反应级数英文.pptVIP

Reaction Kinetics (3) Xuan Cheng Xiamen University 半衰期法确定反应级数 孤立法确定反应级数 速率决定步骤 速率决定步骤 稳态近似 氢与碘的反应 用稳态近似法求碘原子浓度 用平衡假设法求碘原子浓度 从反应机理导出速率方程必须作适当近似，稳态近似是方法之一。 假定反应进行一段时间后，体系基本上处于稳态，这时，各中间产物的浓度可认为保持不变，这种近似处理的方法称为稳态近似，一般活泼的中间产物可以采用稳态近似。 Physical Chemistry 分别用稳态近似和平衡假设来求中间产物[I]的表达式，并比较两种方法的适用范围。 Physical Chemistry * * Physical Chemistry Determination of the Rate Law Physical Chemistry The rate law (17.48) 1. Half-life method (17.49) (17.29) For n ? 1 (17.49) Reaction Kinetics 用半衰期法求除一级反应以外的其它反应的级数。 以lnt1/2～ln[A]o作图从直线斜率求n值。从多个实验数据 用作图法求出的n值更加准确。 根据 n 级反应的半衰期通式： 取两个不同起始浓度[A]o，[A]o’作实验，分别测定半衰期为t1/2和 ，因为同一反应，常数相同，所以： Physical Chemistry Reaction Kinetics Physical Chemistry Determination of the Rate Law 2. Powell-plot method (17.50) the fraction of A unreacted For n ? 1 (17.28) For n ? 1 (17.13) For n = 1 For n = 1 (17.51) Reaction Kinetics Physical Chemistry Determination of the Rate Law For n ? 1 For n = 1 (17.51) For a given n, there is a fixed relation between ? and ? for every reaction of order n. Plot ? versus log10? for commonly occurring values of n to give a series of master curves. (Fig. 17.6) The Powell-plot method requires the initial investment of time needed to make the master plots. Table 17.1 gives the data needed to make the master plots. Reaction Kinetics Physical Chemistry Determination of the Rate Law 3. Initial-rate method The rate law (17.48) The ratio of initial rates for runs 1 and 2 Measure r0 for two different initial concentrations [A]0,1 and [A]0,2 while keeping [B]0, [C]0, …fixed. ? can be found The orders ?,…? can be found similarly Reaction Kinetics Physical Chemistry Determination of the Rate Law 4. Isolation method The rate law (17.48) Make initial concentrations of reactant A much less than the concentrations of all other species [B]0 [A]0, [C]0 [A]0, … The rate law becomes (17.52) Where j is essentially constant. The reaction has the pseudo-order ?. The orders ?,…? can be found similarly. Reac