Collision Theory

When reactants come together the kinetic energy they possess means their particles will collide and some of these collisions will result in chemical bonds being broken and some new bonds being formed
Increasing the number of successful collisions means that a greater proportion of reactant particles collide to form product molecules
We can use collision theory to explain why these factors influence the reaction rate
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

Collision Energy, downloadable IB Chemistry revision notes

Collision theory helps to explain the energy process when particles react in chemical changes

Explaining Rates

There are several factors that can affect the rate of a reaction. These are:
- Concentration of the reactants in solution
- Temperature at which the reaction is carried out
- Surface area of solid reactants
Changes in these factors directly influence the rate of a reaction

Concentration of a Solution

Reaction Kinetics Concentration on Rate of Reaction, downloadable AS & A Level Chemistry revision notes

The diagram shows a higher concentration of particles in (b) which means that there are more particles present in the same volume than (a) so the number of collisions between reacting particles is increased causing an increased rate of reaction

Explanation:

Increasing the concentration of a solution will increase the rate of reaction
This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second
If you double the number of particles you will double the number of collisions per second
The number of collisions is proportional to the number of particles present

Temperature

Effect of temperature, downloadable IB Chemistry revision notes

An increase in temperature causes an increase in the kinetic energy of the particles. The number of successful collisions increases

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
The effect of temperature on collisions is not so straight forward as concentration or surface area; a small increase in temperature causes a large increase in rate
For aqueous and gaseous systems, a rough rule of thumb is that for every 10 degree (Kelvin) increase in temperature the rate of reaction approximately doubles

Surface area of a solid

Effect of surface area, downloadable IB Chemistry revision notes

An increase in surface area means more collisions per second

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
If you double the surface area you will double the number of collisions per second

Surface Area and Particle Size

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

Diagram showing that surface area increase as particle size decreases. A 2 cm³cube has a surface area of 24 cm²and the same cube cut up into 8 cubes has a surface area of 48 cm²

Examiner Tip

You should be able to recall how changing the concentration, pressure, temperature and surface area affect the rate of reactions

Collision Theory (Edexcel GCSE Chemistry: Combined Science)

Revision Note