Geoscience Reference
In-Depth Information
1.4. Chemical Reactions and
Photoprocesses
Many of the pollution problems today are exacerbated
by atmospheric chemical reactions. Reactions are initi-
ated by sunlight, lightning, changes in temperature, or
molecular collisions. In this section, chemical reactions
are briefly discussed.
Gas-phase chemical reactions are conveniently
divided into photolysis reactions (also called photopro-
cesses, photodissociation reactions, or photolytic reac-
tions) and chemical kinetic reactions. Photolysis reac-
tions, which are
unimolecular
(involving one reactant),
are initiated when solar radiation strikes a molecule and
breaks it into two or more products. An example of a
photolysis reaction
is
NO
2
(g)
Nitrogen
dioxide
than at low temperatures. Isomerization reactions are
similar to Reaction 1.3, except that an isomerization
reaction has one product, which is another form of the
reactant. An example of an
isomerization reaction
is
H
H
O
*
M
(1.4)
CO
CO
H
H
O
*
Excited formic
acid
Excited Criegee
biradical
The bimolecular
collision reaction
,themostcommon
type of kinetic reaction, may occur between any two
chemically active reactants that collide. A prototypical
collision reaction is
OH(g)
Hydroxyl
radical
CH
3
(g)
Methyl
radical
NO(g)
Nitric
oxide
O(g)
Atomic
oxygen
CH
4
(g)
Methane
+
→
+
H
2
O(g)
Water
vapor
(1.5)
+
h
→
+·
<
420 nm
(1.1)
where the h
v
implies a photon of solar radiation and
is
the wavelength of the radiation (defined in Chapter 2).
Chemical kinetic reactions are usually
bimolecular
(involving two reactants) and include thermal decom-
position, isomerization, and standard collision reac-
tions. Thermal decomposition and isomerization reac-
tions occur when a reactant molecule collides with an
air molecule. The kinetic energy of the collision elevates
the reactant to an energy state high enough that it can
thermally decompose or isomerize.
Thermal decom-
position
occurs when the excited reactant dissociates
into two or more products.
Isomerization
occurs when
the excited reactant changes chemical structure, but not
composition or molecular weight.
An example of a bimolecular
thermal decomposi-
tion reaction
is
In some cases, bimolecular reactions result in
colli-
sion complexes
that ultimately break into products.
Such reactions have the form A
+
AB
∗
→
+
F,
where AB
∗
is a molecule that has weak bonds and is
relatively unstable, and the double arrow indicates that
the reaction is
reversible
.
Termolecular
(involving three reactants) collision
reactions are rare because the probability that three trace
gases collide simultaneously and change form is not
large. For descriptive purposes, however, pairs of reac-
tions can be written as termolecular
combination reac-
tions
.For example, the combination of the bimolecular
kinetic reaction NO
2
(g)
B
D
N
2
O
5
(g)
∗
with the
+
NO
3
(g)
isomerization reaction N
2
O
5
(g)
∗
+
M
N
2
O
5
(g)
+
Mgives
NO
2
(g)
Nitrogen
dioxide
N O
3
(g)
Nitrate
radical
+
+
M
N
2
O
5
(g)
+
M
(1.6)
NO
2
(g)
Nitrogen
dioxide
N O
3
(g)
Nitrate
radical
N
2
O
5
(g)
+
M
→
+
+
M
(1.2)
Dinitrogen
pentoxide
Dinitrogen
pentoxide
In this case, M is any molecule, whose purpose is to
carry away energy released during the reaction. The
purpose of M in Reaction 1.6 differs from its purpose in
Reaction 1.2, where it provided collisional energy for
the reaction. In both cases, M is usually either N
2
(g)
or O
2
(g). Reactions 1.2 and 1.6 are pressure dependent
because the concentration of M is proportional to the air
pressure. Because M in Reaction 1.6 does not change
concentration, Reaction 1.6 can also be written as
where M, which can be any molecule, provides the
collisional energy. Because molecular oxygen [O
2
(g)]
and nitrogen [N
2
(g)] together comprise more than 99
percent of the gas molecules in the air today, M is most
likely tobeO
2
(g) or N
2
(g).
Because M in Reaction 1.2 does not change concen-
tration, the reaction can also be written as
M
→
NO
2
(g)
Nitrogen
dioxide
N O
3
(g)
Nitrate
radical
N
2
O
5
(g)
Dinitrogen
pentoxide
+
+
(1.3)
M
NO
2
(g)
Nitrogen
dioxide
N O
3
(g)
Nitrate
radical
+
N
2
O
5
(g)
Dinitrogen
pentoxide
(1.7)
Thermal decomposition reactions are temperature
dependent. At high temperatures, they proceed faster
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