# Lecture 33 ```Lon-Capa



8th (final) HW assignment due Tuesday,
December 2 by 5 pm.
Quiz #6 opens today at 5 pm. Due Tuesday,
December 2 by 10 pm.
Exams and Practice Exams on website
1
Evaluating Mechanisms


The rate law comes from the ratedetermining step (the “slow” step).
Fast equilibrium.



Rate forward = rate reverse.
Rate law cannot include intermediates.



[intermediate] = constant.
d[intermediate]/dt = 0
Rate law cannot include intermediates.
2
Clicker Question
The reaction 2A + B  C has the following
proposed mechanism
A+B
D
(fast equil.)
D+BE
(slow)
E + A  C + B (fast)
The rate law from this mechanism is
a) rate = k[A][B]
d) rate = k[A][B]2
b) rate = k[A]2[B]
e) I don’t know
c) rate = k[A]2[B]2
d) rate = k[A][B]2
3
Reaction Profiles
4
Recall for
st
1
Lecture
5
Transition State
6
Rate constant (k) vs. T
7
ln(k) vs. 1/T
8
Catalyzed Pathways Have a
Lower Activation Energy
9
Catalyzed Pathways Have a
Lower Activation Energy
10
Zero-order (catalyst)
11
Chapter 15: 115
Consider the hypothetical reaction:
A + B + 2C  2D + 3E
In a study of this reaction, three
experiments were run at the same
temperature. The rate is defined as
–d[B]/dt.
Expt 1: [A]0 = 2.0 M, [C]0 = 1.0 M,
[B]0 = 1.0 x 10-3 M
Write the rate law for this reaction,
and calculate the rate constant.
[B]
Time (s)
(mol/L)
2.7 x 10-4 1.0 x 105
1.6 x 10-4 2.0 x 105
1.1 x 10-4 3.0 x 105
8.5 x 10-5 4.0 x 105
6.9 x 10-5 5.0 x 105
5.8 x 10-5 6.0 x 105
12
Chapter 15: 115
Consider the hypothetical reaction:
A + B + 2C  2D + 3E
In a study of this reaction, three
experiments were run at the same
temperature. The rate is defined as
–d[B]/dt.
Expt 2: [B]0 = 3.0 M, [C]0 = 1.0 M,
[A]0 = 1.0 x 10-2 M
Write the rate law for this reaction,
and calculate the rate constant.
[A]
Time (s)
(mol/L)
8.9 x 10-3
1.0
7.1 x 10-3
3.0
5.5 x 10-3
5.0
3.8 x 10-3
8.0
2.9 x 10-3
10.0
2.0 x 10-3
13.0
13
Chapter 15: 115
Consider the hypothetical reaction:
A + B + 2C  2D + 3E
In a study of this reaction, three
experiments were run at the same
temperature. The rate is defined as
–d[B]/dt.
Expt 3: [A]0 = 10.0 M, [B]0 = 5.0 M,
[C]0 = 5.0 x 10-1 M
Write the rate law for this reaction,
and calculate the rate constant.
[C]
(mol/L)
0.43
Time (s)
1.0 x 10-2
0.36
2.0 x 10-2
0.29
3.0 x 10-2
0.22
4.0 x 10-2
0.15
5.0 x 10-2
0.08
6.0 x 10-2
14
```