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Collision Theory : Definition, Explanation, and FAQs

Nov 8, 2022

Collision Theory

In today’s world of rapid change, the collision theory of chemical processes and associated kinetics has substantially advanced. Chemical reactions are present in everything from drinking water containers to steel mills, the quickest cars, and artificially created biological implants. 

William Lewis and Max Trautz, 1916-1918, invented “collision theory definition for chemical reactions” to better understand these chemical reactions. So let’s go over the collision theory in depth. 


What is Collision Theory?

Collision theory determines the speeds of chemical reactions, especially those involving gases. The collision theory assumes that for a chemical reaction to take place, the atoms or molecules of chemically reacting compounds and elements must meet together or collapse with each other.

But not every contact leads to a chemical adjustment. Only when the components are brought together to have a minimum level of internal energy equivalent to the reaction’s activation energy will a collision result in chemical change. 


Secondly, the colliding atoms or molecules must be interlinked to allow room for the essential atom and electron rearrangement. Thus, the collision hypothesis states that the pace of a chemical reaction is proportional to the frequency of collisions between particles.

Since the molecular or atomic collision of compounds and elements can only be determined with some certainty for gases, the collision theory only applies to gas-phase reactions.


Collision theory describes how chemical reactions take place and why various reactions occur at different speeds. The theory explains that:

  • The molecules or atoms of reacting compounds must interact with each other to begin a chemical reaction.
  • To successfully carry out a chemical reaction, the molecules or atoms of interacting compounds must have sufficient energy to produce breakdowns in the molecules’ bonds.
  • Increasing temperature leads molecules to travel faster and collide more violently, significantly raising the chances of bond fragmentation and rearrangements.
  • Neutral molecular reactions cannot occur until they have gained the activation energy required to stretch, bend, or deform one or more bonds.
Curiously Enough


Activation energy is the total amount of energy that must resist a chemical reaction. It is the least amount of energy needed to initiate a chemical reaction.

Collision Theory Explanation 

As explained previously, the collision theory definition explains qualitatively how chemical reactions happen and why various chemical reactions occur at different rates. Check out the following simple bimolecular step:

Consider there to be two molecules, A and B. They must get close enough for their existing bonds to dissolve and the development of new ones that are required to create new products to result in a reaction. 


In a gas, the collision rate between A and B is proportional to their concentrations. When A’s concentration is doubled, the frequency of A and B collisions doubles. Increasing the concentration of B has the same effect.

It is not sufficient for the molecules to collide. They must be orientated in a precise way for the process to take place. The molecules must collide from the appropriate side. The contact will not result in a chemical reaction if they do not.


For a chemical reaction to take place, the molecules’ collision energies must be greater than or equal to the reaction’s activation energy. The chemical reaction won’t take place if this intensity doesn’t exist. The energy required for the molecules to break down their existing bonds and make new bonds plays an important role in creating new products.

This energy is known as the molecules’ kinetic energy to carry out a chemical reaction. The chemical reaction will not proceed if this kinetic energy is not equal to or higher than the activation energy.


Collision Theory’s Temperature Dependency

Adequate thermal energy correlates with the direction of mobility of the atoms and molecules in the compounds or elements.

A higher thermal temperature causes molecules or atoms to move more swiftly and collide more violently, increasing the likelihood of bond breaks. Thermal energy is frequently used to give activation energy. 

As the chemical reaction finishes and the new product is formed, the activation energy is collected as vibrational energy and promptly discharged as heat. As a result, the molecules must collide with energies larger than or equivalent to the reaction’s activation energy.

Collision Theory predicts the Rate of Reaction.

For a bimolecular fundamental chemical reaction, A + B → New Product

the rate of reaction is, Rate = ZAB e-Ea/RT

Where ZAB indicates the frequency of collision between the two reactants A and B and e -Ea/RT indicates the percentage of molecules with energies equal to or higher than the reaction’s activation energy. It is why the rates of different it vary significantly. Each chemical reaction has different reactant frequencies and activation energies.

An effective collision meets all of the requirements in the collision theory and results in the formation of a new product. Thus, appropriate molecular orientation and activation energy are two crucial requirements in collision theory.

Test Yourself

Q. Mention the necessary conditions for a chemical reaction to proceed in the collision theory.

Ans: Following are the necessary conditions for a chemical reaction to proceed:

  • The atoms or molecules must come into contact.
  • The atoms or molecules must be properly oriented, which means they must strike from the right side.
  • The impacting molecules must collide with kinetic energies higher than or equal to the chemical reaction’s activation energy.
  • To increase the likelihood of bond fragmentation between the molecules, they must have enough kinetic energy to convert it into the form of heat energy.

As a result, if all of the criteria are met, the collision will occur.

Arrhenius Equation

The Arrhenius equation is a mathematical statement that quantifies the influence of temperature on the acceleration of a chemical reaction and serves as the foundation for all inferential expressions used to calculate reaction-rate constants. The reaction-rate constant is k, the rate at which molecules and atoms contact in a way that causes a reaction is A, the activation energy for the chemical reaction is E, the general gas constant is R (8.314 joules/kelvin/mole), and the actual temperature is T. The equation is often expressed as an exponential formula, 

k = Aexp(-E/RT), 

indicates that a minor rise in chemical reaction temperature will significantly increase the size of the reaction rate constant. 

J.J. Hood developed the Arrhenius equation after studying the fluctuation of rate constants of various processes with temperature. This equation is named after the Swedish chemist Svante Arrhenius, who demonstrated that it applies to practically all types of reactions. In contrast with the equation for the thermodynamic equilibrium constant, he also offered a theoretical basis for the equation. Later, the transition-state and collision theories of chemical reactions demonstrated that the numeric constants A and E were quantities reflective of the underlying chemical processes.


Thus, after reading the preceding article, students should understand what is collision theory. It is the theory used to estimate the speeds of chemical reactions, particularly those involving gases.

According to the definition of the collision hypothesis, in order for a chemical reaction to occur, the constituents (atoms or molecules) of the reacting elements or compounds must come into contact with one another. 

However, the collision theory definition does not have to result in a chemical change. A collision will only produce chemical change if the components are brought together to have a minimum amount of internal energy equal to the reaction’s activation energy.

Frequently Asked Questions

1. What exactly is a chemical reaction?

Ans. A chemical reaction is a process or a technique that transforms one or more compounds or elements, (also called as chemical reactants), into one or more different new compounds or elements, known as final products. Examples of these substances are chemical compounds and elements. A chemical reaction restructures the component atoms of the reactant molecules to produce various products.

Chemical reactions are essential to innovation, society, and life. Earth’s origin is based on these chemical reactions, the environment and oceans, and various other complex reactions that operate in all biological systems are dependent on these chemical reactions.

2Explain the law of mass action.

Ans. According to the law of mass action, a chemical reaction is always proportional to the masses of the reactive components products, with each mass increased to a power equal to the exponent that occurs in the chemical equation. The law was invented by the Norwegian scientist Peter Waage and Cato M. Guldberg between 1864 and 1879, although it is now solely of historical relevance. This law was beneficial for determining the right equilibrium equation for a chemical reaction, but it is now known that the rate expresses it exclusively for elementary reactions.

3. How does the collision theory function?

Ans. For the collision theory to function effectively, the molecules of reactants are supposed to be rigid spheres, and the chemical reactions are considered to proceed only when all these rigid molecular spheres interact with each other. But it should be noted that all collisions do not necessarily result in the formation of new products; the appropriate orientation and activation energy of the interacting molecules establish the prerequisite for a collision that will contribute to the emergence of products.

Collision theory


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