Difference Between Activation Energy and Threshold Energy

The key difference between activation energy and threshold energy is that the activation energy describes the potential energy difference between the reactants and the activated complex whereas the threshold energy describes the energy required by reactants to collide with each other successfully to form the activated complex.

Energy is the ability to do work. If there is sufficient energy we can use that energy to do some work we desire; in chemistry, this work can be either a chemical reaction or a nuclear reaction. activation energy and threshold energy are two terms that we use in chemistry to define two different forms of energy.

1. Overview and Key Difference
2. What is Activation Energy
3. What is Threshold Energy
4. Side by Side Comparison – Activation Energy vs Threshold Energy in Tabular Form
5. Summary

What is Activation Energy?

Activation energy is a form of energy that we need to activate a chemical or nuclear reaction or any other reaction. Most of the times, we measure this energy form in the unit kilojoules per mole (kJ/mol). This form of energy is the potential energy barrier that avoids a chemical reaction from progressing. This means it prevents the reactants from converting into the products. Moreover, in order to progress a chemical reaction in a thermodynamic system, the system should reach a high temperature that is enough to provide the reactants with an energy that is either equal or greater than the activation energy barrier.

Difference Between Activation Energy and Threshold Energy

Figure 01: Reaction Rate in the absence and presence of a Catalyst

If the system gets enough energy, then the reaction rate increases. However, in some cases, the reaction rate decreases when we increase the temperature. This is due to the negative activation energy. We can calculate the reaction rate and the activation energy using the Arrhenius equation. It is as follows:

K = Ae^-E_a/(RT)

Where k is the reaction rate coefficient, A is the frequency factor for the reaction, R is the universal gas constant and T is the absolute temperature. Then E_a is the activation energy.

In addition to that, catalysts are substances that can lower the activation energy barrier for a reaction. it does so by modifying the transition state of reaction. Moreover, the reaction does not consume the catalyst while progressing the reaction.

What is Threshold Energy?

The threshold energy is the minimum energy that a pair of particles must have in order to undergo a successful collision. This term is highly useful in particle physics rather than in chemistry. Here, we talk about the kinetic energy of particles. This collision of particles forms the activated complex (intermediate) of a reaction. Therefore, the threshold energy equals the sum of kinetic energy and activation energy. Hence, this form of energy is always either equal to or greater than the activation energy.

What is the Difference Between Activation Energy and Threshold Energy?

Activation energy is a form of energy that we need to activate a chemical or nuclear reaction or any other reaction. It describes the potential energy difference between the reactants and the activated complex. Moreover, its value is always either equal to or lower than the threshold energy of the same thermodynamic system. The threshold energy , on the other hand, is the minimum energy that a pair of particles must have in order to undergo a successful collision. It describes the energy required by reactants to collide with each other successfully to form the activated complex. In addition to that, the value of this energy is always either equal to or greater than the activation energy of the same thermodynamic system. The below infographic presents the difference between activation energy and threshold energy in tabular form.

Summary – Activation Energy vs Threshold Energy

We can define both threshold energy and activation energy for a thermodynamic system. The key difference between activation energy and threshold energy is that the activation energy describes the potential energy difference between the reactants and the activated complex whereas the threshold energy describes the energy required by reactants to collide with each other successfully to form the activated complex.

Reference:

1. “Activation Energy.” Wikipedia, Wikimedia Foundation, 27 July 2018. Available here
2. “Threshold Energy.” Wikipedia, Wikimedia Foundation, 9 Aug. 2018. Available here