What is the concept of the collision theory?

Collision theory in chemistry is the concept that guides the rates of chemical reactions.

The collision theory states that before a chemical reaction will occur there must be collisions between reactant particles and only a fraction of these collisions leads to the formation of products.

Why is it only a fraction that can lead to formation of products?

This is because before a collision can form products, there must be a minimum amount of energy that must be attained or overcome before products are formed.

This minimum amount of energy is otherwise called activation energy.

Activation energy is thus defined as the minimum amount that must be equalled or overcome before products are formed.

This energy serves as a barrier energy that prevents or delays reactions from occurring.

In the same vein, that type of collision that Keady to the formation of products by overcoming the activation energy is called effective collision.

So, it is important to say again that not all collisions form products but only a fraction of the collisions.

Consequently, we have two types of collisions: effective collisions and ineffective collisions.

Effective collision leads to formation of products by equaling or overcoming the activation energy.

Ineffective collision is the type of collision that do not lead to formation because it cannot overcome Activation energy.

Let me explain a little what I mean by barrier energy and how it functions.

Consider this reaction equations for instance.

Case A

Mg + Cl2===MgCl2

If takes 40J of energy barrier before this reaction occurs.

Case B

Ca + Cl2==CaCl2

If it takes 30J of energy barrier before this reaction occurs

Case C

4Na + O2 ====2Na2O

If it takes 20J of energy barrier before thus reaction occurs


The first implication is that the reaction with the highest energy barrier (40J) required to equal or overcome will be the slowest of them all.

The reason is not far-fetched  ut simply because it will take time for the particles to be able to generate minimum energy that at least will equal the energy barrier or overcome it as the case maybe.

The second implication is that the reaction with smallest energy barrier (20J) needed to be equaled or overcome will be the fastest of them all and the reason is still similar.  It will take little for the collisions to generate energy equal or more than the activation energy.

Why is it only a fraction of collisions that form products?

Recall that according to collision theory, particles must collide before products are formed.

So, particles of reactants start colliding once we set up the reaction apparatus.

Though we recommend having enough particles to increase concentration of reactants so products can be formed however only effective collision leads to the formation of products.

But we have discovered that a high concentration of reactants will lead to high effective collision and that’s why increase in concentration leads to increase in rate of reaction.

Collision theory and Temperature

You can agree with me that collision theory is built on the principle of frequency of collision..

But there are certain factors that affect frequency of collision and thus rate of reaction.

Temperature affects collision theory and thus affects rate of reaction.

It is important to note that anything that affects collision theory will slowly affect rate of reaction.

How does temperature affect collision theory and rate of reaction?

Picture 31
Effect of Temperature

When you increase the heat from the Bunsen burner (you increase temperature) this supplies heat energy to the reactant particles thus increasing the average kinetic energy of the particles which increase the collision rate helping reactant particles collide faster with each other resulting to an increase in rate of reaction.

So, in conclusion, it implies that if other factors that affect rate of reaction is kept constant, chemical reactions will occur faster at higher temperatures than at lower temperatures.

Collision theory and concentration

Concentration is another factor that affects collision theory and by default rate of reaction.

Picture 33
Effect of concentration on rate of reaction

But how does this happen?

When you increase concentration of reactant particles. frequency of collisions increases.,

For example, mass of solid, volume of a gas and concentration in mol per dm3 of an aqueous reactant, the resultant effect is that you will have more of the particles crowded in the same space. 

This effect makes particles to become closer to each other and thus increases the frequency at which they hit each other therefore increasing collision rate and consequently rate of reaction.

Collision theory and Pressure

When you talk about concentration of gases, we use the term pressure.

Picture 20
Effect of Pressure on rate of reaction

Pressure is observed when you increase force on an area containing the gaseous reactants, the volume decreases making the gaseous reactants to be very close to each other and thus making the gaseous reactants to collide often increasing frequency of collisions and thus rate of reaction.

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