Chemistry Reference
In-Depth Information
The relative contribution of the different internal states of the reagents A and B in the
formation of the product C is considered by the two factors
)
an
A
(
n
A
(
a
)
bn
B
(
n
B
(
b
x
A
(
a
)
=
and
x
B
(
b
)
=
.
(3.178)
a
)
b
)
3.5.7 O
RDER OF
C
HEMICAL
R
EACTIONS
The type of chemical reactions is usually classified by the reaction order which
depends on the number of simultaneous interacting particles in a collision process.
In nonthermal plasmas the reactions of zero, first, second, and third order may be
considered characterized by the following rate equations and corresponding rate
coefficients.
1.
Zero-order chemical reaction:
For example, spontaneous dissociation of the
molecule AB:
dn
AB
dt
=−
k
(
0
)
AB
−→
k
(
0
)
AB
AB
A
+
B
(3.179)
k
(
0
)
=
k
(
0
)
AB
m
−
3
s
−
1
.
n
AB
(
t
)
=−
·
t
+
n
AB
(
0
)
·
(3.180)
2.
First-order chemical reaction:
For example, dissociation of metastable
molecules by collision with background gas
n
N
n
AB
:
dn
AB
dt
=−
k
(
1
)
AB
−→
AB
m
k
(
1
)
AB
+
N
A
+
B
+
N
·
n
AB
(3.181)
k
(
1
)
=
k
(
1
)
AB
s
−
1
.
n
AB
(
t
)
=
n
AB
(
0
)
·
exp
(
−
·
t
)
(3.182)
The neutral density (
n
N
n
AB
) is considered as a constant and included in
the rate coefficient.
Here, the special case of the consecutive reactions is exemplarily dis-
cussed starting with concentration
n
A
and the formation of an intermediate
product
n
B
which is consumed to form the end product
n
C
(see Figure 3.25).
k
(
1
)
A
−→
k
(
1
)
B
−→
A
B
C.
(3.183)
dn
A
dt
=−
k
(
1
)
A
·
n
A
.
(3.184)
dn
B
dt
=
k
(
1
)
A
k
(
1
)
B
·
n
A
−
·
n
B
.
(3.185)
dn
C
dt
=
k
(
1
)
B
·
n
B
.
(3.186)
