Using Collision Theory to Explain the Factors that Affect the Rate of Reaction

  • Collision theory states that chemical reactions occur only when the reactant particles collide with sufficient energy to react.
  • The minimum amount of energy needed is called the activation energy , which is different for each reaction.
  • Particles that collide with insufficient energy have unsuccessful collisions and just bounce off each other.
  • The rate of a reaction is therefore also dependent on the energy of collisions as well as the number of collisions.
  • To increase the rate of a reaction then the number of successful collisions needs to be increased.

Collision Theory - Particles Colliding, IGCSE & GCSE Chemistry revision notes

Diagram showing a successful and an unsuccessful collision

Factors & Proportionality

  • Increasing the number of successful collisions means that a greater proportion of reactant particles collide to form product molecules.
  • The various factors achieve this in different ways.
  • The effects of changing the following parameters are discussed
    • Increasing concentration / pressure
    • Increase the surface area of a solid reactant
    • Increasing temperature
    • Use of a catalyst

Concentration of a Solution / Pressure of a Gas

Reaction rate-increase in concentration, IGCSE & GCSE Chemistry revision notes

Diagram showing increase in concentration of solution

Explanation:

  • Increase in the concentration of a solution, the rate of reaction will increase.
  • This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second, increasing the rate of reaction.
  • For a gaseous reaction, increasing the pressure has the same effect as the same number of particles will occupy a smaller space, increasing the concentration.

Surface area of a solid

Surface Area of a Solid Reactant, IGCSE & GCSE Chemistry revision notes

Diagram showing surface area to volume ratio of various sized cubes

Explanation:

  • With an increase in the surface area of a solid reactant, the rate of reaction will increase.
  • This is because more surface area of the particles will be exposed to the other reactant, producing a higher number of collisions per second.
  • For the diagram shown above, the large cube has a surface area of 2 x 2 x 6 = 24cm2.
  • When broken up into 8 smaller cubes, the new surface area is 1 x 1 x 6 x 8 = 48cm2.
  • Therefore the same mass of substance has doubled its surface area by being broken up into smaller pieces.

Temperature

Reaction rate-effect of temperature on particles, IGCSE & GCSE Chemistry revision notes

Diagram showing the effect of temperature on particles

Explanation:

  • Increase in the temperature, the rate of reaction will increase
  • This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more frequent and successful collisions per second, increasing the rate of reaction

Need help?

AQA GCSE Chemistry Notes

Want to aim for a Level 9?

See if you’ve got what it takes. Test yourself with our topic questions.

Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.