文档介绍：Reaction ics (6)
Xuan Cheng
Xiamen University
Physical Chemistry
1
Physical Chemistry
Theories of Reaction Rates
Hard-Sphere Collision Theory of Gas-Phase Reactions
Assumptions
The molecules are hard spheres
For a reaction to occur between B and C, the two molecules must collide
Not all collisions produce reaction. Reaction occurs if and only if the reactive translational ic energy along the line of centers of the colliding molecules exceeds a threshold energy thr
The Maxwell-Boltzmann equilibrium distribution of molecular velocities is maintained during the reaction
Reaction ics
2
(ii)反应分子必须通过碰撞才可能发生反应;
(iii)并非所有碰撞都能发生反应,相互碰撞的两个分子—碰撞分子对的能量达到或超过某一定值thr—称为阈能时,反应才能发生,这样的碰撞叫活化碰撞;
(iV)在反应过程中,反应分子的速率分布始终遵守麦克斯韦—玻耳兹曼(Maxwell-Boltzmann)分布。
简单碰撞理论的基本假设
该理论的基本假设(即理论模型):
(i)反应物分子可看作简单的硬球,无内部结构和相互作用;
Physical Chemistry
Reaction ics
3
Physical Chemistry
Theories of Reaction Rates
Hard-Sphere Collision Theory of Gas-Phase Reactions
The number of B reacting in a bimolecular reaction
B + C Products
()
The predict rate constant
()
The use of () for ZBC
()
for B C
Reaction ics
4
Physical Chemistry
Theories of Reaction Rates
Hard-Sphere Collision Theory of Gas-Phase Reactions
For the bimolecular reaction 2B Products
The rate of disappearance of B
The use of () for ZBB
()
for B = C
Reaction ics
5
Physical Chemistry
Theories of Reaction Rates
Hard-Sphere Collision Theory of Gas-Phase Reactions
()
for B = C
()
for B C
()
()
Reaction ics
6
Physical Chemistry
Theories of Reaction Rates
Hard-Sphere Collision Theory of Gas-Phase Reactions
()
()
()
for B C
()
for B C
1/2RT is small
The hard-sphere threshold energy is nearly the same as the activation energy. The simple collision theory gives only the pre-exponential factor A (but not for the calculation of Ethr)
Reaction ics
7
K + Br2 KBr + Br 600 0 10
CH3 + CH3 C2H6 300 0
2NOCl 2