Order and molecularity of reactions

Molecularity can be defined as the number of atoms or molecules that must simultaneously collide together for a reaction to occur. Depending upon the number of molecules involved in the reaction, reactions are classified in terms of molecularity as unimolecular, bimolecular, and termolecular.

1. Unimolecular reaction

PCl5 PCl3 + Cl2

2. Bimolecular reaction

CH3COOC2H5 + NaOH   CH3COONa + C2H5OH

3. Termolecular reaction

2 FeCl3 + SnCl2 2 FeCl2 + SnCl4

Only a few termolecular reactions are known. However, no reaction having a molecularity of >three is known because more than three molecules are unlikely to collide with each other at the same time. 

Determination of Molecularity

1. Molecularity is simply the sum of the molecules or different reactants as expressed by the balanced chemical equation in the case of simple reactions (elementary reactions).

1. O2F2 O2 + F2 (Unimolecular)

2. 2 HI H2 + I2 (Bimolecular)

3. 2 NO + O2 2 NO2 (Termolecular)

• The atoms, molecules, or ions participating in the rate-determining step decide the molecularity in the case of complex reactions (Multi-step reactions). The slowest step, among many steps in a reaction, is the rate-determining step and so the molecules or ions participating in this step, determine the molecularity of the reaction.

Let us consider the reaction.

HBrO3 + 6HI HBr + 3H2O + 3I2

The following are the steps of the reaction through which it occurs. 

HBrO3 + HI HBrO2 + HIO2 

HBrO2 + 4 HI HBrO2 + HIO

HIO + HI H2O + I2

Here, two molecules participate in the slowest step. Thus, the reaction is a bimolecular type.

Characteristics of molecularity

1. The Molecularity of a reaction can never be zero or a fraction. It is always a whole number.

2. The molecularity of a reaction does not provide any idea about the rate or mechanism of the reaction.

3. Molecularity is meaningful only for simple reactions or individual steps of complex reactions.

Order of reaction 

The concentration’s exponent or power for a specific reactant as specified in the law of rate determines the order of a reaction.. 

In the rate expression,

The sum of x + y = Order of reaction

Characteristics of the order of reaction

• The order of the reaction is a purely experimental quantity.

• It may be zero, a whole number, or infractions depending on whether x + y is equal to 0, 1, 2, or 3. The reactions are set to be zero order, first order, second order, or third order, respectively.

• The order of reaction can change with changes in conditions such as temperature, pressure, etc.

• When one of the reactants is present in excess, the second-order rate expression becomes a first-order rate expression because the value of change in concentration for the other reactant is taken as negligible.

Zero-order reaction

Zero-order reactions are the reactions in which the concentration of reactants does not change with respect to time and concentration rates.

For example, photosynthesis of HCl

In a zero-order reaction, It is the amount of reactant that changes and not the concentration of the reaction.

The expression for the rate constant for a zero-order reaction is;

The unit of rate constant for the zero-order reaction is mol L-1s-1.

First-order reaction

First-order reactions are defined as reactions in which the rate of reaction is directly proportional to the reactants’ concentration.

For example, the decomposition of

The expression for the rate constant for a first-order reaction is;

The unit of rate constant for a first-order reaction is s-1.

Second-order reaction

The second-order reaction is defined as a reaction in which the reaction rate is decided by the change in two concentration terms of reactants.

For example, the dissociation of Hydrogen iodide  

The expression for rate constant for a second-order reaction is-

The unit of rate constant for second-order reaction is mol-1 L s-1.

Third-order reaction

A third-order reaction is defined as a reaction in which the rate is determined by the variation of three concentration terms. The third and higher orders of reaction are rare and do not occur commonly.  

Differences between the order and molecularity of the reaction

Conclusion

In this article, we learned about the order and molecularity of a reaction. Molecularity can be defined as the number of atoms/molecules/ions that must simultaneously collide together for the reaction to occur. The total number of molecules or atoms whose concentration determines the rate of reaction is known as the order of the reaction. We also learned about zero, first, second, and nth-order reactions.