* Molecularity of rxn :- No. of atom , ions or molcularity that must collide with one another simultaneous to result into chemical rxn.

eg. 2NO₂ + O₂ 2NO₂

Molecularity = 2 +1 = 3 Note :- Molecularity is defined for elementary rxn. not for complex.

Complex rxn. takes place in sequence of step & slowest step is c/d rate determining step.

Molecularity is always whole no. & or due may be traction.

A series of step rxn. account for over all rxn. c/d mechanism of rxn.

eg. 2N₂O₅ 4NO₂ + O₂

Rate = K[N₂O₅]


Step 1:- N₂O₅ NO₂ + NO₃

Step 2:- N₂O₅ 3NO₂ + O₂

(1) Rxns involving two I^st order consecutive ateps:-

Step 1:- R X

Step 2:- X P





Illustration :- 2NO(I) + O2/sub>(I) 2NO₂ (I)



Predict rate law, if mechanism is

2NO₂ + O₂ NO₃(Fast)


NO₃ NO NO₂ + NO₂ (Slow)



Ans :- From Slow Step:- Rate = K₁ [NO₃][NO₂] ................................ (1)

From Fast Step:- eqm. const. K =

Substituting value of [NO₃] from (11) in (1), we get


Rate = K'[NO]²[O₂]


Integrating Rate expression for zero order rxn A P dts

Rate = = K[A]⁰ = K

d[A] = - K dt

[A] = - kt + C ............................................ (1)

C = [A]₀

Substituting value of C in eq. (1)

[A] = - Kt + [A]₀ ................................................. (11)






Half life:- It time is which half of substitution has reacted.









Illustration:- Rate of decomposition of NH₃ on platenium surface is zero order. What is rate of production on of N₂ ?



Rate =

For zero order rxn = K



A Products

After time t

Rate rxn



At t = 0, x = 0,

- ln a = C .................................................. (111)

Putting value of C in eq. (11)

- ln (a - x) = Kt + (- ln a)

Kt = ln



Putting a = [A]₀ & (a - x) = [A]







Half life :-

Dumb question:- What is the difference between and order?

Solⁿ:- Moleularity is a theoretical concept while order is determined using experiments. Moleularity is always a whole number & it cannot be zero while order can take any positive value and zero also.


Question:-


K = 6.2 X 10^-4S^-1

Calculaten rate when [N₂O₅] = 1.25 mol/L

Ans:- Rate = K[N₂O₅]

= 6.2 X 10^-4 [1.25] = 7.75 X 10^-4 mol^-1LS^-1


Integrated law for II^nd order :-

B + RA Products



Rate (a - x)(b - x)





At t = 0, x = 0



Putting in (1) the value of C



Half Life Period:-

order of rxn.