《物理化学(Physical Chemistry)》课件 第十一章 化学动力学二.pptVIP

According to advanced mathematics, at maximum or minimum point of a curve: At tmax, how much is the concentration of B? §8 反应机理初步 认识反应的关键控制步骤，以便控制整个化学反应朝着有利于我们所需要的方向进行； 反应机理是关于化学反应历程的一种推断或假设； 假设或推断必须经过严格的实验验证； 由于机理研究的复杂性，有时需要采用一些估计的方法 Stationary-state approximation; r. d. s. approximation Equilibrium approximation In 1919, J. A. Christiansen, K. F. Herzfeld and M. Polanyi independently proposed the mechanism consisting of five elementary reactions as follows: （1） stationary-state approximation For reaction: H2 + Br2 ? 2 HBr 1、Three approximate treatment methods ? Is it correct? For consecutive reaction, once the reaction take place, active intermediate rapidly attains their Cmax and their concentration remains unchanged during the whole reaction. Stationary-state approximation eq.(1) = 0 adding these two equations yields: eq.(4) eq.(2) eq.(3) = 0 From eq. (3): eq.(5) By substituting eq.(4) and (5) into eq.(1): 稳定态处理法： 假定高活性的中间产物的浓度不随时间变化，即 dci / dt=0; 根据这个假设，中间物的微分方程简化为代数方程，可以从可能的反应机理中寻求与实验结果相符的动力学方程。 （2） rate-determining step (r. d. s.) approximation Its proposed mechanism is as follows: Reaction: Rate equation Reaction (1) is a reaction between two molecules whose activation energy is much larger than reaction (2) between radical and molecules. Therefore, reaction (1) may be a eligible rate-determining step of the whole reaction, and the rate equation acquires the form of rate-determining step approximation provides a easier way to write the rate equation from a single step (r. d. s.) in the complicated mechanism. （3）Equilibrium approximation When rapid opposing reaction occurs in the series of mechanism, the equilibrium approximation applies. Its mechanism is: Rapid equilibrium r.d.s. Rapid reaction What is the rate equation according to this mechanism ? The second elementary reaction being the r.d.s, therefore, the rate equation attains the form: Because the first re