Factors that Affect the Rate of Reactions (Oxford AQA IGCSE Combined Science Double Award)

Temperature

• Increasing the temperature increases the rate of reaction  because the particles have more kinetic energy

  • This causes more collisions per second

  • Leading to more frequent successful collisions per second

  • Therefore, the rate of reaction increases

Diagram showing the effect of increasing temperature

• The effect of temperature on collisions is not as 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

Graph for increasing temperature

Graph showing the effect of temperature on rate of reaction

• Compared to a reaction at a low temperature, the line graph for the same reaction at a higher temperature:

  • Has a steeper gradient at the start

  • Becomes horizontal sooner

  • Forms the same amount of product

• This shows that increasing the temperature increases the rate of reaction

Pressure

• Increasing the pressure of a gas means that there are the same number of reactant particles in a smaller volume

  • This causes more collisions per second

  • Leading to more frequent successful collisions per second

  • Therefore, the rate of reaction increases 

Diagram showing the effect of increasing pressure

Graph for increasing concentration / pressure

Graph showing the effect of concentration on rate of reaction

• Compared to a reaction with a reactant at a low pressure / concentration, the line graph for the same reaction at a higher pressure / concentration:

  • Has a steeper gradient at the start

  • Becomes horizontal sooner

  • Forms the same amount of product

• This shows that increasing the pressure / concentration increases the rate of reaction 

Concentration

• Increasing the concentration of a solution means that there are more reactant particles in a given volume

  • This causes more collisions per second

  • Leading to more frequent 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

• The graph for how increasing concentration affects the rate of a reaction is the same as the graph for increasing pressure

• A higher concentration means that the graph:

  • Has a steeper gradient at the start

  • Becomes horizontal sooner

  • Forms the same amount of product

• This shows that increasing the concentration (or pressure) increases the rate of reaction 

Surface Area

• Increasing the surface area means that more particles will be exposed to the other reactant

  • This causes more collisions per second

  • Leading to more frequent 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

  • The number of collisions is proportional to the surface area

• Increasing surface area can sometimes be described as decreasing solid particle size

Surface area and particle size

Diagram showing the effect of increasing surface area

Graph for increasing surface area

Graph showing the effect of surface area on rate of reaction

• Compared to a reaction with lumps of reactant, the line graph for the same reaction with powdered reactant:

  • Has a steeper gradient at the start

  • Becomes horizontal sooner

  • Forms the same amount of product

• This shows that increasing the surface area increases the rate of reaction