This document discusses theories of unimolecular reaction kinetics, including the Lindemann-Christiansen theory, Hinshelwood theory, RRK theory, and RRKM theory. It notes limitations of earlier theories in explaining experimental data. The RRKM theory, developed by Marcus in 1951-1952, redefined the RRK treatment and addressed prior limitations. RRKM theory is now widely used to interpret thermal and photochemical reactions and allows calculating reaction rates from known vibrational frequencies of molecules.

1. 1
Dr. Y. S. THAKARE
M.Sc. (CHE) Ph D, NET, SET
Assistant Professor in Chemistry,
Shri Shivaji Science College, Amravati
Email: yogitathakare_2007@rediffmail.com
CHEMICAL DYNAMICS
RRK AND RRKM THEORY

2. Arrhenius Equation
k: rate constant
Ea: activation energy (minimum required)
T: absolute temperature
R: universal gas constant=8.314JK-1mol-1
A: orientation factor or Arrhenius constant
Frequency factor or pre-exponential factor
Energy & orientation requirements for reaction
RT
Ea
Ae
k



4. Van-Hoff Rule:
10
1
2
1
2
t
t
t
t
t
k
k 

 


If t2t1
t
t
k
k 10



Temperature coefficient:
The ratio of rate constants of a reaction at two different temperatures
separated by 10°C'

5. THEORIES OF UNIMOLECULAR REACTION
OR
LINDEMANN THEORY
Lindemann (1922)

8. Limitation of Lindemann-Christiansen Theory and its explanation

12. Limitations or Drawbacks of Hinshelwood Theory
1) It does not gives satisfactory explanation for the observed variation of the rate
constant with pressure
2) This failure may be in larger & rather unrealistic assumption that the activation
energy per molecule may be distributed among all or many of the vibrational mode of
the molecule & that excess if energy in this bond above the activation energy does not
increase the probability of reaction.
O.K. Rice
H.C.
Rampsberger
L.S. Kassel,
( 1928)

17. Limitation of RRK Theory

18. Born March 15, 1930
He completed his Ph.D. in 1953 under Nobel
Laureate LINUS PAULING
Prof. Martin Karplus
Nobel Laureate 2013
Marcus’s Extension of the RRK (Rice-Rampsberger-Kassel) Treatment
OR
RRKM Theory
1951-1952 Marcus redefined the RRK theory

21. Applications of RRKM Theory
1) used to interpret the behaviour of thermal and photochemical
reactions.
2) Thermal unimolecular reaction usually exhibit first order
kinetics at high pressures.
3) It is widely used and is valid for the chosen model of a set of
coupled, classical harmonic oscillators
4) The reaction rate was obtained by summing over each of the
accessible quantum state of the transition complex
5) It is a great advance over previous theory because it can be
applied it real molecule, counting the states from the known
vibrational frequencies.