The key difference between activated complex theory and collision theory is that activated complex theory is more accurate and reliable when describing the reaction rates, whereas collision theory is less reliable.
Activated complex theory and collision theory are two important concepts that describe the thermodynamics of chemical reactions. These theories can be used to predict the reaction rates of chemical reactions. The activated complex theory is also named as transition state theory. However, the activated complex theory is considered to be more accurate compared to collision theory.
CONTENTS
1. Overview and Key Difference
2. What is Activated Complex Theory
3. What is Collision Theory
4. Side by Side Comparison – Activated Complex Theory vs Collision Theory in Tabular Form
5. Summary
What is Activated Complex Theory?
The activated complex theory is a thermodynamic theory which describes the presence of a transition state between the reactants and the final products. Therefore, it is also named as the transition state theory or TST theory. This theory provides more accurate and reliable information compared to collision theory. This theory was developed by Henry Eyring in 1935.
The activated complex theory describes the activation energy (Ea) of a reaction mixture and the thermodynamic properties involving the transition state. This theory is a development of the collision theory and uses the basis of the Arrhenius equation. Also, this theory describes the statistical frequency factor, v, which is a fundamental factor of this theory.
According to activated complex theory, there is an intermediate state between the state of the reactants and the state of the products of a reaction mixture. It is called the transition state, which contains an activated complex compound. This activated complex forms by the combination of reactants. According to this theory, there are major facts we need to consider in order to determine whether the reaction will occur. The facts are as follows:
- The concentration of the activated complex in the transition state
- The rate of the breakdown of this activated complex
- The way of the breakdown of the activated complex (the complex may break down forming the products or it may form reactants again)
Apart from that, the activated complex theory also suggests the concept of activation energy regarding a chemical reaction. The activation energy is the energy barrier of the reaction; a certain amount of energy is essential for a chemical reaction to occur. The activated complex is a high energy complex which is unstable, and it has the maximum energy of the reaction process. If the reaction mixture gets an energy amount that is equal to this activation energy, then the reaction mixture can overcome the energy barrier and give products of the reaction.
What is Collision Theory?
Collision theory is a thermodynamic theory which describes the progression of a chemical reaction through the collision between reactants to form products. According to this theory, if two molecules collide with each other for a chemical reaction to take place, the factors influencing the ease of the collision are important in predicting the reaction progression. E.g. the more energy supplied to the reaction mixture, the faster the reactants move, colliding with each other. Similarly, high-temperature conditions can cause more collisions between reactants and high reaction rates.
In the collision theory, the rate at which the reactant molecules collide with each other is named as the collision frequency, z. It gives the units of collisions per unit of time. According to the collision theory, the energy of the reactant mixture and the concentration of the reactants affect the reaction rate. However, in order for a successful collision between reactants to occur, the reactants must collide with each other with enough kinetic energy to break down the chemical bonds in the reactants and to form new chemical bonds, forming the final product. This amount of energy is named as the activation energy.
What is the Difference Between Activated Complex Theory and Collision Theory?
Activated complex theory and collision theory are important thermodynamic theories. The key difference between activated complex theory and collision theory is that the activated complex theory is more accurate and reliable when describing the reaction rates, whereas collision theory is less reliable.
Below infographic tabulates the differences between activated complex theory and collision theory in more detail.
Summary – Activated Complex Theory vs Collision Theory
Activated complex theory and collision theory are important thermodynamic theories. The key difference between activated complex theory and collision theory is that the activated complex theory is more accurate and reliable when describing the reaction rates, whereas collision theory is less reliable.
Reference:
1. “Activated Complex.” Wikipedia, Wikimedia Foundation, 18 June 2020, Available here.
2. “9.7: Theories of Reaction Rates.” Chemistry LibreTexts, Libretexts, 1 Sept. 2020, Available here.
Image Courtesy:
1. “Quasi-equilibrium1” By Chem540grp1f08 – Own work (Public Domain) via Commons Wikimedia
2. “Molecular-collisions” By Sadi_Carnot – (Public Domain) via Commons Wikimedia