Unit
Objectives
4
Chemical Kinetics
After studying this Unit, you will be
able to
• define the average and Chemical Kinetics helps us to understand how chemical reactions
instantaneous rate of a reaction; occur.
• express the rate of a reaction in
terms of change in concentration Chemistry, by its very nature, is concerned with change.
of either of the reactants or
Substances with well defined properties are converted
products with time;
by chemical reactions into other substances with
• distinguish between elementary
different properties. For any chemical reaction, chemists
and complex reactions;
try to find out
• differentiate between the
molecularity and order of a (a) the feasibility of a chemical reaction which can be
reaction; predicted by thermodynamics ( as you know that a
• define rate constant; reaction with ∆G < 0, at constant temperature and
• discuss the dependence of rate of pressure is feasible);
reactions on concentration, (b) extent to which a reaction will proceed can be
temperature and catalyst; determined from chemical equilibrium;
• derive integrated rate equations (c) speed of a reaction i.e. time taken by a reaction to
for the zero and first order reach equilibrium.
reactions;
Along with feasibility and extent, it is equally
• determine the rate constants for
zeroth and first order reactions;
important to know the rate and the factors controlling
the rate of a chemical reaction for its complete
• describe collision theory.
understanding. For example, which parameters
determine as to how rapidly food gets spoiled? How
to design a rapidly setting material for dental filling?
Or what controls the rate at which fuel burns in an
auto engine? All these questions can be answered by
the branch of chemistry, which deals with the study
of reaction rates and their mechanisms, called
chemical kinetics. The word kinetics is derived from
the Greek word ‘kinesis’ meaning movement.
Thermodynamics tells only about the feasibility of a
reaction whereas chemical kinetics tells about the rate
of a reaction. For example, thermodynamic data
indicate that diamond shall convert to graphite but
in reality the conversion rate is so slow that the change
is not perceptible at all. Therefore, most people think
2022-23
, that diamond is forever. Kinetic studies not only help us to determine
the speed or rate of a chemical reaction but also describe the
conditions by which the reaction rates can be altered. The factors
such as concentration, temperature, pressure and catalyst affect the
rate of a reaction. At the macroscopic level, we are interested in
amounts reacted or formed and the rates of their consumption or
formation. At the molecular level, the reaction mechanisms involving
orientation and energy of molecules undergoing collisions,
are discussed.
In this Unit, we shall be dealing with average and instantaneous
rate of reaction and the factors affecting these. Some elementary
ideas about the collision theory of reaction rates are also given.
However, in order to understand all these, let us first learn about the
reaction rate.
4 . 1 Rate of a Some reactions such as ionic reactions occur very fast, for example,
Chemical precipitation of silver chloride occurs instantaneously by mixing of
aqueous solutions of silver nitrate and sodium chloride. On the other
Reaction hand, some reactions are very slow, for example, rusting of iron in
the presence of air and moisture. Also there are reactions like inversion
of cane sugar and hydrolysis of starch, which proceed with a moderate
speed. Can you think of more examples from each category?
You must be knowing that speed of an automobile is expressed in
terms of change in the position or distance covered by it in a certain
period of time. Similarly, the speed of a reaction or the rate of a
reaction can be defined as the change in concentration of a reactant
or product in unit time. To be more specific, it can be expressed in
terms of:
(i) the rate of decrease in concentration of any one of the
reactants, or
(ii) the rate of increase in concentration of any one of the products.
Consider a hypothetical reaction, assuming that the volume of the
system remains constant.
R →P
One mole of the reactant R produces one mole of the product P. If
[R]1 and [P]1 are the concentrations of R and P respectively at time t 1
and [R]2 and [P]2 are their concentrations at time t2 then,
∆t = t2 – t1
∆[R] = [R]2 – [R]1
∆ [P] = [P]2 – [P]1
The square brackets in the above expressions are used to express
molar concentration.
Rate of disappearance of R
Decrease in concentration of R ∆ [R ]
= =− (4.1)
Time taken ∆t
Chemistry 96
2022-23
, Rate of appearance of P
Increase in concentration of P ∆ [P ]
= =+ (4.2)
Time taken ∆t
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.
Equations (4.1) and (4.2) given above represent the average rate of
a reaction, rav.
Average rate depends upon the change in concentration of reactants
or products and the time taken for that change to occur (Fig. 4.1).
{ }
Fig. 4.1: Instantaneous and average rate of a reaction
Units of rate of a reaction
From equations (4.1) and (4.2), it is clear that units of rate are
concentration time–1. For example, if concentration is in mol L–1 and
time is in seconds then the units will be mol L-1s–1. However, in gaseous
reactions, when the concentration of gases is expressed in terms of their
partial pressures, then the units of the rate equation will be atm s–1.
From the concentrations of C4H9Cl (butyl chloride) at different times given Example 4.1
below, calculate the average rate of the reaction:
C4H9Cl + H2O → C4H9OH + HCl
during different intervals of time.
t/s 0 50 100 150 200 300 400 700 800
[C4H9Cl]/mol L–1 0.100 0.0905 0.0820 0.0741 0.0671 0.0549 0.0439 0.0210 0.017
We can determine the difference in concentration over different intervals Solution
of time and thus determine the average rate by dividing ∆[R] by ∆t
(Table 4.1).
97 Chemical Kinetics
2022-23
Objectives
4
Chemical Kinetics
After studying this Unit, you will be
able to
• define the average and Chemical Kinetics helps us to understand how chemical reactions
instantaneous rate of a reaction; occur.
• express the rate of a reaction in
terms of change in concentration Chemistry, by its very nature, is concerned with change.
of either of the reactants or
Substances with well defined properties are converted
products with time;
by chemical reactions into other substances with
• distinguish between elementary
different properties. For any chemical reaction, chemists
and complex reactions;
try to find out
• differentiate between the
molecularity and order of a (a) the feasibility of a chemical reaction which can be
reaction; predicted by thermodynamics ( as you know that a
• define rate constant; reaction with ∆G < 0, at constant temperature and
• discuss the dependence of rate of pressure is feasible);
reactions on concentration, (b) extent to which a reaction will proceed can be
temperature and catalyst; determined from chemical equilibrium;
• derive integrated rate equations (c) speed of a reaction i.e. time taken by a reaction to
for the zero and first order reach equilibrium.
reactions;
Along with feasibility and extent, it is equally
• determine the rate constants for
zeroth and first order reactions;
important to know the rate and the factors controlling
the rate of a chemical reaction for its complete
• describe collision theory.
understanding. For example, which parameters
determine as to how rapidly food gets spoiled? How
to design a rapidly setting material for dental filling?
Or what controls the rate at which fuel burns in an
auto engine? All these questions can be answered by
the branch of chemistry, which deals with the study
of reaction rates and their mechanisms, called
chemical kinetics. The word kinetics is derived from
the Greek word ‘kinesis’ meaning movement.
Thermodynamics tells only about the feasibility of a
reaction whereas chemical kinetics tells about the rate
of a reaction. For example, thermodynamic data
indicate that diamond shall convert to graphite but
in reality the conversion rate is so slow that the change
is not perceptible at all. Therefore, most people think
2022-23
, that diamond is forever. Kinetic studies not only help us to determine
the speed or rate of a chemical reaction but also describe the
conditions by which the reaction rates can be altered. The factors
such as concentration, temperature, pressure and catalyst affect the
rate of a reaction. At the macroscopic level, we are interested in
amounts reacted or formed and the rates of their consumption or
formation. At the molecular level, the reaction mechanisms involving
orientation and energy of molecules undergoing collisions,
are discussed.
In this Unit, we shall be dealing with average and instantaneous
rate of reaction and the factors affecting these. Some elementary
ideas about the collision theory of reaction rates are also given.
However, in order to understand all these, let us first learn about the
reaction rate.
4 . 1 Rate of a Some reactions such as ionic reactions occur very fast, for example,
Chemical precipitation of silver chloride occurs instantaneously by mixing of
aqueous solutions of silver nitrate and sodium chloride. On the other
Reaction hand, some reactions are very slow, for example, rusting of iron in
the presence of air and moisture. Also there are reactions like inversion
of cane sugar and hydrolysis of starch, which proceed with a moderate
speed. Can you think of more examples from each category?
You must be knowing that speed of an automobile is expressed in
terms of change in the position or distance covered by it in a certain
period of time. Similarly, the speed of a reaction or the rate of a
reaction can be defined as the change in concentration of a reactant
or product in unit time. To be more specific, it can be expressed in
terms of:
(i) the rate of decrease in concentration of any one of the
reactants, or
(ii) the rate of increase in concentration of any one of the products.
Consider a hypothetical reaction, assuming that the volume of the
system remains constant.
R →P
One mole of the reactant R produces one mole of the product P. If
[R]1 and [P]1 are the concentrations of R and P respectively at time t 1
and [R]2 and [P]2 are their concentrations at time t2 then,
∆t = t2 – t1
∆[R] = [R]2 – [R]1
∆ [P] = [P]2 – [P]1
The square brackets in the above expressions are used to express
molar concentration.
Rate of disappearance of R
Decrease in concentration of R ∆ [R ]
= =− (4.1)
Time taken ∆t
Chemistry 96
2022-23
, Rate of appearance of P
Increase in concentration of P ∆ [P ]
= =+ (4.2)
Time taken ∆t
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.
Equations (4.1) and (4.2) given above represent the average rate of
a reaction, rav.
Average rate depends upon the change in concentration of reactants
or products and the time taken for that change to occur (Fig. 4.1).
{ }
Fig. 4.1: Instantaneous and average rate of a reaction
Units of rate of a reaction
From equations (4.1) and (4.2), it is clear that units of rate are
concentration time–1. For example, if concentration is in mol L–1 and
time is in seconds then the units will be mol L-1s–1. However, in gaseous
reactions, when the concentration of gases is expressed in terms of their
partial pressures, then the units of the rate equation will be atm s–1.
From the concentrations of C4H9Cl (butyl chloride) at different times given Example 4.1
below, calculate the average rate of the reaction:
C4H9Cl + H2O → C4H9OH + HCl
during different intervals of time.
t/s 0 50 100 150 200 300 400 700 800
[C4H9Cl]/mol L–1 0.100 0.0905 0.0820 0.0741 0.0671 0.0549 0.0439 0.0210 0.017
We can determine the difference in concentration over different intervals Solution
of time and thus determine the average rate by dividing ∆[R] by ∆t
(Table 4.1).
97 Chemical Kinetics
2022-23
