Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

8.1 Experimental observations 176
(4) There are many similar other reactions, however, for which the rate law is secondorder,
i.e.,
[A]
= − 0 [A] [M] (8.7)
Examples are
Br 2 → Br + Br (8.8)
H 2 O 2 → OH + OH (8.9)
(5) More detailed measurements show that at a given temperature the order of the
reaction may depend on pressure (see Fig. 8.2), for instance for the reaction
one has found that
CH 3 NC → CH 3 CN (8.10)
a) for → 0:
[CH 3 NC]
= − 0 [CH 3 NC] [M] (8.11)
This is called the “low pressure range” for the reaction.
b) for →∞:
[CH 3 NC]
= − ∞ [CH 3 NC] (8.12)
This is called the “high pressure range” for the reaction”.
(6) The transition from the low to the high pressure range depends
a) onthesizeofthemolecule(seeFig.8.3),
b) for a given molecule, it depends on temperature.
(7) The activation energy in the low pressure regime is much smaller than in the high
pressure regime:
= 2 ln
(8.13)
where
0 ∞ (8.14)
and
∞ ≈ 0 (8.15)
(8) The preexponential factors in the low pressure regime are much higher than the
collision frequencies, i.e.,
0 (8.16)
(9) For all these reactions, the molecules must somehow be “energized”. There are
other reactions, in which the molecules are energized chemically (“chemically
activated reactions”) or by photons. At the microscopic, i.e., molecular level,
these reactions should have related mechanisms.