Concepts from Chemical Reaction Kinetics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 224

When either [X] 0 or [A] 0 is low, the rate is linear in both. At high concentrations, the
rate is independent of both [X] and [A].
Now that we have seen how a catalyst affects the reaction rate, it is easy to
understand how it affects the energy of the reaction. As an example, consider the
decomposition of hydrogen peroxide in the aqueous phase. Under normal con-
ditions the activation energy is

∼ ∼

57 kJ/mol in the
presence of a little bromide in the aqueous phase. The enhancement in rate is
exp

76 kJ/mol. This reduces to

2140. In some cases k

k b , and hence the first energy
barrier E 1 is rate controlling; this is termed an Arrhenius complex mechanism. In the
second case k b k , and a second barrier controls the rate, which is termed the van't
Hoff complex mechanism. In either case the catalyst provides an alternate route of
low energy for the reaction to occur.

[ ( − 57 + 76 )/RT ]=

5.7.2 H OMOGENEOUS C ATALYSIS
Homogeneouslycatalyzedgasphasereactionsthatdonotinvolvechainreactionshave
nogeneralmechanisms.Anexampleofsuchareactioninatmosphericchemistryisthe
combination of NO and Cl 2 catalyzed by Br 2 molecules. The reaction mechanism is

k f
k b 2NOBr

2NO + Br 2

(5.124)

k
−→ 2NOBr + Cl 2 2NOCl + Br 2 .

Bromine is regenerated in the process, and is therefore the catalyst in this reaction.
If the second reaction is rate-controlling the rate is given by

k f
k b [

k

2

r = NO ] [ Cl 2 ][ Br 2 ] . (5.125)

The experimentally determined rate expression is in agreement with the above.
Anothergasphasehomogeneouscatalysisofsignificanceinatmosphericchemistry
isthedecompositionofozoneintheupperatmospherecatalyzedbyoxidesofnitrogen
and other chlorine-containing compounds such as Freon:

catalyst
−−−−→ 2O 3 3O 2 . (5.126)

The above reaction has serious consequences since ozone plays a significant role
in moderating the amount of UV light that reaches the earth. We shall discuss this
reaction in detail in Chapter 6.
In the aqueous environment, and particularly in surface waters and soil-sediment
porewaters, an important reaction is the hydrolysis of organic pollutants such as alkyl
halide, ester, aromatic acid ester, amide, carbamate, and so on. Many pesticides and
herbicides are also hydrolyzable. The extent of hydrolysis plays an important role in
deciding how nature tends to cleanse itself.