Lindeman Theory Of Unimolecular Reactions
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| Lindeman Theory Of Unimolecular Reactions |
Lindeman proposed the theory of unimolecular reactions in 1922 which has three major postulates.
Postulates
2. The activated
molecule does not decompose into product immediately. There is a time lag
between activation and decomposition (10-10 to 10-13
seconds).
3. The
molecule remains in activated state for some time, and then there are two
chances.
(i) It may decompose into product
(ii) It may collide with a molecule having less energy and get deactivated.
Rate of
reaction will be of that in which product formation occurs. So
Rate= K3 [A*] e.q.10.1
If it is a short lived or intermediate specie, then it is a difficult task to find its concentration. Hence, it is of little use. To make this equation useful, the short lived specie of reaction must be replaced with a stable specie (reactant or product).
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For that
purpose, we use steady Stale hypothesis: It states that at equilibrium,
the rate of formation of an intermediate species is equal to its rate of decomposition.
So, net rate is zero.
Kinetics of Lindeman theory of unimolecular reactions
Rate of decomposition = R2+R3
=
K2[A][A*] + K3[A*]
At equilibrium rate of formation = rate of decomposition
R1=R2+R3
K1 [A]2= K2[A][A*] + K3[A*]
K1 [A]2= [A*] [K2[A]+K3]
Put this value of [A*]
in e.q.10.1
Rate constant at low and high pressure
|
Pressure |
Collisions |
|
Rate of reaction |
|
low |
Collisions less Concentration less |
K2<<K3 K2[A] negligible |
 |
|
high
|
Collisions high Concentration high |
K2>>K3 K3 negligible |
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