MOLECULARITY OF REACTION

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

OR

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.

Note:        

(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

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