物理化学厦门大学.ppt

Theories of Reaction Rates
Theories of Reaction Rates
目的:主要针对基元反应,建立反应过程的模型,根据反应物的非动力学性质计算或预测反应速率或速率常数
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Introduction
Chapter 9 Rate Theories for Elementary Reactions
Transition-state theory 过渡态理论
Unimolecular reaction theory 单分子反应理论
Potential-energy surfaces 势能面
Collision theory 碰撞理论
Reaction dynamics 分子反应动态学(自学)
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Introduction
与热力学的经典理论相比,动力学理论发展较迟。先后形成的碰撞理论、过渡态理论都是20世纪后建立起来的,尚有明显不足之处。
理论的共同点是:首先选定一个微观模型,用气体分子运动论(碰撞理论)或量子力学(过渡态理论)的方法,并经过统计平均,导出宏观动力学中速率常数的计算公式。
由于所采用模型的局限性,使计算值与实验值不能完全吻合,还必须引入一些校正因子,使理论的应用受到一定的限制。
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Collision Theory
Collision density (Collision frequency) 碰撞频率
Fraction of effective collision 有效碰撞分数
Expression of rate constant
Basic assumptions
Focuses
Threshold energy 阈能
Probability factor 概率因子
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Basic Assumptions of Simple Collision Theory (SCT)
The molecules are hard spheres
For a reaction to occur, the two molecules must collide
Not every collision will lead to reaction. Reaction occurs only if the relative ic energy along the line of centers of the colliding molecules exceeds a threshold energy c (阈能)
The Maxwell-Boltzmann distribution of energy is maintained during the reaction
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How to Express the Rate from Collision Theory
Based on the SCT theory, the rate can be expressed as:
L: Avogadro constant
Key point: How to obtain ZAB and F?
Consider a bimolecular reaction: A + B → P
NA: molecular density (molecules/m3)
ZAB: collision density (frequency) 单位时间单位体积内A与B的碰撞次数
(m-3 s-1)
F: fraction of effective collision 有效碰撞分数
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Collision Density, ZAB
Based on ic theory of gases,
ZAB = s <u> NA NB
NA,NB : molecular density
m = MAMB/(MA+MB)
s: Collision cross-section, s = p dAB2
碰撞截面
<u> : relative mean speed
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Collision Density, ZAB and ZAA
Unit of ZAB: m-3 s-1
The collision density for like molecules
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Fraction of Effective Collisions—The Energy Requirements
According to collision theory, only the