1. The document discusses theories of chemical kinetics including collision theory and the effect of temperature on reaction rates. 2. It introduces the Arrhenius equation and the concept of activation energy, which is the minimum energy required for a reaction to occur. 3. Collision theory states that molecules must collide with sufficient energy and proper orientation to overcome the activation energy barrier for an effective reaction.

1. TEJASVI NAVADHITAMASTU
“Let our (the teacher and the taught) learning be radiant”
Let our efforts at learning be luminous and filled with joy, and endowed with the force of purpose
Paper III: PHYSICAL CHEMISTRY
Dr. Prabhakar Singh. D.Phil. Biochemistry
Department of Biochemistry, VBSPU, Jaunpur
Unit-III: A. Chemical Kinetics Theories of chemical kinetics effect of temperature on rate of
reaction, Arrhenius equation, concept of activation energy. Simple collision theory based on hard
sphere model, transition state theory (equilibrium hypothesis), Expression for the rate constant
based on equilibrium constant and thermodynamic aspects.

2. Chemical kinetics is the study of the rates of chemical reactions or how fast reactions occur. The primary
requirement for a reaction to occur is that the reactant particles (atoms or molecules) must collide and
interact with each other in some way. This is the central idea of the collision model, which is used to
explain many of the observations made about chemical kinetics.
Collision theory states that the rate of a chemical reaction is proportional to the number of collisions
between reactant molecules. The more often reactant molecules collide, the more often they react with
one another, and the faster the reaction rate. In reality, only a small fraction of the collisions are effective
collisions. Effective collisions are those that result in a chemical reaction.
In order to produce an effective collision, reactant particles must possess some minimum amount of
energy. This energy, used to initiate the reaction, is called the activation energy. For every sample of
reactant particles there will be some that possess this amount of energy. The larger the sample, the
greater the number of effective collisions, and the faster the rate of reaction. The number of particles
possessing enough energy is dependent on the temperature of the reactants. If reactant particles do not
possess the required activation energy when they collide, they bounce off each other without reacting.
Some chemical reactions also require that the reactant particles be in a particular orientation to produce
an effective collision. Unless the reactant particles possess this orientation when they collide, the
collision will not be an effective one. The reaction of ozone with nitrogen monoxide is an example of how
orientation can be important. To summarize, the requirements for an effective collision (for a chemical
reaction to occur):
1. The reactants must collide with each other.
2. The molecules must have sufficient energy to initiate the reaction (called activation energy).
3. The molecules must have the proper orientation.
CHEMICAL KINETICS THEORY

3. EFFECT OF INCREASE OF TEMPERATURE ON
REACTION RATE

4. ARRHENIUS EQUATION

6. CONCEPT OF ACTIVATION ENERGY
On the other hand, if the potential
energy of the products is greater
than that of the reactants, the
energy released in going from the
activated complex to products will
be less than the activation
energy and the reaction will be
endothermic

8. Ea, that is the minimum energy necessary to cause
a reaction between the colliding molecules. Only the
molecules that collide with a kinetic energy greater
than Ea, are able to get over the barrier and react. The
molecules colliding with kinetic energies less that Ea
fail to surmount the barrier. The collisions between
them are unproductive and the molecules simply
bounce off one another.
COLLISION THEORY

14. Expression for the rate constant based on equilibrium
constant and thermodynamic aspects