What is the order and Molecularity of a reaction..?
please explain it with examples...
Answers
The order of reaction is an empirical quantity determined by experiment from the rate law of the reaction. It is the sum of the exponents in the rate law equation.
Molecularity, on the other hand, is deduced from the mechanism of an elementary reaction, and is used only in context of an elementary reaction.
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Molecularity of Reaction JEE
IIT JEE
Molecularity of Reaction JEE

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Order of Reaction and its Differences

Introduction
Chemical reaction involves one or more than one molecule. A single-step reaction which takes place in one single step is called as an elementary reaction. While the chemical reaction that involves series of two or more step is called as a complex or complicated reaction. In the complicated reaction, the sequence of steps through which it takes place is called as the mechanism of chemical reaction. Each sequential step in the mechanism is an elementary step reaction. During the course of chemical reaction, these molecules constantly bump into each other. Based on that, the chemical reaction takes place.
Definition
The molecularity of reaction can be defined as the number of molecules reacting with each other in the reaction. In other words, it can be defined as the number of molecules which undergo collision that ultimately resulting in to a chemical reaction. Hence, molecularity of reaction indicates the number of molecules that takes part in the chemical reaction.
The minimum number of reactants (atoms, ions, or molecules) required for the reaction to occur is called as the molecularity of an elementary reaction. The molecularity of the chemical reaction is equal to the sum of the stochiometric coefficients of the reactants in the chemical equation of the reaction. It is also defined as the number of reactant molecules taking part in a single step of the reaction.
Examples:
1. The Simple Case of Unimolecular Reaction
In case, when there is only one molecule in the reaction, the reaction is known as unimolecular reaction and its molecularity is one. This is a case of single-step chemical reaction. In this reaction, only one molecule transforms in to products. For example, degradation of dinitrogen tetroxide.
N2 O4 (g)→ 2 NO2 (g)
This reaction is of first order reaction as the reaction rate is directly proportional to the concentration of dinitrogen tetroxide raised to the first power.
Another example of unimolecular reaction is:
P Cl5 → P Cl3 + Cl2
2. Bimolecular Reaction
In this case, there are two molecules of reactant that take part in the chemical reaction.
2 HI → H2 + I2
There are 2 molecules of hydrogen iodide take part in this reaction, and hence, it is an example of bimolecular reaction.
Another example of bimolecular reaction is:
NO + O3 → NO2 + O2
3. Trimolecular Reaction
In this case, there are three molecules of reactant that take part in the chemical reaction.
2 SO2 + O2→ 2 SO3
There are 2 molecules of sulfur dioxide and one molecule of oxygen that collide and take part in the chemical reaction. Hence, it is called trimolecular reaction.
Other examples of this type are mentioned as follow:
2 CO + O2 → 2 CO2
2 FeCl3 + SnCl2 → SnCl4 + 2 FeCl2
“The molecularity of chemical reaction can also be defined as the minimum number of reacting particles (either molecules, atoms, or ions) that come closer and collide in a rate determining step to form one product or more than one product.”
Complex Example of Molecularity of Chemical Reaction
The decomposition of hydrogen peroxide (H2O2) is a complex reaction and it involves more than one step. This reaction takes place in two different steps.
The overall decomposition reaction can be represented as:
H2 O2