2. A chain reaction is one where an intermediate formed in one step
generates an intermediate in a subsequent step, then that
intermediate generates another intermediate, and so on. The intermediates
in a chain reaction are called chain carriers.
Chain Reaction:
In radical chain reactions, radicals (i.e. species with unpaired electrons) act
as carriers. Other carriers include neutrons (in nuclear fission) and ions.
- the step where the chain carriers are initially formed.
Example: Cl atoms are formed by the dissociation of Cl2 molecules either:
1.through vigorous intermolecular collisions in a thermolysis, i.e. a
reaction initiated by heat, or,
2.through absorption of a photon in a photolysis , i.e. a reaction
stimulated by the absorption of a electromagnetic radiation.
The initiation step
3. The steps where chain carriers produced in the initiation step attack
other reactant molecules, each attach results in the formation of a new
carrier.
Propagation steps
Steps which result the attach of a chain carrier on a reactant molecule
produces more than one chain carrier.
Example:
Branching steps
Steps in which a chain carrier attack a product molecule formed earlier in
the reaction, thus reducing the net rate of product formation
Retardation steps
Example: The attack of a methyl radical on ethane:
4. A step where carriers combine and end the chain.
Example: The attack of a methyl radical on ethane:
Termination step
A step where carriers are removed by a process other then termination (i.e.
not by having two carriers combining and ending the chain.)
Inhibition step
Possible ways for inhibition include reaction of the chain carriers with the walls
of the reaction vessel or foreign species.
E.g.:
5. The rate laws of chain reactions:
The Rice-Herzfeld mechanism for the pyrolysis of ethanal in
the absence of air.
The pyrolysis (i.e. thermal decomposition) of ethanal (acetaldehyde,
CH3CHO) is found to exhibit a simple rate law where:
This reaction proceeds according the Rice-Herzfeld mechanism where:
6. According to steady state approximation, the net rate change of the
intermediates may be set to zero, i.e.:
0
]
CH
[
]
CO
CH
[
]
CH
[
]
CHO
CH
[
]
CHO
CH
[
dt
]
CH
d[
) 2
3
3
3
3
3
3
d
c
b
a k
k
k
k
i
0
]
CO
CH
[
]
CH
[
]
CHO
CH
[
dt
]
CO
d[CH
) 3
3
3
3
c
b k
k
ii
Eq. (i) + Eq. (ii):
0
]
CH
[
]
CHO
CH
[ 2
3
3
d
a k
k
Therefore, the steady state concentration of the methyl radical is:
2
/
1
3
3 ]
CHO
CH
[
]
CH
[
d
a
k
k
7. According to eq.(b), the rate of formation of methane (i.e. the rate of
the overall reaction ) is :
2
/
1
3
3 ]
CHO
CH
[
]
CH
[
d
a
k
k
]
CH
[
]
CHO
CH
[
dt
]
d[CH
3
3
4
b
k
]
CHO
CH
[
dt
]
d[CH 2
/
3
3
4
d
a
b
k
k
k
Q. Write down the Rice-Herzfeld mechanism for the decomposition of
acetaldehyde and show that the order of this reaction is three halves.
