Key Difference – Collision Theory vs Transition State Theory
Collision theory and transition state theory are two theories that are used to explain the reaction rates of different chemical reactions at the molecular level. Collision theory describes the collisions of gas molecules in gas-phase chemical reactions. Transition state theory explains the reaction rates by assuming the formation of intermediate compounds that are transition states. The key difference between collision theory and transition state theory is that collision theory relates to the collisions between gas molecules whereas transition state theory relates to the formation of intermediate compounds in transition states.
CONTENTS
1. Overview and Key Difference
2. What is Collision Theory
3. What is Transition State Theory
4. Side by Side Comparison – Collision Theory vs Transition State Theory in Tabular Form
5. Summary
What is Collision Theory?
The collision theory explains that gas-phase chemical reactions occur when molecules collide with sufficient kinetic energy. This theory is built based on the kinetic theory of gases (the kinetic theory of gases describe that gases contain particles having no defined volumes but with defined masses and there are no intermolecular attractions or repulsions between these gas particles).
Figure 01: If there are many gas particles in a small volume, then the concentration is high, then the probability of colliding two gas particles is high. This results in a high number of successful collisions
According to the collision theory, only a few collisions between gas particles cause these particles to undergo considerable chemical reactions. These collisions are known as successful collisions. The energy required for these successful collisions is known as activation energy. These collisions can cause breakage and formation of chemical bonds.
What is Transition State Theory?
Transition state theory indicates that, in between the state where molecules are reactants and the state where molecules are products, there is a state known as the transition state. The transition state theory can be used to determine the reaction rates of elementary reactions. According to this theory, the reactants, products and transition state compounds are in chemical equilibrium with each other.
Figure 02: A Diagram Showing Reactants, Products and Transition State Complexes
The transition state theory can be used to understand the mechanism of an elementary chemical reaction. This theory is a more accurate alternative to the Arrhenius equation. According to the transition state theory, there are three major factors affecting the mechanism of a reaction;
- The concentration of the transition state compound (known as activated complex)
- The rate of the breakdown of the activated complex – this determines the rate of the formation of the desired product
- The way of the breakdown of the activated complex – this determines the products formed in the chemical reaction
However, according to this theory, there are two approaches to a chemical reaction; the activated complex may return to the reactant form, or it can break apart in order to form product(s). The energy difference between reactant energy and transition state energy is known as the activation energy.
What is the Difference Between Collision Theory and Transition State Theory?
Collision Theory vs Transition State Theory |
|
| The collision theory explains that the gas-phase chemical reactions occur when molecules collide with sufficient kinetic energy. | Transition state theory indicates that, in between the state where molecules are reactants and the state where molecules are products, there is a state known as the transition state. |
| Principle | |
| Collision theory states that chemical reactions (in the gas phase) occur due to collisions between reactants. | Transition state theory states that chemical reactions occur via going through a transition state. |
| Requirements | |
| According to the collision theory, only successful collisions cause chemical reactions to occur. | According to transition state theory, a chemical reaction will progress if the reactants can overcome the activation energy barrier. |
Summary – Collision Theory vs Transition State Theory
Collision theory and transition state theory are used to explain the reaction rates and mechanisms of different chemical reactions. The difference between collision theory and transition state theory is that collision theory relates to the collisions between gas molecules whereas transition state theory relates to the formation of intermediate compounds in transition states.
Reference:
1.“Collision Theory.” Chemistry LibreTexts, Libretexts, 22 May 2017. Available here
2.“Transition State Theory.” Wikipedia, Wikimedia Foundation, 28 Feb. 2018. Available here
3.“9.7: Theories of Reaction Rates.” Chemistry LibreTexts, Libretexts, 21 July 2016. Available here
Image Courtesy:
1.’Molecular-collisions’By Sadi_Carnot (Public Domain) via Commons Wikimedia
2.’Rxn coordinate diagram 5’By Chem540grp1f08 – Own work, (CC BY-SA 3.0) via Commons Wikimedia
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