Factors That Affect Reaction Rates (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

• Factors that can affect the rate of a reaction are:

  • The temperature of the reaction 

  • The concentration of the reactants in solution or the pressure of reacting gases

  • The surface area of solid reactants

  • The presence of a catalyst

• Changes in these factors directly influence the rate of a reaction

The effect of temperature on 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 is not as straightforward as concentration or surface area

  • A small increase in temperature causes a large increase in rate

  • A rough rule of thumb: for every 10°C increase in temperature, the rate of reaction approximately doubles

Graph showing the effect of temperature on rate of reaction

Explanation

• Compared to a reaction at a lower 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 

The effect of concentration and pressure on rate

Concentration

• 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

Pressure

• 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 

Graph showing the effect of concentration/pressure on rate of reaction

Increasing the concentration of a solution or gas pressure increases the rate of reaction

Explanation

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

  • 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 

The effect of surface area on rate

• Increasing the surface area increases the rate of reaction

• Increasing the surface area means that a greater surface area of particles is 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

• Doubling the surface area doubles the number of collisions per second

• Breaking up a solid reactant into smaller pieces increases the surface area in contact with any solution it reacts with

• Increasing the surface area to volume ratio increases the rate of reaction for a given mass of solid reactant

Graph showing the effect of surface area on rate of reaction

Increasing the surface area increases the rate of reaction

Explanation

• 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 

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