The order of a reaction refers to the sum of the powers of the concentration terms in the rate equation. It represents the number of molecules or atoms involved in the rate determining step. The document discusses zero, first, second, and third order reactions, providing examples and equations for determining the rate constant and half-life for each order. Molecularity refers to the actual number of reactant species involved in the elementary reaction step and can only be 1, 2, or 3, whereas order is a measurable property determined from the rate equation.

2. Order of a reaction
The sum of the powers of
concentration terms in rate equation is known
as order of reaction.
(or)
Order of reaction may be given
with respect to all participating reactants in
which case it is called order of reaction.

3. Consider a reaction mA + nB product
Rate eq (R)=k[A]m[B]n
Order = m + n

4. Zero Order Reaction
When the reaction rate is independent
of concentration of the reacting substance, it
depends on the zero power of the reactant and
therefore is zero order reaction.
4

5.  The unit of K is concentration time-1
with typical units of mole L-1 s-1.
 Half-life is given by equation;
t1/2 = Co/2k
5

6. First order Reaction
First order reaction consists of one reactant.
A product
R = k[A]1
Rate constant for first order reaction:
k = 2.303 . log(a/a-x)
t
where, a=initial conc. of A in mol/lit
a-x=change in initial conc. of A in
mol/lit in time interval ’t’

7. Half Life Time (t1/2)
Time taken for initial conc. of
reactant to become half of its volume
k= 0.693/t1/2
t1/2=0.693/k
Units of rate constant for first order reaction:
sec-1

8. Examples of first order reaction
 Thermal decomposition of N2O5
N2O5 N2O3+O2
 Hydrolysis of hydrogen
peroxide
H2O2 H2O +1/2O2
 Decomposition of Calcium
carbide
CaCo3 Cao + Co2

9. Second Order Of Reaction
Second order reaction consists
of two reactants.
2A products
A+B products
Rate = k[A]2 ; n=2
Rate = k[A] [B] ; n=1+1=2

10. Half Life Time
Half life time for second order reaction:
t1/2= 1/a n-1
=1/a as n=2
Units:
k = mol -1 lit sec -1 (or)
= conc-1. sec -1

11. Examples Of Second Order Reaction
Thermal decomposition of Nitrous oxide
2N2O 2N2 +o2
Decomposition of NO2
2NO 2 2NO +O 2
Decomposition of Hydrogen Iodide
2HI H 2 + I2

12. Third Order Reaction
Third order reaction includes three
reactants
3A Products
r = k [A] 3
A+B+C Products
r = k [A] [B] [C]
2A+B Products
r = k [A]2 [B]

13. Half Life Time
Half life time for third order reaction
t ½ = 1/a n-1
= 1/a 3-1 = 1/ a2
[as n=3]
Units:
k = mol -2 lit 2 sec -1
= conc. -2 sec -1

14. Examples Of Third Order Reaction
 2NO + O2 2NO2
 2NO + Cl2 2NOCl
 2FeCl3 + SnCl2 2FeCl2 + SnCl4

15. Molecularity of the reaction
The number of ions or atoms
or molecules involve in the rate
determining step or rate limiting step is
called molecularity . The molecularity
will be only whole number and never be
zero or fraction.

16. Molecularity can be determined by
reaction mechanism but it cannot be determined
experimentally.
In no chemical reaction reactants
are directly converted into products in single step,
initial reactants are converted to final products
occurred by different internal steps . These steps are
called elementary step reactions . The sequential
representation of elementary reaction step in overall
chemical reaction is called reaction mechanism.

17. Example:
A B
Reaction Mechanism:
A 2x
2x 2y
2y B (slow)
Among all three steps there is a elementary
step with least reaction . This step is called rate
determining step or rate-limiting step.

18.  Molecularity cannot exceed ‘3’ . If one molecule is
involved in elementary reaction then the reaction is
uni-molecular in nature.
CH2 CH2
2C2H4
CH2 CH2
 In bimolecular reaction the two reactant
molecules(same or different) participate to cause a
chemical change.
2HI H2 + I2

19. In tri-molecular there will be collision of
three molecules.
2NO + O2 2NO2

20. Molecularity Order of reaction
 It is defined as number
of molecules or atoms or
ions participating in rate
determining step.
 It is a theoretical
quantity, which can be
determined through
reaction mechanism.
 It is always whole
number or integer. It can
never be fraction or zero.
 Order is defined as sum
of powers of
concentration terms in a
rate equation.
 Order of reaction can be
determined
experimentally.
 Order of reaction is zero
, fraction , integer &
whole number.