Revision Notes
Class – 12 Chemistry
Chapter 4 – Chemical Kinetics
INTRODUCTION
Chemical Kinetics
(Kinesis: Movement)
The branch of chemistry which deals with the study of the rates of chemical
reactions, the factors affecting the rates of the reactions and the mechanism by
which the reactions proceed is called Chemical Kinetics.
Classification of reactions
On the basis of rates:
Very fast reactions e.g. precipitation of AgCl
Very slow reactions e.g. rusting of iron
Reactions taking place at moderate speeds e.g. hydrolysis of starch
1. RATE OF A CHEMICAL REACTION
The rate of a reaction can be defined as the change in concentration of a reactant
or a product in unit time.
1.1 Average rate
The rate of reaction measured over a definite time interval is called average rate of
a reaction. Consider a hypothetical reaction,
R P
Class XII Chemistry www.vedantu.com 1
, Average rate of reaction =
Decrease in concentration of R
Time taken
=
R
t
Or
Average rate of reaction =
Increase in concentration of P
Time taken
=
P
t
1.2 Instantaneous rate
Rate of change of concentration of any one of the reactants or products at that
particular instant of time is called instantaneous rate.
As
Δt 0
or
-d R d P
rinst = =
dt dt
IMPORTANT: Rate of a reaction is always positive. Since, [R] is a negative
quantity (as concentration of reactants is decreasing), it is multiplied with –1 to
make the rate of the reaction a positive quantity.
1.3 Units of rate of a reaction
Units of rate are concentration time–1
–1 –1 –1
e.g. mol L s ; atm s (for gaseous reactions)
1.4 Overall rate of a reaction
When there are several reactants and products the individual rates of the various
components may differ as they would depend on the stoichiometric coefficients.
Class XII Chemistry www.vedantu.com 2
, For a reaction,
A + 2B 3C + 4D
Rate of disappearance of B = 2 × Rate of disappearance of A (2:1)
Rate of formation of C = 3 × Rate of disappearance of A
(3:1)
Rate of formation of D = 4 × Rate of disappearance of A
(4:1)
To define a unique value for the overall rate of the reaction we divide the individual
rates by the respective coefficients and equate their signs.
Overall Rate = – A / t = –1/2 B/ t = +1/3 C/ t
= +1/4 D / t
2. DEPENDENCE OF RATE ON CONCENTRATION
Factors Influencing Rate of a Reaction
concentration of reactants (pressure in case of gases),
temperature and
catalyst.
Dependence on Concentration:
2.1 Rate law
Consider a general reaction
aA + bB cC + dD
The rate expression for this reaction is
Class XII Chemistry www.vedantu.com 3
Class – 12 Chemistry
Chapter 4 – Chemical Kinetics
INTRODUCTION
Chemical Kinetics
(Kinesis: Movement)
The branch of chemistry which deals with the study of the rates of chemical
reactions, the factors affecting the rates of the reactions and the mechanism by
which the reactions proceed is called Chemical Kinetics.
Classification of reactions
On the basis of rates:
Very fast reactions e.g. precipitation of AgCl
Very slow reactions e.g. rusting of iron
Reactions taking place at moderate speeds e.g. hydrolysis of starch
1. RATE OF A CHEMICAL REACTION
The rate of a reaction can be defined as the change in concentration of a reactant
or a product in unit time.
1.1 Average rate
The rate of reaction measured over a definite time interval is called average rate of
a reaction. Consider a hypothetical reaction,
R P
Class XII Chemistry www.vedantu.com 1
, Average rate of reaction =
Decrease in concentration of R
Time taken
=
R
t
Or
Average rate of reaction =
Increase in concentration of P
Time taken
=
P
t
1.2 Instantaneous rate
Rate of change of concentration of any one of the reactants or products at that
particular instant of time is called instantaneous rate.
As
Δt 0
or
-d R d P
rinst = =
dt dt
IMPORTANT: Rate of a reaction is always positive. Since, [R] is a negative
quantity (as concentration of reactants is decreasing), it is multiplied with –1 to
make the rate of the reaction a positive quantity.
1.3 Units of rate of a reaction
Units of rate are concentration time–1
–1 –1 –1
e.g. mol L s ; atm s (for gaseous reactions)
1.4 Overall rate of a reaction
When there are several reactants and products the individual rates of the various
components may differ as they would depend on the stoichiometric coefficients.
Class XII Chemistry www.vedantu.com 2
, For a reaction,
A + 2B 3C + 4D
Rate of disappearance of B = 2 × Rate of disappearance of A (2:1)
Rate of formation of C = 3 × Rate of disappearance of A
(3:1)
Rate of formation of D = 4 × Rate of disappearance of A
(4:1)
To define a unique value for the overall rate of the reaction we divide the individual
rates by the respective coefficients and equate their signs.
Overall Rate = – A / t = –1/2 B/ t = +1/3 C/ t
= +1/4 D / t
2. DEPENDENCE OF RATE ON CONCENTRATION
Factors Influencing Rate of a Reaction
concentration of reactants (pressure in case of gases),
temperature and
catalyst.
Dependence on Concentration:
2.1 Rate law
Consider a general reaction
aA + bB cC + dD
The rate expression for this reaction is
Class XII Chemistry www.vedantu.com 3
