k À4:9 \ \k 4:10 ½MŠ
ð 2:4:22Þ
which is equivalent to [M] << k -4.91 /k 4.10 = 10
10
=3 Á 10
À10 = 3 Á 10
19 cm
−3 and
[M] >> 3 Á 10
19 cm
−3 , i.e., p M << 1 atm and p M >> 1 atm, respectively. In the
low-pressure limit, this is a purely termolecular reaction, its constant is equal to:
k 4:8 ¼
k 4:9
k À4:9
Á k 4:10 ¼ K 4:9 Á k 4:10 cm
6
=s
ð2:4:23Þ
and rate of the AB molecule formation
r 4:8 ¼ K 4:9 Á k 4:10 Á ½AŠ Á ½BŠ Á ½MŠ cm
À3 s
À1
ð2:4:24Þ
is proportional to [M]. In the high-pressure limit, the kinetics of the reaction is
bimolecular, and its rate constant is simply equal to the bimolecular rate constant of
the (A…B)
# formation. (All (A…B)
# quasimolecules are stabilized, the reaction
(-2.4.9) do not occur). The rate of formation of AB does not depend on [M] under
these conditions. The order of reaction (2.4.8) relative to [M] is 0. Under intermediate conditions, the kinetics is intermediate between the termolecular and
bimolecular and has to be described using the bimolecular rate constant k 4.8 ([M])
depending on [M]. The reaction (2.4.8) order relative to [M] is fractional. The
k 4.8 = f([M]) function has the form of a curve with saturation (Fig. 2.5).
That is why the author prefers the way of writing
A þ B !
ðMÞ
AB
and
A þ B !
M AB
[M]
k II
k II ([M])
tgα=k III
α
Fig. 2.5 The bimolecular rate
constant of a termolecular
recombination k II ([M]) as a
function of M concentration
34
2 General Kinetic Rules for Chemical Reactions, Collisional …