Difference between order and

molecularity of a reaction

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.

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

Order = m + n

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

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

First order ReactionFirst 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’

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

Examples of first order reactionThermal decomposition of N2O5

N2O5 N2O3+O2

Hydrolysis of hydrogen peroxide

H2O2 H2O +1/2O2

Decomposition of Calcium carbide

CaCo3 Cao + Co2

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

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

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

Third Order ReactionThird 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]

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

Examples Of Third Order Reaction

2NO + O2 2NO2

2NO + Cl2 2NOCl 2FeCl3 + SnCl2 2FeCl2 + SnCl4

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.

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.

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.

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

In tri-molecular there will be collision of three molecules.

2NO + O2 2NO2

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.

Thank You