1. Xuan Cheng Xiamen University
Physical Chemistry
Reaction Kinetics (2)
Xuan Cheng
Xiamen University

2. Simple Order Reactions
Physical Chemistry
Reaction Kinetics
Simple Order Reactions
Order
Reaction
Rate Law
t1/2
A  P 1 2
n2

3. Typical Complex Reactions
Physical Chemistry
Reaction Kinetics
Typical Complex Reactions
Reversible reaction
Opposing reaction kb
(1)
Consecutive reaction
(2)
Competing reaction
Parallel reaction
(3)

4. Reversible First-Order Reactions
Physical Chemistry
Reaction Kinetics
Reversible First-Order Reactions
(17.1)
Consider the reverse reaction (opposing reaction) kb
the first order in both forward and back directions

5. Reversible First-Order Reactions
Physical Chemistry
Reaction Kinetics
Reversible First-Order Reactions
(17.31)
(17.32)

6. Reversible First-Order Reactions
Physical Chemistry
Reaction Kinetics
Reversible First-Order Reactions
(17.32)
At equilibrium,
and
(17.33)
(17.34)

7. Reversible First-Order Reactions
Physical Chemistry
Reaction Kinetics
Reversible First-Order Reactions
(17.34)
(17.14)*
[A]/[A]o
[C]/[A]o
kf / kb=2
A  C jt
Fig. 17.2
When [A]eq=0, kb=0
[A]/[A]o t
Fig. 17.1(a)

8. Consecutive First-Order Reactions
Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
Consider two consecutive irreversible first-order reactions
(17.35)
(17.36)

9. Consecutive First-Order Reactions
Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
Let only A be present in the system at t = 0
(17.37)
(17.36)
(17.11)
(17.38)
(17.39)
Integration
(17.40) ?

10. Consecutive First-Order Reactions
Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
At all times (conservation of matter)
(17.38)
(17.40)
(17.41)

11. Consecutive First-Order Reactions
Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
The intermediate’s concentration rises to a maximum, and then falls to zero
(the position of maximum: k1/k2) ?
The concentration of the product C rises from zero and reaches [A]o

12. Consecutive First-Order Reactions
Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
Suppose that in an industrial batch process a substance A produces the desired product B which goes to decay to a worthless product C, each stage of the reaction being first-order. At what time will product B be present in greatest concentration?
The time dependence of [B]
(17.40)
A maximum should occur at

13. Consecutive First-Order Reactions
Physical Chemistry
Reaction Kinetics
Consecutive First-Order Reactions
Since [A]o  0, k1  0, so
The maximum concentration of B ?
For a given value of k1 > k2, as k2 increases, both the time at which [B] is a maximum and the yield of B increase.

14. Competing First-Order Reactions
Physical Chemistry
Reaction Kinetics
Competing First-Order Reactions
Consider two competing irreversible first-order reactions
(17.42) A C D k1 k2
Parallel reactions
(17.43)

15. Competing First-Order Reactions
Physical Chemistry
Reaction Kinetics
Competing First-Order Reactions
(17.43)
Compare with
(17.11)
(17.14)*
For C
(17.44) ?

16. Competing First-Order Reactions
Physical Chemistry
Reaction Kinetics
Competing First-Order Reactions
(17.44)
similarly
(17.45)
Division of (17.44) and (17.45)
At any time during the reaction
(17.46)

17. Competing First-Order Reactions
Physical Chemistry
Reaction Kinetics
Competing First-Order Reactions
(17.42)
Consider the reverse reactions
(17.47)
Moreover
At equilibrium
Thermodynamic control
When any reverse reactions or interconversion of C and D can be neglected
(17.46)
Kinetic control

18. Homework Supplemental Material Page 591 Page 145, Prob. 16 Prob. 17.13
Physical Chemistry
Reaction Kinetics
Homework
Supplemental Material
Page 591
Page 145, Prob. 16
Prob
Page 149, Prob. 30
Page 158, Prob. 68