Page 207 - Instant notes

The kinetics of real systems 193

reactions in flames, the reactions that contribute to ozone destruction in the upper
atmosphere, nuclear fission, or the formation of polymers in solution.
A chain reaction mechanism is illustrated by the gas-phase reaction between hydrogen
and bromine:
H 2(g)+Br 2(g)→2HBr(g)

The following reaction scheme explains the complex observed empirical rate law (see
Topic F2),

1) Initiation. The initiation step is the unimolecular dissociation of Br 2 to produce the
first free radical chain carriers. (Free radicals are reactive species containing unpaired
electrons in their valence shells.)
Br 2→2Br rate=k 1[Br 2]

2) Propagation. Propagation reactions convert reactive intermediates from a preceding
elementary reaction into another reactive intermediate. The total number of reactive
intermediates is unaltered. There are two different propagation reactions in the HBr
mechanism:
Br+H 2→HBr+H rate=k 2a[Br][H 2]

and
H+Br 2→HBr+Br rate=k 2b[H] [Br 2]

Although not present in the HBr mechanism, branching reactions are a specific type of
propagation reaction in which more chain carriers are produced than are consumed.
3) Retardation. The attack of an H radical on a product HBr molecule formed in a
previous propagation step, although still generating another free radical, has the effect of
decreasing, or retarding, the overall rate of product formation.
H+HBr→H 2+Br rate=k 3[H][HBr]
4) Termination. Elementary reactions in which radicals combine to reduce the total
number of radicals present are called termination steps.
2
Br+Br+M→Br 2+M rate=k 4[Br]
In this association reaction, the third body M represents any species present which
removes the energy of the recombination collision between the Br atoms to form the
stabilized Br 2 molecule. The concentration of M (which is a constant for given reaction
conditions) is included in the value of the rate constant k 4. Although other chain
termination reactions are possible, e.g. recombination of two H radicals, it turns out that
only Br recombination is significant in this mechanism.
The rate law is formulated using the procedures described in Topic F4. Product HBr is
formed in the two propagation reactions but consumed in the retardation reaction, so: