Collision Theory (DP IB Chemistry): Revision Note

Collision theory

Kinetic energy and temperature

• The kinetic theory of matter accounts for the properties of solids, liquids and gases in terms of the interactions of particles and their relative energies

For more information on kinetic theory, see our revision note on The Kinetic Molecular Theory

• Kinetic energy refers to the energy associated with movement or motion.

• It is determined by the following equation:

KE = ½mv²

mass = m

velocity = v

• As the kinetic energy of the particles at the same temperature is equal, this means there is an inverse relationship between mass and velocity

  • This is why substances with a lower mass diffuse more quickly than those with greater mass at the same temperature

  • Particles in a substance have a range of kinetic energies due to their random motion

To learn more about how particle energies vary at a given temperature, see our revision note on the Maxwell-Boltzmann energy distribution curve

What is collision theory?

• Collision theory explains how chemical reactions happen

• For a reaction to occur, reactant particles must collide with enough kinetic energy

• The rate of a chemical reaction depends on how often successful collisions happen

• This is affected by four key factors:

  • Collision frequency – how often particles collide

  • Collision energy – the energy each particle has when they collide

  • Activation energy – the minimum energy needed for a reaction to occur

  • Collision geometry – the orientation or angle at which particles collide

Collision frequency

• If a chemical reaction is to take place between two particles, they must first collide

• The number of collisions between particles per unit time in a system is known as the collision frequency

• The collision frequency of a given system can be altered by:

  • Changing the concentration of the reactants

  • Changing the total pressure

  • Changing the temperature

  • Changing the surface area of the reacting particles

Collision energy

• Not all collisions result in a chemical reaction

  • Most collisions just result in the colliding particles bouncing off each other

  • Collisions which do not result in a reaction are known as unsuccessful collisions

• Unsuccessful collisions happen when the colliding species do not have enough energy to break the necessary bonds

• If they do not have sufficient energy, the collision will not result in a chemical reaction

• If they have sufficient energy, they will react, and the collision will be successful

  • The combined energy of the colliding particles is known as the collision energy

Collision energy

Activation energy

• The minimum energy the colliding particles need in order to react is known as the activation energy

• If the collision energy of the colliding particles is less than the activation energy, the collision will be unsuccessful

• If the collision energy is equal to or greater than the activation energy, the collision will be successful, and a reaction will take place

• The activation energy can be changed by the addition of a catalyst 

Collision geometry

• Particles have to have the right orientation when they collide for the reaction to be successful

  • This is particularly the case with large molecules with complex shapes

• Most collisions do not result in a reaction because they do not reach the activation energy rather than not having the correct collision geometry

• Ultimately, the rate of reaction depends on the number of successful collisions that happen per unit time

  • A successful collision is where the particles collide in the correct orientation and with sufficient energy for a chemical reaction to occur

• An unsuccessful 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

Successful and unsuccessful collisions