Order and Molecularity of Reaction

The order and molecularity of a reaction are fundamental properties that determine the reaction kinetics and rate. This article explores the concepts of Order of Reaction and Molecularity, their types, differences, and other relevant details.

1.0Order of a Reaction

In chemical kinetics, the order of a reaction concerning a specific reactant is defined as the power to which the concentration of that reactant is raised in the rate equation.

The order of a reaction is essential in determining the rate of the reaction using the rate constant.

Different Values of the Order of Reaction:

• Zero Order: The rate of reaction is independent of the concentration of the reactant.
• First Order: The rate of reaction is directly proportional to the concentration of one reactant.
• Second Order: The rate of reaction is proportional to the square of the concentration of a reactant or the product of the concentrations of two reactants.

The overall order of a reaction can be determined by summing the individual orders (or powers) of the concentration of each reactant. 

For example, if reactant A is first order (power of 1) and reactant B is first order (power of 1), then the overall reaction would be second order:

Overall Order = 1 (A) + 1 (B) = 2

2.0Determination 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 powers 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 reaction rate is to changes in the concentration of A and B.

For Example:

• Decomposition of Hydrogen Peroxide (H₂O₂)

H₂O₂ → H₂O + 1/2 O₂

Rate = k [H₂O₂]

Order of reaction = 1

• Saponification of Ethyl Acetate

CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

Rate = k[CH₃COOC₂H₅][NaOH]

Order of reaction = 2

3.0 Characteristics of order of a reaction

• The reaction order does not depend on the stoichiometric coefficients of the species in the balanced chemical equation.
• The order of the reaction is usually an integer but can also be fractional or zero.
• The reaction order is determined experimentally, providing insights into the reaction mechanism and rate.
• The reaction order gives the reaction rate dependency on the reactants' concentration.
• It depends on experimental conditions.
• The order of reaction for both complex and elementary reactions can be determined.

4.0Molecularity of a Reaction

The molecularity of a reaction is the number of molecules (or atoms or ions) that must collide simultaneously for a reaction to occur. It refers to the number of reacting species involved in an elementary reaction.

In the case of the general reaction:

aA+bB→cC+dD

The molecularity would be the sum of the coefficients of the reactants, which is:

Molecularity=a+b

Here, a represents the number of molecules of A, and b represents the number of molecules of B involved in the reaction.

Molecularity is always a whole number and applies exclusively to elementary reactions.

Types

Unimolecular Reactions: Reactions with molecularity one are called unimolecular reactions. In these reactions, only one reactant molecule is involved in the rate-determining step. Examples include:

• NH₄NO₂ → N₂ + 2H₂O
• Br₂ → 2Br

Bimolecular Reactions: Reactions with molecularity two are called bimolecular reactions. These involve the collision of two reactant molecules. An example is:

• 2HI → H₂ + I₂

Trimolecular Reactions: Reactions with molecularity three are called trimolecular reactions. These involve the collision of three reactant molecules, such as:

• 2NO + O₂ → 2NO₂

However, the probability of three particles colliding simultaneously and reacting is relatively low, making trimolecular reactions rare. Therefore, molecularity higher than three is generally not observed in elementary reactions.

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

The concept of molecularity applies only to elementary reactions, which occur in a single step with a specific number of reactants.

Characteristics: Always a whole number and applicable only to elementary reactions.

5.0Difference between Order and Molecularity of a Reaction 

Molecularity is different from the order of reaction, as the order refers to the sum of the exponents of the concentrations in the rate law, while molecularity pertains specifically to the number of reactants in an elementary reaction. Molecularity is always an integer and can only be 1, 2, or 3 for elementary reactions.