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Chemistry > Chemical Kinetics > Collision Theory of Chemical Reactions
Chemical Kinetics

Collision Theory of Chemical Reactions

Do you ever wonder how reactions actually happen? What are the conditions necessary for a reaction to happen? Is it certain that a reaction will take place even after the conditions are fulfilled? The answer to all these questions is Collision Theory. Let’s understand what collision theory is and learn about the conditions mentioned in the collision theory.

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Collision Theory

Collision theory basically explains how reactions occur and why different reactions have different reactions rates. It states that:

  • Molecules must collide in order to react.
  • In order to effectively initiate a reaction, the molecules in the collisions must have sufficient energy to bring about disruptions in the bonds of molecules.
  • A rise in temperature will cause the molecules to move faster and collide more vigorously, increasing the likelihood of bond cleavages and rearrangements greatly.
  • The reactions containing neutral molecules cannot take place at all until they have acquired the activation energy needed to stretch, bend or distort one or more bonds.

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Activation Energy

Activation energy is the energy that must be overcome in order for a reaction to occur. It is the minimum energy that is required to start a chemical reaction.

collision theory

Explanation of Collision Theory

As we discussed, collision theory qualitatively explains how chemical reactions occur and why different reactions have different rates. Consider a simple biomolecular step:

  • Clearly, if two molecules A and B are to react, they must approach closely enough to disrupt some of their existing bonds and to permit the creation of new bonds that are required to form products. We call this a collision. The frequency of collision between A and B in a gas will be proportionate to the concentration of each. If we double the concentration of A, the frequency of A-B collision will double. Doubling the concentration of B will have the same effect.
  • It is not enough that the molecules just collide. They need to be oriented in a specific manner that is appropriate for the process to occur. The molecules must collide with one another from the correct side. If they do not do so, the collision will not lead to the reaction.
  • The molecules must collide with energies greater than or equal to the activation energy of the reaction. If this does not happen the reaction will not take place. The molecules need the energy to break their existing bonds and form new bonds. This is the kinetic energy that the molecules possess. If this energy is not equal to or greater than the activation energy, the reaction will not proceed.

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Temperature dependence of Collison theory

Thermal energy relates direction to motion at the molecular level. As the temperature rises, the molecules move faster and collide more vigorously, therefore causing more collisions and increasing the likelihood of bond cleavages. In most cases, the activation energy is supplied in the form of thermal energy.

As the reaction is completing and products are being formed, the activation energy is returned in the form of vibrational energy which is quickly released as heat. Therefore, it very important for the molecules to collide with energies greater than or equal to the activation energy of the reaction.

collision theory

Rate of Reaction according to Collison theory

For a bimolecular elementary reaction, A + B → Products, the rate of reaction is,

Rate = ZAB e -Ea/RT  

where ZAB represents the collision frequency of reactants A and B and e Ea/RT represents the fraction of molecules with energies equal to or greater than the activation energy of the reaction. This is why different reactions have different reactions rates. Different reactions have different frequencies of reactants and different activation energies.

A collision that satisfies all the conditions in the collision theory and succeeds in forming a new product is known as an effective collision. Thus, the two important criteria in collision theory are the activation energy and proper orientation of molecules.

A Solved Question for You

Q: Mention the important criteria in order for a reaction to occur.

Solution:

  • The molecules must collide.
  • The molecules must have correct orientation, that is, they must collide from the correct side.
  • The colliding molecules must collide with energies greater than or equal to the activation energy of the reaction.
  • The molecules can be given kinetic energy in the form of thermal energy in order to increase the chance of bond cleavages.

Therefore, if all the criteria are fulfilled, the collision will lead to fruition.

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