Types Of Reaction Based On The Molecularity

Let us now understand about the Molecularity of the reaction. 

What is the molecularity of a chemical reaction and how is it defined? How are chemical reactions classified on the basis of molecularity???

Chemical reactions occur when atoms, ions and molecules collide with each other to bring about a chemical change and form a new bonded molecule called a product. Therefore, it is important to know the number of reactive species that bring about a chemical change. This brings us to the concept of molecularity of a reaction.

...."The molecularity of a reaction is equal to the sum of number of reactant molecules taking part in a balanced chemical reaction (or which takes part in an elementary step or rate determining step)".......

Molecularity of a reaction does not depend on the external factors such as temperature or pressure.

Depending on the number of reactive molecules which means how many species (atoms, molecules or ions) are reacting with each other to form the product, chemical reactions are of different types-

(a) Uni-molecular reaction

(b) Bi-molecular reaction

(c) Tri-molecular reaction

Uni-molecular Reaction 

A reaction in which a single reactant is taken part in the reaction and converted into products in order to complete the reaction then the reaction is called as an uni-molecular reaction.

For Example

        • NH₄NO₂ ➝ N₂ + 2 H₂O

        •     P-Cl₅   ➝ P-Cl₃ + Cl₂

        •     N₂O₅   ➝ N₂O₄ + 1/2 O₂

        •     H₂O₂   ➝ H₂O + 1/2 O₂

All the above reactions have a single reactant therefore these are called as Uni-molecular reaction whose molecularity is equal to 1.

            ( A ➝ product's )

Bi-molecular Reaction

A reaction which involves collision of two reactant species simultaneously in order to achieve a chemical change is called as Bimolecular reaction (a reaction is called bimolecular if it's molecularity 

equal to 2). 

For Example

         •        2HI    ➝  H₂ + I₂

         •  CH₄ + Cl₂ ➝   2HCl + CH₃

         •   NO + O₃  ➝ NO₂ + O₂

         •     2NOCl   ➝ 2NO + CO₂

         •  CO + NO₃ ➝ NO₂ + CO₂

All the above examples are of Bi-molecular reaction having two reactant species.

        ( A + B ➝ products / A + A ➝ products ) 

  

Tri-molecular Type Reaction

A reaction which involves the collision of three reactant species ( it may be molecules, ions or atoms) simultaneously in order to form a product are alled as a tri-molecular type reaction ( or when the molecularity of reaction is equal to 3 then the reaction is defined as tri-molecular ). 

For Example

       • 2SO₂ + O₂ ➝ 2 SO₃

       • 2FeCl₃+ Sn-Cl₂ ➝ Sn-Cl₄+ 2FeCl₂

       • 2CO + O₂ ➝ 2CO₂

All the above reactions are examples of tri-molecular type reaction having three reactant species. 

☆ With the proper configuration, more than three molecules are less likely to react together after a collision. Therefore, the reactions of tri-molecular molecularity are very low and increase at a slower rate. Thus, it is clear that complex reactions that have more than three molecules in their stoichiometry equation, must occur in more than one step.

For Example, 

   K-ClO₃ + 6Fe₂SO₄ + 3H₂SO₄ ➝ K-Cl + 3Fe₂(SO₄)₃ + 3H₂O

From the above reaction it appears that its order is about 10. But in reality, it is a second-order reaction. Therefore, It states that the reaction takes place in more than one step.

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