
57-6 Chemical Variables
dominates the reaction. In that situation, it is possible to utilize the diusion theory of random motion
in a m
edium
to d
erive
A s
uch
t
hat
A
r D D N
ij i j
diff
1000
=
+4
0
π
( )( )
(5
7
.20
)
where
N
0
is Avogadro’s number
D
i
and D
j
are the diusion constants for species i and species j
r
ij
is the encounter distance
For
D of th
e
or
der
of 1.5 × 10
−5
cm
2
s
−1
, which is a value appropriate for small molecules, A
di
is
10
10
M
−1
s
−1
. Reactions involving protonation or the OH
−
ion proceed at this rate, but only a few
en
zyme–substrate
co
mplex-formation
re
actions
ap
proach
th
e
di
usion
li
mited
ra
te
[6
]
.
57.3 Experimental
e kinetic methods ...