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Difference Between Unimolecular and Bimolecular Reactions

February 20, 2018 Posted by Madhu

Key Difference – Unimolecular vs Bimolecular Reactions
 

In chemistry, the term molecularity is used to express the number of molecules that come together to react in an elementary reaction. An elementary reaction is a single step reaction that gives the final product directly after the reaction between reactants. This means that the elementary reactions are chemical reactions that have no intermediate steps before the formation of the final product. Unimolecular and bimolecular reactions are such elementary reactions. The key difference between Unimolecular and bimolecular reactions is that unimolecular reactions involve only one molecule as a reactant whereas bimolecular reactions involve two molecules as reactants.

CONTENTS

1. Overview and Key Difference
2. What are Unimolecular Reactions
3. What are Bimolecular Reactions
4. Similarities Between Unimolecular and Bimolecular Reactions
5. Side by Side Comparison – Unimolecular vs Bimolecular Reactions in Tabular Form
6. Summary

What are Unimolecular Reactions?

Unimolecular reactions are elementary reactions that involve only one molecule as a reactant. There, the reaction is a rearrangement reaction. The single molecule rearranges to form more different molecules as final products. But this occurs in a single step. There are no intermediate steps that the reactant molecule undergoes in the formation of the final product. It directly gives the final products. The equation for the reaction can be given as

A    →    P

Here A is reactant and P is the product. According to the first order of rate law, the rate of reaction can be given as below.

Rate = k [reactant]

Some examples of unimolecular reactions include followings:

Difference Between Unimolecular and Bimolecular Reactions

Figure 01: Cyclopropane rearrangement to form propane.

  1. Conversion of N2O4 into two NO2 molecules
  2. Conversion of cyclopropane into propene
  3. Conversion of PCl5 into PCl3 and Cl2

What are Bimolecular Reactions?

Bimolecular reactions are elementary chemical reactions that involve two molecules as reactants. It can be described as the collision of two molecules or particles. These are common chemical reactions in organic and inorganic chemistry. The two molecules can be of the same or different types. For example, the two molecules can be two NOCl molecules with the same atomic arrangement or can be C and O2 having different atomic combinations. The equations for the bimolecular reactions is given as below.

A    +   A   →     P

A    +   B   →     P

Key Difference Between Unimolecular and Bimolecular Reactions

Figure 02: Energy diagram for a bimolecular reaction.

Since there are two reactants, these reactions are explained as second-order reactions. Therefore, these bimolecular reactions are described by the second order rate law;

Rate = [A]2

Or

Rate = [A][B]

In which the overall order is always 2. Some examples of bimolecular reactions are given below.

  1. Reaction between CO and NO3
  2. Reaction between two NOCl molecules
  3. Reaction between Cl and CH4

What are the Similarities Between Unimolecular and Bimolecular Reactions?

  • Both Unimolecular and Bimolecular Reactions are elementary reactions.
  • Both Unimolecular and Bimolecular reactions give the product in a single step.
  • Both Unimolecular and Bimolecular reactions have no intermediate steps.

What is the Difference Between Unimolecular and Bimolecular Reactions?

Unimolecular vs Biomolecular Reactions

Unimolecular reactions are elementary reactions that involve only one molecule as a reactant. Bimolecular reactions are elementary chemical reactions that involve two molecules as reactants.
 Reactants
Unimolecular reactions have one reactant Bimolecular reactions have two reactants.
Order of Rate Law
Unimolecular reactions are explained via first order of rate law. Bimolecular reactions are explained via second-order rate law.
 Overall Order
The overall order of the rate equation of unimolecular reactions is always 1. The overall order of the rate equation of bimolecular reactions is always 2.

Summary – Unimolecular vs Bimolecular Reactions 

Unimolecular and bimolecular reactions are elementary reactions. These reactions give the product in a single step. These reactions can be expressed using rate laws as well. The difference between Unimolecular and bimolecular reactions is that unimolecular reactions involve only one reactant whereas bimolecular reactions involve two molecules as reactants.

Download the PDF of Unimolecular vs Bimolecular Reactions

You can download the PDF version of this article and use it for offline purposes as per citation note. Please download the PDF version here: Difference Between Unimolecular and Bimolecular Reactions

Reference:

1.“Elementary Reactions.” Chemistry LibreTexts, Libretexts, 26 May 2017. Available here
2.“Elementary reaction.” Wikipedia, Wikimedia Foundation, 10 Feb. 2018. Available here
3.“Molecularity.” Wikipedia, Wikimedia Foundation, 13 Feb. 2018. Available here

Image Courtesy:

1.’Izomerization of cyclopropane’By Djokili92 – Own work, (CC BY-SA 4.0) via Commons Wikimedia  
2.’Rxn coordinate diagram 5’By Chem540grp1f08 – Own work, (CC BY-SA 3.0) via Commons Wikimedia

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Filed Under: General Chemistry Tagged With: Bimolecular Reactions, Bimolecular Reactions Definition, Bimolecular Reactions Order, Bimolecular Reactions Process, Bimolecular Reactions Reactants, Compare Unimolecular and Bimolecular Reactions, Unimolecular and Bimolecular Reactions Differences, Unimolecular and Bimolecular Reactions Similarities, Unimolecular Reactions, Unimolecular Reactions Definition, Unimolecular Reactions Order, Unimolecular Reactions Process, Unimolecular Reactions Reactants, Unimolecular vs Bimolecular Reactions

About the Author: Madhu

Madhu is a graduate in Biological Sciences with BSc (Honours) Degree and currently persuing a Masters Degree in Industrial and Environmental Chemistry. With a mind rooted firmly to basic principals of chemistry and passion for ever evolving field of industrial chemistry, she is keenly interested to be a true companion for those who seek knowledge in the subject of chemistry.

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