文档介绍：Reaction ics (4)
Xuan Cheng
Xiamen University
Physical Chemistry
1
Physical Chemistry
Temperature Dependence of Rate Constants
Arrhenius Equation
()*
A & Ea: constants characteristic of the reaction (Arrhenius parameters)
Ea: Arrhenius activation energy
A: pre-exponential factor (Arrhenius A factor) (frequency factor)
()
Some reactions are not Arrhenius-like,
If Ea independent of temperature
()
()
Reaction ics
2
Physical Chemistry
Temperature Dependence of Rate Constants
()
Slope = -Ea/R
Plot lnk (log10k) vs. 1/T
Reaction ics
3
Physical Chemistry
Temperature Dependence of Rate Constants
Arrhenius Equation
()*
Tolman 用统计平均的概念对基元反应的活化能下了一个定义:活化分子的平均能量与反应物分子平均能量之差值,称为活化能。
设基元反应为 A P
正、逆反应的活化能可以用图表示。
Reaction ics
4
复杂反应的活化能
复杂反应的活化能无法用简单的图形表示,它只是组成复杂反应的各基元反应活化能的数学组合。
这表观活化能也称为总包反应活化能或实验活化能。
组合的方式决定于基元反应的速率系数与表观速率系数之间的关系,这个关系从反应机理推导而得。例如:
Physical Chemistry
Reaction ics
5
温度对反应速率影响的类型
r
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(2)
(3)
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(5)
通常有五种类型:
(1)反应速率随温度的升高而逐渐加快,它们之间呈指数关系,这类反应最为常见。
(2)开始时温度影响不大,到达一定极限时,反应以爆炸的形式极快的进行。
Physical Chemistry
Reaction ics
6
温度对反应速率影响的类型
(3)在温度不太高时,速率随温度的升高而加快,到达一定的温度,速率反而下降。如多相催化反应和酶催化反应。
(4)速率在随温度升到某一高度时下降,再升高温度,速率又迅速增加,可能发生了副反应。
(5) 温度升高,速率反而下降。这种类型很少,如一氧化氮氧化成二氧化氮。
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(4)
(5)
Physical Chemistry
7
Chain Reactions & Free-Radical Polymerizations
Physical Chemistry
A series of steps: a reactive intermediate is consumed
Chain reaction
Reactants are converted to products
The intermediate is regenerated
Cycle is repeated
The structure of chain reactions
Chain carriers: the intermediates responsible for the propagation of a chain reaction
Radical chain carriers: the chain carriers are radicals
Radicals: Species with unpaired electrons
Ions, neutrons (in nuclear fission)
Reaction ics
8
Physical Chemistry
Chain Reactions & Free-Radical Polymerizations
A chain reaction leads to plicated rate law.
Derive the