Explaining Rates Using Collision Theory (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

Extended tier only

• Collision theory can explain why concentration, pressure, surface area, temperature and catalysts affect the rate of a chemical reaction 

How increasing concentration affects rate

• Increasing the concentration of a solution increases the rate of reaction 
• Increasing the concentration means that there are more reactant particles in a given volume
  • This causes more collisions per second
  • Leading to more frequent and successful collisions per second
  • Therefore, the rate of reaction increases 
• 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

Diagram showing the effect of increasing concentration

A higher concentration of particles in (b) means that there are more particles present in the same volume than (a) so the number of collisions and successful collisions between particles increases causing an increased rate of reaction

How increasing pressure affects rate

• Increasing the pressure of a gas increases the rate of reaction 
• Increasing the pressure means that there are the same number of reactant particles in a smaller volume
  • This causes more collisions per second
  • Leading to more frequent and successful collisions per second
  • Therefore, the rate of reaction increases 

Diagram showing the effect of increasing pressure

The higher pressure (b) means that there are the same number of particles present in a smaller volume than (a) so the number of collisions and successful collisions between particles increases causing an increased rate of reaction

How increasing the surface area affects rate

• Increasing the surface area increases the rate of reaction 
• Increasing the surface area means that a greater surface area of particles will be exposed to the other reactant
  • This causes more collisions per second
  • Leading to more frequent and successful collisions per second
  • Therefore, the rate of reaction increases
• If you double the surface area, you will double the number of collisions per second

Diagram showing the effect of increasing surface area

An increase in surface area means more collisions per second

How increasing temperature affects rate

• Increasing the temperature increases the rate of reaction 
• Increasing the temperature means that the particles have more kinetic energy
  • This causes more collisions per second
  • Leading to more frequent and successful collisions per second
  • Therefore, the rate of reaction increases
• The effect of temperature on collisions is not so straightforward 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 ^oC increase in temperature, the rate of reaction approximately doubles

Diagram showing the effect of increasing temperature

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

How a catalyst affects rate

• Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction
• Different processes require different types of catalysts but they all work on the same principle
• A catalyst provides a different reaction pathway that has a lower activation energy
  • This means a higher proportion of the reactant particles have energy greater than the activation energy and will result in more successful collisions per second

A catalyst lowers the activation energy of a reaction by providing an alternative reaction pathway

• An important industrial example is iron in the production of ammonia
  • Iron beads are used to increase the surface area available for catalysis
• Enzymes are biological catalysts, they work best at specific temperature and pH ranges
• Normally only small amounts of catalysts are needed to have an effect on a reaction