Molecularity vs. Order of Reactions

In any chemical reaction, the number of molecules involved is called the molecularity of the reaction. Whereas, the sum of exponents of the rate equation is known as the order of the reaction.

In other words, molecularity is a theoretical value and is independent of the reaction conditions. On the other hand, the order of the reaction is calculated experimentally.

Molecularity vs order of reactions

Molecularity	Order of reaction
The number of ions or molecules of reactant present in the reaction	It is the total no of molecules or ions on which the rate of the reaction depends
It is sum of the coefficients of reactant species in a reaction	It is the sum of exponents in the rate equation of that chemical reaction
It is a theoretical value	It is an experimental value
It can be easily evaluated from a stoichiometric chemical equation	It is the molecularity of the rate-determining step
It is never zero	It can be zero, e.g all metal surface catalyzed reactions and photocatalytic reactions are zero-order reactions
It is always in the whole number	It can be in fractions
Molecularity may be more than 3 but reactions with higher molecularity are rare	The highest value of the order of the reaction is 3

Molecularity of a chemical reaction

Chemical reactions proceed in different steps. Some reactions are single steps and are called elementary reactions. While others may have more than one step. The step which is the slowest is known as the rate-determining step and that determines the molecularity. So, in elementary reactions, the number of molecules their only step involves is the molecularity. On the other side, in reactions with more than one step, the rate-determining step must be known first.

Elementary reactions are further divided as:

Unimolecular reactions

The reaction in which only one molecule is known as a unimolecular reaction.

Bimolecular reactions

When the reaction proceeds with two molecules of reactant.

Ter molecular reactions

The reaction in which three different molecules collide successfully, according to collision theory.

Order of a reaction

Order of reaction is an important aspect of reaction kinetics. It tells us on what species do the rate of reactions depends. This is important while planning large-scale industrial processes to obtain the maximum yield.

Key Takeaway(s)

Concepts Berg

Can molecularity be fractional?

No, molecularity is the total number of molecules or ions oriented to collide and give products. It cannot be fractional because it’s impossible for a molecule to take part in a reaction in an incomplete manner.

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

Order and molecularity will be equal when the number of molecules involved in the rate-determining step is equal to the sum of the coefficient of the rate equation.

Why are molecularity and order the same in an elementary reaction?

The molecularity and order of the reaction are the same in elementary reactions. For example, consider the below reaction:

A + B → Products

Rate equation = [A]¹ [B]¹

The order of the reaction is the sum of the coefficient of the rate equation.

Therefore,

The order of reaction = 2 and molecularity = 2

Is the order of a complex reaction equal to the molecularity of its slowest step?

In complex reactions, the order of the overall reaction is equal to the molecularity of the slowest step. This is so because the rate of overall reaction depends upon the total number of molecules involved in the slowest step of the reaction. Hence, the molecularity of the slowest step is equal to the order of the overall reaction.

What is the difference between a first-order reaction and a unimolecular reaction?

A first-order reaction is one where the rate of the reaction depends on the concentration of the reactant. A unimolecular reaction is a reaction that involves only one molecule.

So, if a reaction is a unimolecular reaction as the first step and rate-determining, it will also become first order.

What is the overall order of a reaction?

The overall order of the reaction is the sum of the individual orders of reaction of the reactants.

Reference Books