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Collision Theory

Last Updated : 15 Jun, 2023
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Collision Theory says that when particles collide (strike) each other, a chemical reaction occurs. However, this is necessary but may not be a sufficient condition for the chemical reaction. The collision of molecules must be sufficient to produce the desired products following the chemical reaction. The effective collision process, on the other hand, will determine the qualities and properties of the resulting product. As a result, understanding the collision theory is required in order to understand and determine the resulting products.

Max Trautz and William Lewis created the Collision Theory of Chemical Reactions in 1916-1918, which was based on the kinetic theory of gases. The kinetic Theory of Gases explains the behavior of gases by imagining them as a swarm of particles, molecules, or atoms moving in random directions.

Theory of Collision

According to the Theory of Collision, the collision of molecules is a pre-requisite condition for a chemical reaction to occur. It is a simple rule that more molecules lead to more collisions. As a result, the fraction of collision is determined by the number of particles involved in the collision. Collisions should have enough energy called Activation Energy to start a reaction. Since a chemical reaction involves bond breaking and bond formation, hence, bond disruption will occur only if the collision strength is strong. 

Collisions are temperature-dependent—the higher the temperature, the more collisions. Collisions become more violent at higher temperatures. Because neutral molecules have a lower energy level, they cannot break bonds or participate in the collision process, whereas molecules with sufficient energy will. Bending, stretching, and twisting the bond are all part of the reaction process. As a result, the process requires energetic molecules.

Collision of Molecules gives an idea about energy and the mechanism of a chemical reaction.

Collision Theory of Chemical Reactions

According to the Collision Theory of Chemical Reactions, “The molecules of reactants are assumed to be hard spheres, and the reactions are assumed to occur only when these spheres (molecules) clash with each other”. Hence, it became necessary to quantify the number of collisions that occurred in a chemical reaction to produce products in order to have a clear image of the reaction. Hence, the term collision frequency was coined. 

Molecular Collisions

Collision theory of chemical reactions and their kinetics has made significant advances that are critical in today’s fast-paced world. Be it packaged drinking water, water bottles, steel production plants, the fastest motor vehicles, or synthetically engineered biological implants, they all involve a chemical reaction in some form. In order to gain a better understanding of these chemical events, 

The basic postulates of Molecular Collisions are,

  • More molecules result in more molecular collisions.
  • In the reaction, various molecules collide to perform the collision.
  • The increase in the temperature results in more molecular collisions.

Collision Frequency

Collision Frequency is the number of collisions per second per unit volume of the reacting mixture. It is commonly represented by the letter Z.

We already know that the rate of a chemical reaction is affected by Activation Energy, hence, we will establish a relation between the Rate of Reaction, Collision Frequency, and the Activation Energy of a chemical reaction. Consider the following reaction:

P + Q → Product

According to Collision Theory, the Rate of the above reaction is given by:

Rate = ZPQe−Ea/RT


  • ZPQ is collision frequency of reactants P and Q
  • Ea is Activation Energy
  • R is Universal Gas Constant
  • T is Temperature in absolute scale

If we compare the above equation with Arrhenius’s Equation k = A -Ea/RT  we find that A which is the Pre-Exponential Factor in Arrhenius’s equation is similar to ZPQ i.e. Collision Frequency.

Effective Collision

In real scenario especially in the case of complex reaction, not all collision leads to the formation of a product. In order to form a product the collision of molecules must have minimum or sufficient kinetic energy and should also have proper orientation. Such a collision of molecules in which there is minimum energy and proper orientation that leads to the breaking and formation of a bond is called an Effective Collision.

To take account of effective collisions out of total collisions we have a factor ρ which is called the steric factor or the probability factor. Hence, the above equation for the Rate of Reaction can be rewritten as

Rate = ρZPQe−Ea/RT

Thus, we can say that Activation Energy and Proper Orientation are the two most important factors in determining Effective Collision and hence, the Rate of Reaction.

Apart from the above two mentioned factors, the surface area also impacts collision and rate of reaction. We will see its mechanism below:

Collision Theory: Surface Area

When the surface area is large, more molecules are present, and more molecules can react with each other, resulting in a higher collision or reaction rate. As a result, the larger the surface area, the faster the response. Furthermore, according to the collision hypothesis, if the surface area of molecules is greater, it has more energy and boosts the reaction rates.

Since not all collisions lead to the formation of new products based on this collision is classified into two categories. We will learn those types below.

Types of Collision

The types of collision are classified on the basis of the formation of products. Basically, there are two types of collision

  • Elastic Collision
  • Inelastic Collision

Elastic Collision

In Elastic Collision, the system’s kinetic and momentum energy are both conserved. It means the total Kinetic Energy of the two bodies before and after the collision remains the same. The collision of distinct subatomic particles is primarily elastic in this case. The impact of two glass or steel balls, for example, is often elastic. The forces involved in elastic collisions are conservative in nature.

Pictorial Representation of Elastic Collision is given below:

Elastic Collision


Inelastic Collision

An inelastic collision is one in which kinetic energy is not conserved and only momentum is conserved. The Kinetic Energy gets transformed into other forms of energy say Thermal Energy, Sound Energy, etc.  Every day, we encounter numerous collisions that are mostly inelastic. For Example, a ball hitting the ground from a height. Some of the kinetic energy gets transformed into thermal and sound energy.

Pictorial Representation of Inelastic Collision is given below:

Inelastic Collision


The activation energy is another quantity that has a substantial impact on the speeds of chemical processes (Ea). Arrhenius used the term activation energy to describe the least amount of energy that reactants must have in order to generate a product during a chemical reaction.

Activation Energy

Activation Energy is the minimum amount of energy required by the reacting particles in any given reaction for that reaction to occur. Particles do not react unless they collide with enough energy to produce the Activation Energy. Before a reaction may occur, Activation Energy must be given. To begin a chemical reaction, chemical bonds in the reactants must be broken, which takes energy. The energy required to initiate the reaction is referred to as activation energy. When the Activation Energy is low enough, the reaction can begin at ambient temperature without being heated. When the Activation Energy gap is large enough, then the reaction occurs at elevated temperature i.e. external energy is provided to break the barrier of Activation Energy. 

Activation Energy

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FAQs on Collision Theory

Q 1: What is the Collision Theory of Chemical Reactions?


Collision Theory of Molecule states that for a chemical reaction to happen molecules of reactant must collide effectively and in a proper orientation.

Q2: What is Collision Frequency?


Collision Frequency is the number of collision per second per unit volume of reaction mixture.

Q3: What is the relation between Rate of Reaction and Collision Frequency?


The relation between Rate of Reaction and Collision Frequency is given by 

Rate = ρZPQe−Ea/RT 


  • ZPQ is the collision frequency of reactants P and Q
  • Ea is the Activation Energy
  • R is the Universal Gas Constant
  • T is the Temperature in absolute scale
  • ρ is the Steric Factor

Q4: What is Activation Energy?


Activation energy is defined as the minium energy required to initiate a chemical recation.

Q5: What is Effective Collision?


Effective Collision refers to the collision in which reactant molecule collide with a minimum threshold energy and in proper orientation so as there is breaking and formation of bond among the atoms of the molecule.

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