In general, molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation.


The molecularity of a reaction is the number of reactant molecules taking part in a single step of the reaction.

e.g., PCl5  →  PCl3 + Cl2                       (Unimolecular)

        2HI  →  H2 + I2                              (Bimolecular)

        2SO2 + O →  2SO3                       (Trimolecular)

        NO + O3  →  NO2 + O2                    (Bimolecular)

        2CO + O2  →  2CO2                     (Trimolecular)

        2FeCl3 + SnCl2 → SnCl2 + 2FeCl2   (Trimolecular)


The minimum number of reacting particles (molecules, atoms or ions) that come together or collide in a rate determining step to form product or products is called the molecularity of a reaction.

For example, decomposition of H2O2 takes place in the following two steps:


                H2O2 → H2O + 1/2O2          (overall reaction)

Step 1:      H2O2 → H2O + [O]             (Slow)

Step 2:       [O] + [O] → O2                  (fast)


The slowest step is rate-determining. Thus from step 1, reaction appears to be unimolecular.


(i)    Molecularity is a theoretical concept.

(ii)    Molecularity cannot be zero, -ve, fractional, infinite and imaginary.

(iii)    Molecularity cannot be greater than three because more than three molecules may not mutually collide with each other.


There are some chemical reactions whose molecularity appears to be more than three from stoichiometric equations, e.g. in

                 4HBr + O2 → 2H2O + 2Br2

  2MNI4- + 16H+ + 5C2 O42- →  2Mn2+ + 10CO2 + 8H2O

In the first reaction molecularity seems to be '5' and in the second reaction molecularity seems to be '23'. Such reactions involve two or more steps; each step has its own molecularity not greater than three, e.g., in first reaction.

                  HBr + O2 → HOOBr

                  HOOBr + HBr → 2HOBr

                  [HOBr + HBr → H2O + Br2] × 2


                  4HBr + O2 → 2H2O + Br2


Molecularity of each of the above steps is 2.

(a)  Reaction between Br- and H2O2 in acidic medium:

The overall reaction is

2Br- + H2O2 + 2H+ → Br2 + 2H2O

The proposed mechanism is

2Br- + H2O2 + H+ → HOBr + H2O    (slow)

HOBr + H+ + Br- → Br2 + H2O       (fast)

The reaction is trimolecular


(b)  Reaction between NO2 and F2:

The overall reaction is

2NO2 + F2 → 2NO2F

The proposed mechanism is

        NO2 + F2 → NO2 + F            (slow)

        NO2 + F → NO2F                 (fast)

The reaction is bimolecular. 

(c)  Decomposition of H2O2:

The overall reaction is

        H2O2 → H2O + O                (slow)

        H2O2 + O → H2O + O2         (fast)

        Rate = k[H2O2]           

The reaction is unimolecular

Related Resources
Rate of Reaction


Zero Order Reactions

Zero Order Reactions A reaction is said to be of...

Solved Examples

Download IIT JEE Solved Examples for Molecularity...

Reaction Mechanism

REACTION MECHANISM Knowledge about involved steps...

Parallel or Competing Reaction

1 | 2 | 3 Parallel or Competing Reaction The...

Collision Theory of Reaction Rate

Collision theory of reaction rate(Arrhenius Theory...

Rate Constant

RATE CONSTANT Consider a simple reaction A →...

Methods Determination Order of Reaction


Order of Reaction

ORDER OF REACTION Let us consider a good reaction:...

Introduction to Chemical Kinetics

Introduction to Chemical Kinetics Thermodynamics...

First Order Reactions

First Order Reactions A reaction is said to be...

Second Order Reaction

Second Order Reactions A reaction is said to be of...

Factors Affecting Rate of Reaction

Factors Affecting Rate Of Reaction (i) Nature of...

Expression of Rate

Expression of Rate Consider the following reaction...

Law of Mass Action

LAW OF MASS ACTION (Goldberg and Waage, 1864) This...

Third Order Reactions

Third Order Reactions A reaction is said to be of...