Concepts from Chemical Reaction Kinetics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 200

The equilibrium constant is

k f
k b =

[ A ] s W A

C Aw =

K eq =

] w =

K sw . (5.40)

[ A

Similar analogies also apply to air-soil, aerosol-air, and biota-water partition con-
stants. Thus, an equilibrium partition constant is a ratio of the forward and backward
rate constants for the processes. A large value of K sw implies either a large value of
k f or a small value of k b .

E XAMPLE 5.3 R EVERSIBLE R EACTION

= 0.1 h − 1 . The A reaction A B is said to occur with a forward rate constant, k f

concentration of A monitored with time is given:

t ( h ) [A] (mM)

0 1

1 0.9

5 0.65

10 0.48

15 0.40

100 0.33

500 0.33

Find K eq and k b .
As t Π 4, [A] eq =[ A ] 0 ( k b /k f + k b ) . Since [A] eq = 0.33, k b /k f + k b = 0.33 / 1 =
0.33. Hence k b = 0.05 h − 1 and K eq = 0.1 / 0.05 = 2. [B] eq =[ A ] 0 ( k f /k f + k b ) =
0.67. Figure 5.5 plots the change in [A] and [B] with t .

1.2

[A]/mM
[B]/mM

1

0.8

0.6

0.4

0.2

0

-0.2

0 100 200 300 400 500 600 700

t /hr

FIGURE 5.5 Change in concentrations of A and B for a reversible reaction A B.