Molecularity

A chemical reaction that takes place in one and only one step i.e., all that occurs in a single step is called elementary reaction while a chemical reaction occurring in the sequence of two or more steps is called complicated reaction.

The sequence of steps through which a complicated reaction takes place is called reaction – mechanism.
Each step in a mechanism is an elementary step reaction.

The molecularity of an elementary reaction is defined as the minimum number of molecules, atoms or ions of the reactants(s) required for the reaction to occur and is equal to the sum of the stoichiometric coefficients of the reactants in the chemical equation of the reaction. hus, the molecularity of some elementary reactions are as mentioned below.

Elementary reactions      Molecularity

PCl5 ↔ PCl3 + Cl2          1

H2 + I2 ↔ 2HI           1 + 1 = 2

Reactions with molecularity equal to one, two, three etc., are called unimolecular, bimolecular, termolecular, etc., respectively.

A complicated reaction has no molecularity of its own but molecularity of each of the steps (elementary reactions) involved in its mechanism.

For example, consider the reaction: 2NO + 2H2 –> N2 + 2H2O, which is complicated reaction and takes place in the sequence of following three steps.

i) NO + NO <–> N2O2 (fast and reversible)

ii) N2O2 + H2 –> N2O + H2O (slow)

iii) N2O + H2 –> N2 + H2O (fast)

The molecularity of each step in the mechanism is two, so what we say is that the reaction takes in the sequence of three steps each of which is bimolecular. There is another view also. According to which molecularity of a complicated reaction is taken to be equal to the molecularity of the slowest step i.e. rate – determining step (r.d.s.) in the mechanism.

The molecularity of each step in the mechanism is two, so what we say is that the reaction takes in the sequence of three steps each of which is bimolecular. There is another view also. According to which molecularity of a complicated reaction is taken to be equal to the molecularity of the slowest step i.e. rate – determining step (r.d.s.) in the mechanism.

Also Read :

→ Chemical Kinetics
→ Rate of Reaction
→ Determination Of rate of reaction
→ Order of Reaction
→ Order Of Reaction
→ Difference between Order and Molecularity
→ Kinetics of First order Reaction
→ Half-life period of a first order reaction
→ Graphical Representation for order reaction
→ Effect of Temperature on the Reaction Rate
→ Arrhenius Equation
→Catalyst

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