文档介绍：Reaction ics (5)
Xuan Cheng
Xiamen University
Physical Chemistry
1
Key Words
Pyrolysis
Acetaldehyde
Methane
Polymerization
Monomer
Initiator
Relaxation
高温分解
乙醛
甲烷
聚合
单体
引发剂
迟豫
Physical Chemistry
Reaction ics
2
Physical Chemistry
Rice-Herzfeld Mechanisms
Simple rate laws can follow from plex chain mechanisms.
(a) Initiation:
(b) Propagation:
(c) Retardation:
(d) Termination:
The Rice-Herzfeld mechanism for the pyrolysis of acetaldehyde is
A chain reaction can lead to a simple rate law.
Pyrolysis of acetaldehyde
Reaction ics
3
Physical Chemistry
Rice-Herzfeld Mechanisms
rates of the formation of the two intermediates are
The sum of the two equation is
Reaction ics
4
Physical Chemistry
The rate of formation of CH4 is
Rice-Herzfeld Mechanisms
in agreement with the three-halves order observed experimentally.
However, the true mechanism is plicated than R-H mechanism.
Other products (acetone, CH3COCH3, and propanaldehyde, CH3CH2CHO) can be formed.
Prob.
Reaction ics
5
Free-Radical Polymerizations
Physical Chemistry
Let I and M stand for the initiator and monomer
Chain polymerization
Results in the rapid growth of an individual polymer chain for each activated monomer, and often occurs by a radical chain process.
(a) Initiation
(b) Propagation
(c) Termination
Reaction ics
6
Free-Radical Polymerizations
Physical Chemistry
(a) Initiation
(b) Propagation
(fast)
The rate-determining step is the formation of the radicals R.
The chain of reactions propagates quickly,
f is the yield of the initiation step, the fraction of radicals that R essfully initiate a chain.
()
Reaction ics
7
Free-Radical Polymerizations
Physical Chemistry
(c) Termination
Assume that the rate of termination is independent of the length of the chain,
the rate of change of radical concentration by this process is
The total radical concentration is approximately constant throughout the main part of the polymerization.
(the rate at w