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Molecularity of a Reaction

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Molecularity of a Reaction - Lesson Summary

A reaction which takes place in a single step is called an elementary reaction.

Ex: C + O2 → CO2

Reaction which takes place in multiple steps is called a complex reaction. Each step in a complex reaction is called an elementary reaction.


Step (1)                          HBr + O2 → HOOBr
Step (2)                    HOOBr + HBr → 2 HOBr
Step (3)                       HOBr + HBr → H2O + Br2
Step (4)                       HOBr + HBr → H2O + Br2
                     Overall      4 HBr + O2 → 2H2O + 2Br2

The sequential representation of all the elementary reactions in an overall reaction is called the mechanism of the reaction.

The number of reacting species, such as molecules, atoms or ions of an elementary reaction that collide with each other to bring about a chemical reaction is called the molecularity of that reaction.

Molecularity of a reaction is the property that is useful to understand the mechanism of a reaction.

A reaction in which only one molecule takes part is known as a uni-molecular reaction. Ex: Decomposition of ammonium nitrite

NH4NO2 → N2 + 2H2O

Bimolecular reactions involve two molecules.

Ex: Decomposition of nitrogen dioxide

2NO2 → 2NO + O2

Trimolecular reactions involve three molecules.

Ex: Combustion of nitric oxide

2NO + O2 → 2NO2

Collision theory states that as the number of reacting species (or) molecules increases, the probability of simultaneous collision of reacting molecules decreases.

Therefore reactions involving more than three molecules take place in several steps, such reactions are called complex reactions.

A complex reaction involving more than three molecules generally occurs in more than one step. When a reaction involves more than one step then elementary step which proceed no faster than any other step is called the slowest step. The slowest step of the reaction is called the rate determining step.

Step (1)                          HBr + O2 → HOOBr           Very Slow
Step (2)                    HOOBr + HBr → 2 HOBr          Very Fast
Step (3)                       HOBr + HBr → H2O + Br2      Very Fast
Step (4)                       HOBr + HBr → H2O + Br2      Very Fast
                     Overall      4 HBr + O2 → 2H2O + 2Br2

In the above reaction,

HBr + O2 → HOOBr - is the rate determining step.

In a complex reaction the molecularity and order of reaction are same in the slowest step.

       HBr + O2 → HOOBr 

             Order = 2
    Molecularity = 2


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