Page 94 - Bruno Linder Elementary Physical Chemistry

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

Chemical Kinetics 79

7.15. Chain Reactions

Many liquid and gaseous reactions are chain reactions, meaning that
an intermediate produced in one step generates an intermediate in a
subsequent step; the latter generates another intermediate, etc.
It is customary to characterize the various reactions by names, such as
initiation step, propagation step, termination step, etc.
As an example, consider the reaction
H 2 (g) + Br 2 (g) → 2HBr(g) (7.59)

Theratelaw is found to be
d[HBr]/dt = k[H 2 ][Br 2 ] 3/2 /([Br 2 ]+ k [HBr]) (7.60)

The proposed mechanism involves chain reactions and free radicals. A
radical is denoted by a dot after the atomic symbol. The final results must
not contain intermediate free radicals. Again, all reaction constants are in
the forward direction.

1) Initiation

Br 2 → 2Br •
1/2d[Br •]/dt = k a [Br 2 ] (7.61)
2) Propagation

Br • +H 2 → HBr + H •
−d[Br •]/dt =d[H ]/dt = k b[Br •][H 2] (7.62)
•

H • +Br 2 → HBr + Br •
−d[H ]/dt =d[Br •]/dt = k [H ][Br 2 ] (7.63)

•
•
b
3) Retardation
•
H +HBr → H 2 +Br •
−d[H ]/dt =d[Br •]/dt = k c [H ][HBr] (7.64)
•
•
4) Termination
Br • +Br • +M → Br 2 +M
1 2
− d[Br •]/dt = k d[Br •] (7.65)
2