It is clear that if the temperature interval in which one has measured this
dependence is small, then one gets a straight line. It is also clear that in sufficiently
large temperature ranges this simple depende once cannot be observed. The reason
is that we got this dependence within very simple assumptions. An example would
be recombination reactions with a positive activation energy, for example,
Oð
3 PÞ þ CO !
M CO 2 e
X
1
R
þ
g
:
ð2:3:11Þ
At temperatures of about 1000 K and below, the activation energy of this
reaction, due to the presence of a potential barrier at the reaction coordinate, is
positive, *3 kcal/mol. At very high temperatures, obtained in shock tubes, when
the average energy of the species exceeds the height of this potential barrier, this
reaction behaves as ordinary termolecular with negative activation energy.
The representation of the Arrhenius equation even in a rather narrow temperature range
at high activation energies and often even at small ones is a matter of taste: one writes
k ¼ A Á expðÀE a =RTÞ, k ¼ A Á T
1=2 expðÀE a =RTÞ, k ¼ A Á T
n exp ðÀE a =RTÞ.
Getting used to this is necessary.
2.4 Complex Reactions. Consecutive Reactions.
Steady-State Method
As mentioned in Sect. 2.1, there are complex, simple processes (reactions) and
elementary processes. Let the author reminds that if the stoichiometric order of the
process does not coincide with the kinetic, determined in the experiment, then the
process is called complex; if it is, then it is simple. If a simple process proceeds in
one stage, then this is an elementary process. Most often, simple processes do not
proceed in one stage but are a kind of multistage process. The complex reactions are
multistage process, indeed.
The author will not discuss in detail the different types of complex chemical
reactions, since readers who research or study in the fields of molecular
spectroscopy/molecular physics, as well as chemical physics/physical chemistry in
the gas-phase they are rarely encountered. They will be mentioned in passing.
So, distinguish:
– Coupled reactions, the features of which lie in the fact that one of the reactions
A + B 2 can take place only in the presence of the second A + B 1 . The reason is
that the product of one reaction is one of the other reactant:
A þ B 1 ! X þ other products
ð2:4:1Þ
X þ B 2 ! C þ other products
ð2:4:2Þ
28
2 General Kinetic Rules for Chemical Reactions, Collisional …
dependence is small, then one gets a straight line. It is also clear that in sufficiently
large temperature ranges this simple depende once cannot be observed. The reason
is that we got this dependence within very simple assumptions. An example would
be recombination reactions with a positive activation energy, for example,
Oð
3 PÞ þ CO !
M CO 2 e
X
1
R
þ
g
:
ð2:3:11Þ
At temperatures of about 1000 K and below, the activation energy of this
reaction, due to the presence of a potential barrier at the reaction coordinate, is
positive, *3 kcal/mol. At very high temperatures, obtained in shock tubes, when
the average energy of the species exceeds the height of this potential barrier, this
reaction behaves as ordinary termolecular with negative activation energy.
The representation of the Arrhenius equation even in a rather narrow temperature range
at high activation energies and often even at small ones is a matter of taste: one writes
k ¼ A Á expðÀE a =RTÞ, k ¼ A Á T
1=2 expðÀE a =RTÞ, k ¼ A Á T
n exp ðÀE a =RTÞ.
Getting used to this is necessary.
2.4 Complex Reactions. Consecutive Reactions.
Steady-State Method
As mentioned in Sect. 2.1, there are complex, simple processes (reactions) and
elementary processes. Let the author reminds that if the stoichiometric order of the
process does not coincide with the kinetic, determined in the experiment, then the
process is called complex; if it is, then it is simple. If a simple process proceeds in
one stage, then this is an elementary process. Most often, simple processes do not
proceed in one stage but are a kind of multistage process. The complex reactions are
multistage process, indeed.
The author will not discuss in detail the different types of complex chemical
reactions, since readers who research or study in the fields of molecular
spectroscopy/molecular physics, as well as chemical physics/physical chemistry in
the gas-phase they are rarely encountered. They will be mentioned in passing.
So, distinguish:
– Coupled reactions, the features of which lie in the fact that one of the reactions
A + B 2 can take place only in the presence of the second A + B 1 . The reason is
that the product of one reaction is one of the other reactant:
A þ B 1 ! X þ other products
ð2:4:1Þ
X þ B 2 ! C þ other products
ð2:4:2Þ
28
2 General Kinetic Rules for Chemical Reactions, Collisional …