likely!
Order and Disorder in Molecular Systems
Ordered
Disordered
These changes occur spontaneously
Entropy and the Boltzmann Equation
Entropy, S, is a measure of disorder
S = k lnW
W = Number of microstates accessible to system
W will depend on distribution of molecules amongst their
energy levels, as well as the arrangement of the molecules
in space.
The Boltzmann constant, k = R/NA.
Nature maximizes disorder
S > 0
maximizes S
spontaneous
Ionic strength and activities
'I' is a measure of total dissolved charges
in a solution. These charges shield one
another to varying degrees.
The efficiency varies with total concentration,
so concentrations must be replaced by
activities in an equilibrium constant.
aM = [M] M;
are activity coefficients and are tabulated.
Example-- are activity
coefficients
What is the concentration of F anions in a
solution saturated with CaF2 and containing
enough strong electrolyte to make the ionic
strength I=0.05 M?
Ca = 0.485 F = 0.81 Ksp = 3.91011
Solution:
CaF2(s) = Ca2+(aq) + 2 F-(aq);
Ksp = 3.91011
2
K sp = (Ca 2 + )( F )2 = Ca 2+ [Ca 2 + ] F [ F ]2 = 3.9 1011
(Ca 2 + ) : the thermodynamic 'activity' of calcium ion.
[Ca
2+
] : the analytical concentration of calcium ion.
2
Ca and F are activity coefficients.
2+
Solution:
K sp = (Ca 2 + )( F )2
2
K sp = Ca 2+ [Ca 2 + ] F [ F ]2 = 3.9 1011
[Ca
3
2+
s=
3.9 1011
2
] [F ] =
2
F Ca 2+
3.9 1011
0.812 0.485
=
3.9 1011
0.812 0.485
s = 4.97 104 M