Lon-Capa
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8th (final) HW assignment due Tuesday,
December 2 by 5 pm.
Quiz #6 opens Friday at 5 pm. Due Tuesday,
December 2 by 10 pm.
Exams and Practice Exams on website
1
Clicker Question
Determine the missing
time given the following
orders of reaction for
aA  Products
a)
b)
c)
d)
zero 1st
20
20
30
20
15
20
Not sure.
2nd
20
15
30
[A]
Time
1.00 M
0
0.500 M
10 sec
0.250 M
?? sec
2
Chapter 15: 102
The following results were obtained
at 600K for the decomposition of
ethanol on an alumina (Al2O3)
surface,
C2H5OH(g)  C2H4(g) + H2O(g)
a. Predict Ptotal in torr at t = 80. s
b. What is the value of the rate
constant, and what are the units?
c. What is the order of the reaction?
d. Calculate Ptotal at t = 300. s
Time (s)
0
Ptotal
(torr)
250.
10.
265
20.
280.
30.
295
40.
310.
50.
325
3
aA + bB  Products
Given the following data, determine the rate
law and the value for the rate constant.
Expt 1 {[B] = 5.00 M}
Expt 2 {[B] = 10.0 M}
[A]
0.010 M
0.0050 M
0.0025 M
[A]
0.010 M
0.0050 M
0.0025 M
time
0
40. sec
120 sec.
time
0
20. sec
60. sec
4
Mechanisms
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Series of elementary steps.
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Must be chemically reasonable
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Must have the correct stoichiometry. That is,
the steps must add up to the overall reaction.
How can we tell this?
Cannot prove but can support empirically.
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Must agree with the overall rate law.
5
Rate Laws From Elementary
Steps
6
Evaluating Mechanisms
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The rate law comes from the ratedetermining step (the “slow” step).
Fast equilibrium.
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Rate forward = rate reverse.
Rate law cannot include intermediates.
Steady-state approximation.
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[intermediate] = constant.
d[intermediate]/dt = 0
Rate law cannot include intermediates.
7
Chapter 15: 115
Do for next Friday.
8