Theories of Reaction Rate

1. THEORIES OF
REACTION RATE
Ashika.G
198205

2. Reaction rate
The rate of the reaction is defined as the change in
concentration of any reactant or product per unit time.
Rate of the Reaction = Rate of disappearance of A = Rate of
appearance of B
Rate = -d[A]/-dt = d[B]/dt
Theories of reaction rate :
1) Collision theory
2) Transition state theory

3. Collision Theory :
 According to this theory, The molecules of reactants are
assumed to be hard spheres and reactions are assumed to
occur only when these hard spheres(molecules) collide with
each other.
 Not all collisions are effective
 Only a small fractions of the collisions produce a reaction.
Two conditions
1) Kinetic energy of the colliding molecules
2) Orientation factor
Kinetic energy of the colliding molecules:
 The molecules must collide with sufficient kinetic energy
A-A + B-B → 2A-B

4.  Chemical reaction occurs by breaking the bonds between the
atoms of reacting molecules and forming the new bonds in the
product molecules.
 The energy comes from the kinetic energy possessed by the
reacting molecules before the collision.
Energy barrier
Ea
A-A
B-B
Energy Reactant
2A-B
Product
Reaction coordinate

5. Ea = activation energy
Activation energy is the minimum energy necessary to
cause the collision between the reacting molecules.
 Molecules with kinetic energy greater than the Ea are able
to get over the energy barrier and react
Effective collision
 Molecules colliding with Kinetic energy less than Ea fail to
get over the barrier the collision between them are
unproductive and the molecules simply bounce off one
another Ineffective collision

6. Orientation factor:
 The molecules must collide with correct orientation.
 Only the molecules with kinetic energy greater than Ea and
with correct orientation can cause reaction.
 The correct orientation is that which ensure direct contact
between the atoms involved in the breaking and forming of
bonds.
Example: H2 + I2 → 2HI

7. + Molecular approach Bond formation +
+ Molecular approach Bond separation
+
H H
I I
H H
I I
I
H
I
H
HH I I H H II
HH II

8. Rate of the reaction = Collision frequency ∙ energy factor ∙
Orientation factor
Collision frequency is the total number of collision per unit
time per unit volume.
Energy factor is the fraction of collision that have sufficient
energy ( ≥Ea )
Orientation factor is the fraction of collision having proper
orientation .
Rate of the reaction = Z ∙ e-Ea/RT ∙ P
Z =2n2σ2 √ᴨkbT/m
n = number of molecules per unit volume
m = mass of the molecules
kb = Boltzman constant
T = temperature
σ = Diameter of colliding molecules ( A and B )

9. Drawbacks of collision theory:
 The theory is only applicable to simple gas where the
reacting molecules are simple molecules.
 The expression from collision theory only applies to simple
bimolecular reactions.
 In this theory only kinetic energy of the reacting molecules
are considered, while the rotational and vibrational
energies are ignored.
 It considers all the atoms to be hard spheres and ignores
their structures.

10. AB# Transition state
Ea
Energy A+B
Reactant
C
Product
Reaction coordinate
Transition state theory
According to this theory, the reactant molecules is first
transformed into intermediate transition state. The
transition state is formed by loose association or bonding of
reactant molecules. The activated complex is unstable and
breaks into the products at a definite rate.

11. The theory suggests that there are three major factors that determine whether a
reaction will occur :
 The concentration of the activated complex.
 The rate at which the activated complex breaks apart.
 whether it breaks apart to reform the reactants or whether it breaks apart to
form a new complex, the products.
Consider a reaction
A+B →C
The rate law is given by, Rate = k[A][B] (1)
Where k is rate constant. According to activated state model Transition state AB#
is formed
K# k #
A+B [AB]# C
Rate =k#[AB]#
k# is the rate constant of activated complex

12. K# = [AB]#/[A][B]
Rate of the reaction depends on the two factors
1)concentration activated complex and 2) Frequency of
vibration of activated complex that converts activated
complex to products. Hence
Rate= ν[AB#]
Rate= ν[A][B]K#
where, ν = kbT/h
Kb = Boltzman constant
h = planck’s constant
Therefore ,
Rate = K#[A][B]∙kbT/h (2)

13. Comparing equation (1) and (2)
k[A][B]= K#[A][B]∙kbT/h
k = kbT/h∙K#

14. Difference between transition state theory and collision
theory :
Collision theory Transition state
theory
Describes the collision of gas
molecules in gas phase with
sufficient kinetic energy.
Explains reaction rates by
assuming the formation of
intermediate compound that are
transition state
States chemical reaction occur
due to collision between
reactants
States that chemical reaction
occur by going through
transition state.
The collision theory used to
predict the rates of chemical
reactions particularly for gases
TST can be used to determine
the reaction rates of elementary
reactions.

15. References :
 Chemical Kinetics and Reaction dynamics – Santhosh K.
Upadhyay
 Chemical kinetics - Keith J. Laidler
 https://chem.libretexts.org.