Outline:
2/19/07
Pick up Exam #1 - from me
 Pick up CAPA #10 & 11 - outside
 Seminar – Tues @ noon
 Jaecker Applications – Chem Dept.

Today:
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Chapter 16
Chemical Equilibrium
The Equilibrium Constant (Keq)
Manipulating/Calculating Keq
Exam #1
 Average
~74.7
 High: 99,…
Exam 1 (121-03)
14
12
# students
10
8
6
4
2
0
5
5
15 15 2525 3535 4545 5555 65 65 75 75 85 85 95 95
Score
Well done!
Exam #1

What to do if….
Go over test = do a post mortem
Remember it’s only 20%...or less!
Change something…
Come visit…
Chapter 16: Equilibrium
2 NO2(g)  N2O4(g)
Rateformation = kf[NO2]2
N2O4(g)  2 NO2(g)
Ratedecomposition = kd[N2O4]
Ratef = Rated
kf [NO2]2 = kd[N2O4]
At equilibrium:
Define an “equilibrium constant”:
Keq = kf /kd = [N2O4]/[NO2]2
nomenclature: [concentrations] or p
 becomes 
Rxn quotient: Q = [products]/[reactants]
e.g. If Q = Keq system is at equilibrium
If Q > Keq products  reactants
If Q < Keq reactants  products
The Equilibrium Constant
For a general reaction
aA + bB
cC + dD
the equilibrium constant expression
for everything in solution is

Cc Dd
K eq 
Aa Bb
where Keq is the equilibrium constant.
The Equilibrium Constant
• An equilibrium can be approached
from any direction.
2NO2(g)
• Example: N2O4(g)
• has
K eq 
P2
NO 2
P
N 2O 4
 6.46
The Equilibrium Constant
• In the reverse direction:
2NO2(g)
P
N2O4(g)
1
N 2O 4
K eq 
 0.155 
6.46
P2
NO 2
Equilibrium Constant Rules:
Magnitude of Keq tells you about
preferred direction of reaction
Keq = 31000
or
Keq = 4.610-8
more products
(at equilibrium)
more reactants
Equilibrium Constant Rules:
Reactions can be written in either
direction at equilibrium
Keq = 1/ K’eq
Reactions can be written as the sum
of other reactions (Hess’ law)
Keq = K’eq K”eq
A specific example (chem 113):
AgCl(s)  Ag+ + Cl-
Keq = 1.8  10-10
Ag+ + 2 NH3  Ag(NH3)2+
Keq = 1.6  107
AgCl(s) + 2 NH3  Ag(NH3)2+ + ClKeq = 2.9  10-3
Equilibrium Constant Rules:
Solids and (pure) liquids are left out
of the Keq expression
Example:
H3PO4(aq) + 3 H2O(l)  PO3-4(aq) + 3H3O+(aq)
Keq = [PO3-4 ] [H3O+]3 / [H3PO4]
Equilibrium Constant Rules:
Magnitude of Keq tells you about
the extent of reaction
Reactions can be written in either
direction at equilibrium
Solids and (pure) liquids are left
out of the Keq expression
Units of Keq are defined to be 1….
Let’s Practice:
Worksheet #6 (A)
Keq = (pNO)2(pO2) / (pNO2)2
= (0.0015)20.051 / (0.1)2 = 1.15e-5
Starting from initial concentrations:
Mix: 110-3 Fe3+ and 110-3 SCNKeq = 142
What are the final concentrations?
Fe3+ + SCN-  Fe(SCN)2+
110-3 110-3
-x
-x
0
Initial
+x
Change
(110-3-x)(110-3-x) +x
Equil.
 Initial
concentrations:
Mix: 110-3 Fe3+ and 110-3 SCNWhat are the final concentrations?
Keq = [Fe(SCN)2+ ]/[Fe3+][SCN-]
142 = x / (110-3 - x) (110-3 - x)
(a quadratic equation in x)
x = 1.1110-4
Return to Worksheet #6

Do problems B & C
Practice these!
Finish Friday