AQA A Level Chemistry

Revision Notes

1.7.1 Collision Theory

Collision Theory

Collision theory

  • The collision theory states that for a chemical reaction to take place the particles need to collide with each other in the correct orientation and with enough energy

Collision Theory Table

Reaction Kinetics Table 1_Rate of Reaction Collision Theory, downloadable AS & A Level Chemistry revision notes

  • An ineffective collision is when particles collide in the wrong orientation or when they don’t have enough energy and bounce off each other without causing a chemical reaction

 

Reaction Kinetics Effective and Non-Effective Collisions, downloadable AS & A Level Chemistry revision notes

(a) shows an ineffective collision due to the particles not having enough energy whereas (b) shows an effective collision where the particles have the correct orientation and enough energy for a chemical reaction to take place

Increase in reaction rate

  • The collision frequency is the number of collisions per unit time
  • When more collisions per unit time take place, the number of particles with energy greater than the Ea increases
  • This causes an increase in the rate of reaction

Activation Energy

  • For a reaction to take place, the reactant particles need to overcome a minimum amount of energy
  • This energy is called the activation energy (Ea)
  • In exothermic reactions the reactants are higher in energy than the products
  • In endothermic reactions the reactants are lower in energy than the products
  • Therefore, the Ea in endothermic reactions is relatively larger than in exothermic reaction

 

Reaction Kinetics Exothermic-Reaction-Activation-Energy, downloadable AS & A Level Chemistry revision notes

The diagram shows that the reactants are higher in energy than the products in the exothermic reaction, so the energy needed for the reactants to go over the energy barrier is relatively small

Reaction Kinetics Endothermic Reaction Activation Energy, downloadable AS & A Level Chemistry revision notes

The diagram shows that the reactants are lower in energy than the products in the endothermic reaction, so the energy needed for the reactants to go over the energy barrier is relatively large

  • Even though particles collide with each other in the same orientation, if they don’t possess a minimum energy that corresponds to the Ea of that reaction, the reaction will not take place
  • Therefore, for a collision to be effective the reactant particles must collide in the correct orientation AND possess a minimum energy equal to the Ea of that reaction

Catalysts and activation energy

  • A catalyst is a substance that increases the rate of reaction without taking part in the chemical reaction by providing the particles an alternative mechanism with a lower activation energy

Reaction Kinetics Catalyst Activation Energy, downloadable AS & A Level Chemistry revision notes

A catalyst increases the rate of a reaction by providing an alternative pathway which has a lower activation energy

Exam Tip

The activation energy is the energy needed to ‘activate’ the reactant particles in order for them to collide effectively and cause a chemical reaction.

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