Environmental Engineering Reference
In-Depth Information
This mechanism shows the formation of the
“
growth monomer CH
2S
”
(reactions RX.
5.3
(reaction RX. 5.9), propagation
(RX. 5.11 and RX. 5.14), the termination to a paraffin (alkane, hydrocarbon without
double bonds, (RX. 5.10) and (RX. 5.13)), and the termination to an olefin (alkene,
hydrocarbon with at least one carbon-to-carbon double bond, (RX. 5.12)).
-
5.8), the formation of the
“
chain starter CH
S
”
5.1.1 Order of Reaction and Reaction Rate
As said, in order to understand how a chemical reaction takes place, elementary reac-
tions are needed. For a homogeneous system, a common kinetic behavior exists,
which can be described by the Guldberg and Waage law (or law of mass action). This
law states that the rate of an elementary reaction is proportional to the product of the
concentrations of the participating molecules.
Reactions are classified based on the order of reaction, which is the number
of chemical species (molecules or atoms) that determine, based on their concen-
tration, the reaction rate of the process. In other words, the order of reaction is
the index, or exponent, to which its concentration term in the rate equation
is raised. It is important to highlight that these indexes can be different from
the stoichiometric coefficients as the balanced reaction equation is usually
different from the reaction mechanism, whose rate is determined by the slowest
step.
A generic chemical reaction could be represented in the following way:
X
X
n
i
=1
ν
i
0
M
i
!
n
i
=1
ν
0
i
M
i
ð
RX
:
5
:
15
Þ
ν
i
00
are the stoichio-
metric coefficients of the products, n is the total number of the chemical species
involved in the reaction, and M
i
represents the species. For the reaction
ν
i
0
are the stoichiometric coefficients of the reactants and
where
2H
2
+O
2
!
2H
2
O
ð
RX
:
5
:
16
Þ
of which the order can be determined only experimentally, we clearly have
n=3 H
2
,O
2
,H
2
O
ð
Þ
;
M
1
=H
2
;
M
2
=O
2
;M
3
=H
2
O
ν
1
0
=2;
ν
2
0
=1;
ν
3
0
=0;
ν
0
1
=0;
ν
0
2
=0;
ν
0
3
=2
By convention, the stoichiometric coefficients are positive for products and negative
for reactants, and we can define the reaction rate as
r
=
1
ν
i
dM
½
dt
ð
Eq
:
5
:
1
Þ
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