12

1. Chemical kinetics

2. From this chapter in board exam
• Either 12 marks or 7 marks
• 5 marks numerical sure

3. Chemical kinetics:
The branch of chemistry which deal with the study of speeds or rates of
chemical reaction, the factors affecting the rate of the reaction and the
mechanism by which the reaction proceed is known as chemical
kinetics.

4. Concept of the reaction rate:
The rate of the reaction is a measure of the speed with which reactants
are converted into products.
In terms of amount, the rate of the reaction is defined as the change in
amount of reactant or product per unit time.
Mathematically,
Rate of the reaction=
change in amount of reactants or products
time taken for the changed

5. In terms of concentration, the rate of the reaction is defined as the
change in the concentration of the reactant( or product) per unit time.
Rate of reaction=
change in concentration of reactants or products
time taken for change
In this case, unit of the rate is molL-1time-1 i.e molL-1s-1 or molL-1min-1
or molL-1hr-1.

6. Therefore rate of reaction is defined as the rate of disappearance (or
decrease) in the concentration of the reactants or the rate of appearance
(or increase) in the concentration of the products.
For any hypothetical reaction,
A → B
Rate of reaction=
decrease disappearance in concentration of reactants(A)
time taken
or, =
increase appearance in concentration of products(B)
time taken
Or, rate of the reaction = -
∆[A]
∆T
= +
∆[B]
∆T
=
change in concentration
time needed for the change

7. Types of the reaction:
Depending upon the time interval over when the change in amount of
reactant or product is measured the reaction rates can be classified
into two main classes:
1. Average rate of the reaction
2. Instantaneous rate of the reaction

8. 1. Average rate of the reaction:
It is defined as the rate of change of concentration of the reactants (or
products) over a specified measurable period of time.
For a reaction, A → B
If [A]1 and [A]2 be the concentration of the reactants A at time t1 and t2
respectively, then the rate of the reaction in terms of the reactant in
time interval (t1 – t2) is given by,
Average rate of reaction =
change in concentration of A
time interval
=
A 2
− A 1
t2
−t1
= -
∆[A]
∆t

9. Here [A]1 > [A]2
Therefore negative is used.
Similarly, if [B]1 and [B]2 be the concentration of product B at time t1 and t2
respectively, then the rate of reaction in terms of product B is given by,
Average rate of the reaction =
change in concentration of B (products)
time interval
=
B 2
− B 1
t2
−t1
= +
∆[B]
∆t
Here,[B]2 > [B]1, therefore positive sign is used
Figure……….. in board

10. 2. Instantaneous rate of the reaction:
It is defined as the rate of change in concentration of any one of the reactant or
product at particular time. It gives the tendency of the reaction at a particular point
of time during the course.
Mathematically,
Instantaneous rate =[Average rate]∆t → 0
𝑑𝑥
𝑑𝑡
=
Where dx= very small change in concentration
dt= very small change in time
For instantaneous rate,
t1 ≈ t2
∆t → 0.

11. Calculation of instantaneous rate of reaction:
To know the rate of the reaction at any time , a tangent is drawn to the
curve at the point corresponding to that time and it is extended on
either side so as to cut the axes. Then
Rate of reaction=
change in concentration
time
Figure ……………….in board

12. Expressing the rate of the reaction:
Example,
aA + bB → cC + dD
The expression for the rate of the reaction is
Rate, R = -
1
a
d[A]
dt
= -
1
b
d[B]
dt
= +
1
c
d[C]
dt
= +
1
d
d[D]
dt
Rate of disappearance of A, -
𝐝[𝐀]
𝐝𝐭
= a × rate of the reaction
Rate of disappearance of B, -
𝐝[𝐁]
𝐝𝐭
= b × rate of the reaction
Rate of appearance of C, +
𝐝[𝐂]
𝐝𝐭
= c × rate of the reaction
Rate of appearance of D, +
𝐝[𝐃]
𝐝𝐭
= d × rate of the reaction

13. Numericalfromelementarychemicalcalculations

14. Rate law and rate law equation:
For a general reaction,
aA + bB → products
Then according to the law of mass action,
Rate ∝ A a B b
Or, rate = K[A]a[B]b where K is constant called velocity constant.
However, experimentally is observed that the rate of this reaction may
or may not depend upon all the ‘a’ concentration terms A and all the ‘b’
concentration terms of B.
Suppose, experimentally the rate of the reaction is found to depend
upon ‘x’ concentration terms of A and ‘y’ concentration terms of B.

15. Then we may write,
Rate ∝ A x B y where, x is the order with respect to A and y is the order
with respect to B.
Rate = K[A]x[B]y…………………..(i)
Where [A] and [B] are the molar concentration of A and B respectively and K
is constant called rate constant. The above expression is called rate law.
The rate law is defined as “ The mathematical expression which represents
the experimentally measured rate of the reaction in terms of the
concentration of the reacting species which influence the rate of the
reaction.”

16. Again, if all concentrations are taken as unity i.e.
[A]=[B]= 1 mol/L
Then rate = K(rate constant)
Hence, rate constant ,may be defined as the rate of the reaction when
the concentration of each reactant is taken as unity.

17. Characteristics of the rate constant(K)
i. Rate constant is a measure of the rate of reaction. Higher the value
of rate constant faster is the reaction.
ii. The value of rate constant ‘K’ is different for different reactions.
Hence it is also called specific constant.
iii. For a particular reaction, the value of rate constant is independent
of the concentration of the reactants.(zero order reaction)

18. Order of the reaction:
The sum of the concentration terms on which the rate of a reaction
actually depends as observed experimentally is called order of the
reaction.
Or,
The power to which the concentration of the reactants is raised in the
rate law equation is called order of the reaction.
Examples ,let us consider a general reaction,
mA + nB → product
………….in board

19. Differences between rate of reaction and rate
constant.
Rate of reaction Rate constant
1. It is the change in concentration of the
reactant or product per unit time.
1. It is a constant of proportionality in the rate
law equation and is equal to the rate of the
reaction when the molar concentration of
each reactants is unity.
2. The rate of reaction at any instant of time
depends upon the molar concentration.
2. The rate constant is constant for a
particular reaction at a particular
temperature and may or may not depend
upon the concentration of the reactants.
3. Its units are always molL-1time-1 3. Its units depend upon the order of the
reaction

20. Molecularity of a reaction:
The number of atoms, ions or molecules that must collide with one
another simultaneously so as to result into a chemical reaction. Here
molecularity of a reaction is simply the number of reacting species
taking part in a simple reaction i.e. single step reaction. For example,
1. Decomposition of O2F2: The balanced equation is
O2F2 → O2 + F2 Hence the molecularity of the reaction
is 1 and is called unimolecular.
2. Dissociation of HI: The balanced equation is
2HI → H2 + I2 Hence the molecularity is 2 and the
reaction is called bimolecular reaction.

21. 3. Reaction between NO and O2: The balanced equation is
2NO + O2 → 2NO2 Hence the molecularity is 3 and the
reaction is called termolecular.
However there are many reactions in which all the atoms, ions or molecules
as represented in the balanced chemical reaction may not come together to
collide simultaneously. For example in the reaction
5Br- + BrO3
- + 6H+ → 3Br2 + 3H2
It is impossible for all 12 ions of the reactant to come together
simultaneously to collide. These reactions are supposed to take part in a
sequence of number of steps. Each of these step reactions is called
elementary reaction. Molecularity can be defined only for an elementary
reaction. It has no meaning for the complex reactions.

22. Differences between molecularity and order of
the reaction: (VVI)
Molecularity of the reaction Order of the reaction
1. It is the number of reacting species taking
part in a simple reaction.
2. It is the theoretical value determined from
the balanced single step reaction.
1. It is the sum of the powers of the
concentration terms in the rate law equation.
2. It is the experimentally determined value.
3. It has natural number values i.e. 1, 2, 3 etc. 3. Order of the reaction can be fractional values
also.
4. Molecularity of the reaction cannot be zero. 4. It can be zero
5. Molecularity of a complex reaction has no
meaning. It is expressed for each elementary
step.
5. Order is same for the whole reaction no
matter it is simple or a complex reaction.

23. Pseudo-order reaction:
The reactions which appear to be higher order but follow lower order
kinetics are called pseudo-order reactions. In case of elementary reactions
(which complete in one step), the order and molecularity are identical. Let us
consider a general reaction.
A + B → Products
In which the reactant B is present in a large excess. Since it is elementary, its
rate law can be written as
Rate = K[A][B]
As B is present in large excess, its concentration remains practically constant
during the course of the reaction. Thus the rate law can be written as
Rate = K’[A]

24. Where the new rate constant K’=K[B]. Thus the actual order of the reaction is
second order but in practice it is of first order. Therefore the reaction is said to be
pseudo-first order.
Examples:
a. Hydrolysis of an ester:
CH3COOC2H5 + H2O(Large excess) → CH3COOH + C2H5OH
The rate law for the reaction can be written as
Rate =K[CH3COOC2H5][H2O]
Since water is present in large excess, its concentration remains practically constant
during the course of the reaction. Thus above rate law can be written as
Rate = K’[CH3COOC2H5] The reaction is actually second order but in practice it
follows first- order kinetics. Thus, it is a pseudo-first order reaction.

25. Concept of activation energy:
The excess energy which must be supplied to the reactants to undergo
chemical reactions is called activation energy (Ea).
It is equal to the difference between the threshold energy needed for
the reaction and the average kinetic energy of all the reacting
molecules.
Activation energy= Threshold energy - Average kinetic energy of the
reacting molecules
(The minimum kinetic energy which the colliding molecules must
have in order that the collision between them may be affective is
called threshold energy.)

26. Figure in board:
Activated complex: It is an intermediate state that is formed during the
conversion of reactants into products.

27. According to the concept of activation energy, the reactants do not
change directly into the product. The reactants first absorb energy
equal to the activation energy and thereby form activated complex. At
this state, the molecules have energy at least equal to the threshold
energy. This means that the reaction involves some energy barrier
which must be overcome before products are formed. The energy
barrier is known as activation energy barrier.
Example:

28. Effect of catalyst:
The use of catalyst provides an alternative path for a reaction by
lowering activation energy( positive catalyst) or highering(increasing)
the activation energy(negative catalyst).
Figure in board

29. A catalyst itself is not used in a reaction. So, a small amount of catalyst
is sufficient to catalyse the reaction. After the completion of the
reaction, the chemical composition of the catalyst remain unchanged.
The phenomenon of catalyst is specific i.e. a particular catalyst can
catalyse a particular reaction.

30. Effect of temperature on the reaction rate:
Rate of the reaction increase on increasing the temperature. In general, it
has been found that, an increase in temperature by 10oc doubles the rate of
the reaction.
In 1889 Arrhenius has derived a simple relation between rate constant and
the temperature of the system.
K = Ae-Ea/RT ………………………(i)
where A =collision frequency
Ea= activation energy
R= universal gas constant
T= Kelvin temperature
(Collision frequency: The number of collision that take place per second
per unit volume of the reaction mixture is called collision frequency)

31. In equation (i) the power consists of negative. It means with decrease
in activation energy, the value of the rate constant increases and vice
versa. Similarly, with increase in temperature the value of the rate
constant (K) also increases. It is because with increase in temperature,
the KE of reacting molecules also increases. So more number of
molecules can cross the energy barrier and the rate of reaction
increases.

32. Collision theory of bimolecular reaction:
According to this theory, a chemical reaction takes place only by
collision between the reacting molecules. But not all collision are
effective. Only a small fraction of the collision can be change reactant
into product. Two main criteria for the reaction are:
1. The colliding molecules must possesses sufficient kinetic energy to
cause the reaction.
2. The reacting molecules must collide with proper orientation.

33. To explain collision theory let us consider a
general reaction:
A-A + B-B → 2AB
To give the product, there must be breakdown of bonds between the
atoms in the reactant molecules. The energy for the breakage of bonds
comes from the kinetic energy possessed by the reacting molecules
from collision.
Fig………
Only those molecules which collide with K.E greater than or equal to Ea
are able to cross the barrier and give product. The molecules colliding
with KE less than Ea fail to cross the energy barrier. Such type of
collision between the molecules are unproductive.

34. The reacting molecules must collide with proper orientation. The
correct orientation is that which ensures direct contact between the
atoms involved in the breaking and forming the bonds.
Effective collision: The collision between reacting molecules resulting
in the formation of product is called effective collision.