- 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.
Diagram showing a successful and an unsuccessful collision
- 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
Diagram showing increase in concentration of solution
- 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
Diagram showing surface area to volume ratio of various sized cubes
- 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.
Diagram showing the effect of temperature on particles
- 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