Difference Between Unimolecular and Bimolecular Reactions

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:

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

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?

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.

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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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