Order and Molecularity of Reaction

The order of a reaction and the Molecularity of the reaction is the basic properties of the reaction that helps to get the kinematics and rate of reaction. In this article, we will learn about the Order of Reaction and Molecularity, its types differences, and others in detail.

What is the Molecularity of a Reaction?

The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called the molecularity of a reaction.

Generally for a reaction  aA + bB →  cC + dD

Molecularity = a + b

Unimolecular Reactions: The reactions with molecularity 1 are called Unimolecular Reactions examples of which are,

• NH₄NO₂ →  N₂ + 2H₂O
• Br₂ → 2Br

Bimolecular Reaction: The reactions with molecularity 2 are called Bimolecular Reactions examples of which are,

• 2HI → H₂ + I₂ 

Trimolecular Reaction: The reactions with molecularity 1 are called Trimolecular Reactions examples of which are,

• 2NO + O₂  → 2NO₂ 

The probability of three particles colliding and reacting simultaneously and reacting is very low. Therefore a molecularity higher than three is generally not observed.

An elementary reaction is defined as a reaction that occurs in a single step. Many reactions that follow single rate law actually take place in a series of steps. Such reactions are called complex reactions. Each step of a complex reaction is an elementary reaction. 

The concept of molecularity is valid only for elementary reactions.

What is the Order of Reaction?

For any elementary or complex reaction, the order of a reaction is defined as,

“Sum of the coefficients of the reacting compounds that are involved in the rate equation for the reaction.”

The order of Reaction is an experimental value i.e. it is always calculated experimentally.

Determination of Order of Reactions

Consider a reaction aA + bB →  cC + dD 

The rate law for the above reaction is Rate α  [A]ᵃ [B]ᵇ

The sum of the power a and b is called the overall order of the reaction.

i.e. a + b = overall order.

a and b represent the order with respect to reactants A and B individually.

The values of a and b indicate how sensitive the rate of the reaction is to the change in the concentration of A and B.

For Example:

Decomposition of Ammonium Nitrite (NH₄NO₂)

NH₄NO₂ → N₂ + 2H₂O

Rate = k [NH₄NO₂] 

Order of reaction = 1

Dissociation Of HI

2HI → H₂ + I₂

Rate = k[HI]²

Order of reaction = 2

Characteristics of Order of Reaction

Various characteristics of the order of the reaction are discussed below in the article,

• The order of reaction gives the dependency of the rate of reaction on the concentration of reactants.
• It is an experimentally determined quantity.
• The order of reaction can be zero, fractional, or integral.
• it gives an idea about the rate of reaction.
• It gives information about the reaction mechanism.
• it depends on experimental conditions.
• The order of reaction of both complex and elementary reactions can be determined.

Difference Between Order of Reaction and Molecularity of Reaction

The difference between the Order of Reaction and Molecularity of Reaction is discussed in the table given below,

Order of Reaction	Molecularity of Reaction
The sum of the powers of concentration terms involved in the rate equation is known as the order of the reaction.	The sum of the powers of concentration terms involved in the law of mass action is called as molecularity of a reaction.
it is a property of both, complex and reactions elementary reactions.	It is the property of only elementary reactions that have third-order no meaning for complex reactions.
It is an experimentally determined value.	It is a theoretical value.
It may be zero, fractional, or integral.	It can never be zero or fractional. It always has a positive integral value.
it depends on experimental conditions.	It is independent of experimental conditions.
It gives an idea about the rate of a reaction.	It does not give any idea about the unimolecular rate of the reaction.
According to their order, reactions are classified as first, second, and third-order reactions.	According to their molecularity, reactions are classified as unimolecular, bimolecular, and trimolecular reactions.

Read, More

• Characteristics of Chemical Reactions
• Types of Chemical Reactions
• Rate of a Reaction

Solved Examples on Order of Reaction and Molecularity

Example 1: Find the Order and Molecularity of the reaction N₂O₅ → N₂O₃+O₂

Solution:

For the above reaction,

Molecularity of reaction = 1

Rate = k[N₂O₅]

Order of Reaction = 1

Example 2: Find the Order and Molecularity of the reaction H₂O₂ → H₂O + 1/2 O₂

Solution:

For the above reaction,

Molecularity of reaction = 1

Rate = k[H₂O₂]

Order of Reaction = 1

Example 3: Find the Order and Molecularity of the reaction 2NO + O₂ → 2NO₂ 

Solution:

For the above reaction,

Molecularity of reaction = 3

Rate = k[NO]²[O₂]

Order of reaction = 3

Example 4: Find the Order and Molecularity of the reaction 2HI → H₂ + I₂

Solution: 

For the above reaction,

Molecularity of reaction = 2

Rate = k[HI]²

Order of reaction = 2