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# Order and Molecularity of reaction

## What is the order of reaction?

The order of reaction is defined as

the sum of the coefficients (or power ) of the reacting species that are involved in the rate equation for the reaction.”

The definition of order of reaction is valid both for elementary as well as complex reactions. The order of the reaction is always determined experimentally.

## Determination of order of reactions

Let us consider a general reaction:

aA + bB + cC → Product

Let active moles of A, B and C be α, β and γ respectively. Then the rate of reaction may be given as:

Rate = k [A]α [B]β [C]γ

The order of the reaction is equal to the sum of power of concentration terms involved in rate law expression.

Order = α + β + γ

When α + β + γ = a + b +c, then order of reaction = molecularity of reaction

Note:

(i) Order is an experimentally determined quantity.

(ii) It may be equal to zero, positive, negative, whole number or fractional number.

Examples:

(i) Reaction between H2 and Br2 to form HBr.

H2 + Br2 → 2HBr ; rate = k [H2] [HBr]½ ; order of reaction = 1 + ½ = 3/2.

(ii) Reaction between CO and Cl2 to form COCl2.

CO + Cl2 → COCl2 ; rate = k [CO]2 [Cl2]½ ; order of reaction = 2 + ½ = 5/2.

(iii) Decomposition reaction of HI gives H2 and I2

2HI → H2 + I2 ; rate = k [HI]2 ; order of reaction = 2

## What is the molecularity of reaction?

The number of reacting particles (molecules, atoms, or ions) that collide simultaneously in a rate determining step to form a product is called molecularity of a reaction.

In general, the molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation.

Examples:

PCl5 → PCl3 + Cl2 (Unimolecular reaction)

2SO2 + O2 → 2SO3 (Trimolecular reaction)

NO + O3 → NO2 + O2 (Bimolecular reaction)

Note:

(i) Molecularity of a reaction is always in the whole number.

(ii) It is never fractional.

(iii) Molecularity is a theoretical concept.

(iv) Molecularity cannot be greater than three because more than three molecules may not mutually collide with each other.

## Difference between order and molecularity of reactions

Order Molecularity
(a) Order of a reaction is the sum of the coefficients of the reacting species involved in the rate equation. (a) Molecularity is the number of reacting species involved in simultaneous collisions in an elementary or simplest reaction.
(b) Order of a reaction is determined experimentally. (b) The concept of molecularity is theoretical in nature.
(c) It is derived from the rate equation. (c) It is derived from the mechanism of reaction.
(d) Order of a reaction may be fractional in some cases. (d) Molecularity of the reaction is always a whole number.
(e) Order of a reaction can be zero. (e) Molecularity of a reaction cannot be zero.
(f) The reaction order is applicable in all chemical reactions (f) Is only applicable in simple reactions.

## Frequently Asked Questions on Order and Molecularity of reaction

Q1

### When the order and molecularity of a reaction will be equal?

Order and molecularity of a reaction will be the same if the reaction is elementary. let aA + bB + cC → Product is an elementary reaction then order of reaction is equal to molecularity of reaction = a + b +c .

Q2

### What is the difference between order and molecularity?

Order of a reaction is the sum of the coefficients of the reacting species involved in the rate equation. Molecularity is the number of reacting species involved in simultaneous collisions in an elementary or simplest reaction. Order is an experimentally determined quantity. It may be equal to zero, positive, negative, whole number or fractional number. Molecularity is a theoretical concept, it is always in whole numbers.

Q3

### What is molecularity given as an example?

The molecularity of a reaction is defined as the number of reacting molecules which collide simultaneously to bring about a chemical reaction. In other words, the molecularity of an elementary reaction is defined as the number of reactant molecules taking part in the reaction.For example, consider the reaction H2 + I2 → 2HI , it is a bimolecular reaction.

Q4

### Can molecularity of reaction be zero?

Molecularity is the number of reacting species involved in simultaneous collisions in an elementary or simplest reaction. Since at least one molecule must be present, so that molecularity will be at least one. Hence the molecularity of any reaction can never be equal to zero.

Q5

### Why is molecularity not fractional?

Molecularity of a reaction is the number of reacting species (atoms, ions or molecules) taking part in an elementary reaction. So, the molecularity of a reaction is always a positive integer, it can not be negative or a fraction.