Concepts from Chemical Reaction Kinetics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 221

E XAMPLE 5.11 E FFECT OF I ONIC S TRENGTH ON R ATE C ONSTANT

The ionic strength affects a bimolecular reaction rate constant as follows:

I (mol/kg) k (mol/L/s)

2.5 × 10 − 3

1.05

3.7 × 10 − 3

1.12

4.5 × 10 − 3

1.16

6.5 × 10 − 3

1.18

8.5 × 10 − 3

1.26

A plot of the rate constant versus I gives as intercept k 0

= 0.992 L/mol/s with

r 2 = 0.936 and slope = 31.5. A plot of ln (k/k 0 ) versus I 1 / 2 gives a straight line

with slope 2.2 and a correlation coefficient r 2 = 0.756. The slope 2.2 = 1.02 z A z B .

If one of the ions A has z A =− 1 (e.g., OH − ) , the other ion B must have a
charge z B =− 2. Thus, we can infer the charge of the ions involved in the activated
complex.

5.7 ENVIRONMENTAL CATALYSIS
The rates of environmental reactions are influenced by organic or inorganic species,
solid particles, and liquid surfaces. The resulting change in reaction rates is called
catalysis and the entities responsible for the charge are called catalysts . Catalysis is
prevalent in the natural environment (water, air, and soil) and also in waste treatment
and pollution prevention processes. If the process occurs such that the catalysts are in
the same phase as the reactants, it is termed homogeneous catalysis . Some reactions
are, however, affected by the presence of a separate phase (e.g., solid particles in
water); these are called heterogeneous catalysis . Most surface reactions, in one way
or another, belong to the latter category. A list of typical examples in environmental
engineering is given in Table 5.3.
Catalysts participate in a reaction, but are regenerated in the system such that
there is no net concentration change. The equilibrium constant K eq = k f /k b remains

unchanged. Therefore, it must be noted that both k f and k b are influenced to the
same extent. For most environmental catalysis reactions, the general rate expression
will be

f ( r =[ f ( [ A ] , [ B ] , ...) ]·[ X ]+ [ A ] , [ B ] , ...) , (5.113)

, ...) and f ( where [X] represents the catalyst concentration f ( [ A ] , [ B ] [ A ] , [ B ] , ...)

denote the dependence on the substrate concentration. As [ X ]→ 0, r →

f ( , ...) and in some cases since f (

[ A ] , [ B ] [ A ] , [ B ] , ...) = 0, r → 0.