Difference Between Molecularity and Order of Reaction

Main Difference – Molecularity vs Order of Reaction

The rate of a chemical reaction can mainly depend on the temperature and pressure of the system, the concentration of the reactants present, presence or absence of catalysts and the nature of reactants. However, the rate of the reaction is calculated considering the rate determining step. Some chemical reactions have only one step, but some reactions occur in several steps. In that case, the rate of the reaction is determined by the slowest step. Molecularity and order of reaction are two terms used regarding the rate of reactions. The main difference between molecularity and order of reaction is that molecularity is a theoretical concept whereas order of reaction can be determined experimentally.

Key Areas Covered

1. What is Molecularity
      – Definition, Explanation with Examples
2. What is Order of Reaction
      – Definition, Explanation with Examples
3. What is the Difference Between Molecularity and Order of Reaction
      – Comparison of Key Differences

Key Terms: Bimolecular, First Order Reaction, Molecularity, Order of Reaction, Second Order Reactions, Trimolecular, Unimolecular, Zero Order Reactions

What is Molecularity

Molecularity is the number of molecules or ions that take part in the rate determining step. The rate determining step is the slowest step among the other steps of the reaction mechanism. The slowest step is considered as the rate determining step because the whole reaction rate would be increased if the rate of the slowest step is increased. Molecularity of the reaction is named according to the number of molecules or ions that participate in the rate determining step.

Unimolecular Reactions

In unimolecular reactions, a single molecule undergoes changes. Then, the equation for the rate determining step has only one reactant.

Figure 01: The conversion of N₂O₅ into N₂O₃ and O₂ is Unimolecular

Bimolecular Reactions

These reactions involve two reactants in the rate determining step.

Figure 2: A Bimolecular Reaction

Trimolecular Reactions

These reactions involve three reactants in the rate determining step of a chemical reaction.

What is Order of Reaction

The order of reaction can be defined as the sum of the powers to which the reactant concentrations are raised in the rate law equation. The rate law is the equation that gives the reaction rate with the use of reactant concentrations and constant parameters such as the rate constant.

Order of reaction is the sum of the exponents of the rate law. The order of the reaction may or may not equal to the stoichiometric coefficients of each reactant. Therefore, the order of reaction should be determined experimentally. Order of reaction is a quantitative measurement regarding the rate of a reaction. Unlike molecularity, the order of reaction can be given in fractional values or whole numbers. Order of reaction can also be zero. This means that the reaction rate is independent of the concentration of the reactants. Let us consider an example.

aA   +     bB   +  cC    →        dD    +      eE

The rate law of the above reaction is,

R    =     k [A]^p[B]^q[C]^r

Where,

R is the rate of the reaction

A, B and C are reactants

P, q and r are orders of reaction of to A, B and C, respectively.

The order of the reaction is equal to the sum of p+q+r.

The values of p, q and r should be determined experimentally. Sometimes, these values can be equal to stoichiometric coefficients of each reactant, but sometimes it may not. The order of reaction is calculated considering the overall reaction, not only the rate determining or the slowest step. According to the order of reaction, there can be several types of reactions.

Figure 3: Graph of Reactant Concentration vs. Time of Reaction

Zero Order Reactions

The rate of a zero order reaction is independent of the concentrations of reactants.

First Order Reactions

In first order reactions, the rate of the reaction is dependent on the concentration of only one reactant. This corresponds to a unimolecular reaction.

Second Order Reactions

The reaction rate of second order reactions may depend on the concentration of one second order reactant or two first order reactants.

Difference Between Molecularity and Order of Reaction

Definition

Molecularity: Molecularity is the number of molecules or ions that take part in the rate determining step.

Order of Reaction: Order of reaction is the sum of the powers to which the reactant concentrations are raised in the rate law equation.

Rate Determining Step

Molecularity: The rate determining step is used to obtain the molecularity.

Order of Reaction: The overall reaction is used to obtain the order of reaction.

Value

Molecularity: Molecularity is always a whole number.

Order of Reaction: Order of reaction can be zero, a whole number or a fraction.

Determination

Molecularity: Molecularity is determined by looking at the reaction mechanism.

Order of Reaction: The order of reaction is determined by experimental methods.

Conclusion

The molecularity and order of reaction are two different terms used to explain the rate of a chemical reaction. The molecularity is obtained from the reaction mechanism. The order of reaction is obtained from the rate law of the reaction. The difference between molecularity and order of reaction is that molecularity is a theoretical concept whereas order of reaction is determined experimentally.

References:

1. “Molecularity and Kinetics.” Chemistry LibreTexts. Libretexts, 21 July 2016. Web. Available here. 31 July 2017. 
2. “Order of reaction.” Wikipedia. Wikimedia Foundation, 25 June 2017. Web. Available here.  31 July 2017.