Page 78 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch07 Elementary Physical Chemistry

Chemical Kinetics 63

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If a rate law is not of the form [A] [B] [C] ..., the reaction has no
order. For example, the rate of formation of HBr (in a particular reaction
to be developed later) has the form
d[HBr]dt = {k[H 2 ][Br 2 ] 3/2 }/{[Br 2 ]+ [HBr]} (7.11)

This reaction has no order.

7.4. Determination of the Rate Law
7.4.1. Isolation Method
This method is used when all reactants except one are in great excess so
that their concentrations virtually do not change.

Example 7.1. Consider a reaction which has the rate expression r =
2
k[A][B] . Assuming that [B] is in large excess so that it can be considered
constant and equal to its initial value. So, we could express the rate as
r = k [A] (7.12a)

where

k = k[B] 2 (7.12b)
The rate law is now effectively first order, and is called pseudo-first order.
Had [A] rather than [B] been in excess, the rate would have been pseudo-
second order.

7.4.2. Initial Rate Method

In this approach, one measures instantaneous rates at the beginning of
several different concentrations of a particular reactant, holding all other
reactants constant.
m
Example 7.2. Consider the rate expression r = k[A] , of unknown order
m. Denoting the initial rate of a particular sample as r 1 and of another
sample as r 2 , and the corresponding initial concentrations as [A] 1 and [A] 2 ,
we can write

ln(r 2 /r 1)= m ln{[A] 2/[A] 1 } (7.13)

from which we can obtain the order m and from r the value of k.