文档介绍：Reaction ics (3)
Xuan Cheng
Xiamen University
Physical Chemistry
1
Determination of the Rate Law
Physical Chemistry
The rate law
()
1. Half-life method
()
()
For n 1
()
Reaction ics
2
半衰期法确定反应级数
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 ics
3
Physical Chemistry
Determination of the Rate Law
2. Powell-plot method
()
the fraction of A unreacted
For n 1
()
For n 1
()
For n = 1
For n = 1
()
Reaction ics
4
Physical Chemistry
Determination of the Rate Law
For n 1
For n = 1
()
For a given n, there is a fixed relation between and for every reaction of order n.
Plot versus log10 monly occurring values of n to give a series of master curves. (Fig. )
The Powell-plot method requires the initial investment of time needed to make the master plots.
Table gives the data needed to make the master plots.
Reaction ics
5
Physical Chemistry
Determination of the Rate Law
3. Initial-rate method
The rate law
()
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 ics
6
Physical Chemistry
Determination of the Rate Law
4. Isolation method
The rate law
()
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 es
()
Where j is essentially constant.
The reaction has the pseudo-order .
The orders ,… can be found similarly.
Reaction ics
7
孤立法确定反应级数
孤立法类似于准级数法,它不能用来确定反应级数,而只能使问题简化,然后用前面三种方法来确定反应级数。
[A]>>[B]
先确定β值
[B]>>[A]
再确定α值
Physical Chemistry
Reaction ics
8
Physical Chemistry
Rate Laws and Equilibrium Constants for Elementary Reactions
Show that for a reactio