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Collision Theory of Chemical Reactions

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The term collision refers to the collision of particles or molecules. According to collision theory, when particles collide (strike) each other, a chemical reaction occurs. Furthermore, reactant molecules must collide, although this may not be sufficient 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.

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, Max Trautz and William Lewis created the Collision Theory of Chemical Reactions in 1916-1918, which was based on the kinetic theory of gases. We’ve all heard of the kinetic theory of gases. It explains the behaviour of gases by imagining them as a swarm of particles, molecules, or atoms moving in random directions.

Molecular Collisions: Collision Theory

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. The collision of molecules is required prior to the chemical reaction. Collisions should have enough kinetic energy to start a reaction between them. 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 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,” according to the collision hypothesis. So it was necessary to quantify the number of collisions that occurred in order to produce products in order to have a clear image of the reaction, and so the term collision frequency was coined. The number of collisions per second per unit volume of the reacting mixture is referred to as the collision frequency. It is commonly represented by the letter Z. Take a look at the following bimolecular elementary reaction:

P + Q → Product

According to collision theory, the rate of the preceding reaction is given by:

Rate = ZPQρe−Ea/RT


ZPQ = collision frequency of reactants P and Q

Ea = Activation Energy

R = Universal Gas Constant

T = Temperature in absolute scale

ρ = is the steric factor

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.

According to the Arrhenius Equation, any molecules with energies greater than or equal to the activation energy will collide to produce products. However, this was not the case for all of the reactions. In reactions involving complicated compounds, there was a large level of variance. Some molecules with enough energy (activation energy) did not collide to produce the result. Only a few of them had effective collisions that resulted in the production of products. The researchers discovered that the kinetic energy of the molecules is not the only factor governing the reaction. They found that only molecules with the right orientation and threshold energy (activation energy) during the collision will produce products. To account for effective collisions, they created the probability factor P.

In a nutshell, the activation energy and appropriate orientation of the interacting molecules together provide the condition for an efficient collision, which results in the production of products. In collision theory, the rate of a reaction is governed by both activation energy and effective collision.

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.

Types of Collision Theory

  1. Elastic collision: An elastic accident occurs when the system’s kinetic and momentum energy are both conserved. 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 inelastic collisions are conservative in nature.
  2. Inelastic collision: An inelastic collision is one in which kinetic energy is not conserved and only momentum is conserved. Every day, we encounter numerous collisions that are mostly inelastic.

An ineffective collision (A) is one that does not result in product formation. An effective collision (B) is one in which chemical bonds are broken and a product is formed.

Activation Energy

The activation energy is the smallest 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.

Sample Problems

Question 1: How does collision theory work?


The molecules of reactants are supposed to be hard spheres in the collision theory, and reactions are assumed to occur only when these spheres (molecules) impact with each other. It is not necessary for all collisions to result in the synthesis of products; the activation energy and appropriate orientation of the interacting molecules establish the condition for a collision that will result in the development of products.

Question 2: What is the difference between effective and ineffective collision?


The reacting species in a chemical reaction can only produce products if they come into touch with one other or crash with each other. It is not required for all collisions to result in products. Collisions between reacting species that result in the product are referred to as effective collisions.

Question 3: Is the Arrhenius equation only for the first-order reaction?


The Arrhenius equation expresses a link between the rate constant, absolute temperature, and the A factor (also known as the pre-exponential factor; can be visualized as the frequency of correctly oriented collisions between reactant particles). It sheds light on the relationship between reaction rates and absolute temperature. As a result, it can be applied to any order of reaction.

Question 4: What is activation energy?


Activation energy is defined as the additional energy that must be delivered to the reacting species in their normal energy state in order for their energy to equal threshold energy.

Question 5: Where is the Arrhenius equation used?


The Arrhenius equation is used to calculate the effect of temperature change on the rate constant. Temperature and the rate constant were mathematically related by Arrhenius.

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Last Updated : 28 Feb, 2022
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