Notes On Molecularity of a Reaction - CBSE Class 12 Chemistry
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. Ex: 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

#### 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. Ex: 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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