Several reactions occur in and around us like rusting of iron, digestion of food, etc. Some reactions are speedy, and some are very slow. These reactions may occur between one or more reactants. The number of molecules which undergo simultaneous collisions determine the molecularity of a reaction.

The reactions that take place in only one step is known as elementary reactions, whereas the reactions that take place in several steps are known as complex reactions. In this article, we will discuss about molecularity in detail. Continue reading to learn more!

Molecularity of Reaction: Overview

According to the collision theory, a reaction occurs when the molecules collide with each other. The number of molecules which undergo simultaneous collisions decide the molecularity of a reaction.

The number of atoms, ions, or molecules that collide with one another simultaneously to result in a chemical reaction is called the molecularity of the reaction.

The reaction taking place in one step is called elementary reaction. The reaction that takes place in more than one step is called a complex reaction. The different steps in which a complex reaction takes place are called the mechanism of the reaction. Each step of the mechanism of the reaction is an elementary reaction. 

Molecularity of Elementary Reactions

The molecularity of the elementary reaction or simple reaction is defined as the sum of the molecules of the different reactants as represented by the balanced chemical equation. 

Unimolecular Reactions

When only one reactant molecule is involved in the reaction, the molecularity of the reaction is \(1,\) and the reaction is called a unimolecular reaction.

Let us understand the concept through the molecularity of reaction examples.

Example 1: Decomposition of hydrogen peroxide.

\({{\rm{H}}_{\rm{2}}}{{\rm{O}}_{\rm{2}}} \to {{\rm{H}}_{\rm{2}}}{\rm{O + }}\frac{{\rm{1}}}{{\rm{2}}}{{\rm{O}}_{\rm{2}}}\)

Example 2: Decomposition of dioxygen difluoride.

\({{\rm{O}}_{\rm{2}}}{{\rm{F}}_{\rm{2}}} \to {{\rm{O}}_{\rm{2}}}{\rm{ + }}{{\rm{F}}_{\rm{2}}}\)

Bimolecular Reactions

When two reactant molecules collide together in a reaction, it is called a bimolecular reaction. The molecularity of the reaction is \(2.\)

Example: Decomposition hydrogen peroxide

\({\rm{2HI}} \to {{\rm{H}}_{\rm{2}}}{\rm{ + }}{{\rm{I}}_{\rm{2}}}\)

Trimolecular Reactions

When three reactant molecules collide together in a reaction, the molecularity of the reaction is \(3,\) and the reaction is called trimolecular reaction.

Example: Reaction between nitric oxide and oxygen

\({\rm{2NO + }}{{\rm{O}}_{\rm{2}}} \to {\rm{2N}}{{\rm{O}}_{\rm{2}}}\)

For elementary reactions, the order of the reaction is the same as its molecularity, and the order for each reactant is equal to its stoichiometric coefficient as represented in the balanced chemical equation.

Molecularity of Complex Reactions

A complex reaction proceeds through several steps. Each step in a complex reaction is an elementary reaction. Some elementary steps may be slow, while others may be fast. The overall rate of a complex reaction is governed by the rate of the slowest elementary step. The slowest elementary step is therefore termed the rate-determining step. Thus, the number of atoms, ions, or molecules taking part in the slowest elementary step of a complex reaction is called the molecularity of the complex reaction.

Let us understand the concept through the molecularity of reaction examples.

Example 1: The reaction between nitric oxide and hydrogen is complex.

\(2{\rm{NO}} + 2{{\rm{H}}_2} \to {{\rm{N}}_2} + 2{{\rm{H}}_2}{\rm{O}}\)

The above reaction is taking place in the following two steps:

Step 1: \(2{\rm{NO}} + {{\rm{H}}_2} \to {{\rm{N}}_2} + {{\rm{H}}_2}{\rm{O}}\) (Slowest)

Step 2: \({{\rm{H}}_2}{{\rm{O}}_2} + {{\rm{H}}_2} \to 2{{\rm{H}}_2}{\rm{O}}\) (Fastest)

Step 1 is the rate-determining reaction since it a the slowest step. In complex reactions, molecularity is equal to the slowest elementary reaction. Therefore, the reaction between nitric oxide and hydrogen is trimolecular, and its molecularity is \(3.\)

Example 2: Thermal decomposition of dinitrogen pentoxide.

\(2\;{{\rm{N}}_2}{{\rm{O}}_5} \to 4{\rm{N}}{{\rm{O}}_2} + {{\rm{O}}_2}\)

The above reaction is taking place in the following two steps:

Step 1: \({{\rm{N}}_2}{{\rm{O}}_5} \to {\rm{N}}{{\rm{O}}_2} + {\rm{N}}{{\rm{O}}_3}\) (Slowest)

Step 2: \({{\rm{N}}_2}{{\rm{O}}_5} + {\rm{N}}{{\rm{O}}_3} \to 3{\rm{N}}{{\rm{O}}_2} + {{\rm{O}}_2}\) (Fastest)

The molecularity of the above unimolecular reaction is \(1.\)

Molecularity of Water

When water is taken in excess in the reaction, the molecularity of water is neglected.

Example: Hydrolysis of methyl acetate. 

\({\rm{C}}{{\rm{H}}_3}{\rm{COO}}{{\rm{C}}_2}{{\rm{H}}_5}{\rm{(aq)}} + {{\rm{H}}_2}{\rm{O}}({\rm{l}}) \to {\rm{C}}{{\rm{H}}_3}{\rm{COOH(aq)}} + {{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}}({\rm{aq}})\)

According to the stoichiometric coefficient, it should be a bimolecular reaction, but it is a pseudo unimolecular reaction. Since water taken in excess concentration remains unchanged during the reaction.

Order of Reaction

The power to which the concentration term of a particular reactant in the rate law is raised is called the order of reaction for that reactant.

The sum of all the powers to which all the concentration terms in the rate law are raised to express the observed rate of reaction is called the overall order of the reaction. The overall order of the reaction is generally called the order of the reaction.

The order of the reaction may be first-order, second-order, or third-order based on the overall order \(1, 2,\) and \(3,\) respectively. The order of the reaction is purely an experimental quantity and cannot be known just by the stoichiometry of the balanced equation. The order of the reaction is usually a whole number. However, it can be zero or fractional, or negative.

Example: \(2{\rm{NO}} + {{\rm{O}}_2} \to 2{\rm{N}}{{\rm{O}}_2}\)

Its rate law is Rate \({\rm{2 = k[NO}}{{\rm{]}}^{\rm{2}}}{\left[ {{{\rm{O}}_{\rm{2}}}} \right]^{\rm{1}}}\)

Overall order of reaction\(=2+1=3\)

Therefore, it is the third-order reaction.

Is Order of Reaction and Molecularity Same?

Below we have provided the comparison between the order of a reaction and molecularity:

Order of Reaction	Molecularity of Reaction
1. It is the sum of the concentration terms on which the rate of reaction depends.	1. It is the number of atoms, ions, or molecules that collide with another simultaneously to result in a chemical reaction.
2. It can be a whole number, fractional, or zero.	2. It is always a whole number.
3. It is obtained from the experimental data.	3. It is obtained from the stoichiometric equation.
4. Order applies to elementary as well as complex reactions.	4. Molecularity applies only to elementary reactions. In complex reactions, the molecularity of the slowest step is the same as the overall order of the reaction.

Summary

The molecularity of a reaction is described as the number of atoms, ions, or molecules that collide simultaneously to result in a chemical reaction. The molecularity of the elementary reaction refers to the sum of the molecules of the several reactants as depicted by the balanced chemical equation. The molecularity of the complex reaction is defined as the number of atoms, ions, or molecules taking part in the slowest elementary step of a complex reaction.

FAQs on Molecularity

Q.1. What is the molecularity of elementary reaction?
Ans: The molecularity of the elementary reaction is the sum of the molecules of the different reactants as represented by the balanced chemical equation.

Q.2. What is Molecularity? Explain with example.
Ans: The number of atoms, ions, or molecules that collide simultaneously to result in a chemical reaction is called the molecularity of the reaction.
Example: Decomposition of dioxygen difluoride.
\({{\rm{O}}_2}\;{{\rm{F}}_2} \to {{\rm{O}}_2} + {{\rm{F}}_2}\)
Only one mole of reactant is in the reaction. Hence it is a unimolecular reaction. Its molecularity is 1.

Q.3. Why can molecularity not be fractional?
Ans: Molecularity of the reaction is the number of atoms, ions, or molecules that collide simultaneously to result in a chemical reaction. The atoms, ions, or molecules cannot react as a fraction. Therefore, molecularity cannot be fractional

Q.4. What is the difference between molecularity and order of reaction?
Ans: The number of atoms, ions, or molecules that collide simultaneously to result in a chemical reaction is called the molecularity of the reaction.
The power to which the concentration term of a particular reactant in the rate law is raised is called the order of reaction for that reactant. 

Q.5. Why can molecularity not be more than \(3\)?
Ans: Effective collisions decreases with an increase in the number of reactant molecules. Therefore, molecularity cannot be more than 3.

Q.6. How do you determine the molecularity of a reaction?
Ans: The Molecularity of the elementary reaction is determined by the sum of the molecules of the different reactants as represented by the balanced chemical equation.
The number of atoms, ions, or molecules taking part in the slowest elementary step of a complex reaction determines the molecularity of the complex reaction.

Q.7. What are molecularity and its types?
Ans: The number of atoms, ions, or molecules that must collide simultaneously to result in a chemical reaction is called the molecularity of the reaction.
The types of molecularity are unimolecular, bimolecular, and trimolecular.

Q.8. Is molecularity equal to order?
Ans: Molecularity is equal to order in elementary reactions. Whereas, in complex reactions, molecularity is equal to the slowest step, i.e., rate-determining step. 

Q.9. Can the molecularity of the reaction be zero?
Ans: Molecularity of reaction cannot be zero because colliding molecules in a reaction cannot be zero.

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