第十一章化学动力学一.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 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 reaction is a rapid reversible reaction, its equilibrium keeps during the reaction: * Substituting this equation into the rate equation yields: * Equili