Page 90 - Bruno Linder Elementary Physical Chemistry

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

Chemical Kinetics 75

It is found experimentally that the reaction obeys the rate law

2
d[NO 2 ]/dt = k[NO] [O 2 ] (7.35)
Caution. It is important to distinguish between rate of reaction and rate of
consumption or rate of production. Thus, if r represents the rate of reaction,
then

1 1
r = − d[NO]/dt = −d[O 2 ]/dt = d[NO 2 ]/dt (7.36)
2 2
On the other hand, the rates of consumption of NO and O 2 are respectively

−d[NO]/dt =2r (7.37a)

and

−d[O 2 ]/dt = r (7.37b)

and the rate of formation of NO 2 is

+d[NO 2 ]/dt =2r (7.37c)

Obviously, the rate law, r, does not represent the rate of formation of the
product NO 2.

To explain the observed rate law, the following mechanisms have been
suggested. It is assumed that N 2O 2 is the intermediate and that the
following elementary reactions take place. [Note that reactions are said to
be elementary, when the order of the reactions can be determined from the
stoichiometric coefficients.]

1) NO + NO → N 2O 2
d[N 2 O 2]/dt = k 1[NO] 2 (7.38)

2) N 2 O 2 → NO+NO fast

− d[N 2 O 2 ]/dt = k [N 2 O 2 ] (7.39)
1
3) N 2 O 2 +O 2 → 2NO 2 slow
− d[N 2 O 2 ]/dt = k 2 [N 2 O 2 ][O 2] (7.40)

Note: These k’s are not the k A which are the product of k and a.