Rates of Reaction Factors (CIE IGCSE Chemistry)

• Factors that can affect the rate of a reaction are:
  • The concentration of the reactants in solution or the pressure of reacting gases
  • Surface area of solid reactants
  • The temperature of the reaction 
  • The presence of a catalyst
• Changes in these factors directly influence the rate of a reaction
• It is of economic interest to have a higher rate of reaction as this implies a higher rate of production and hence a more efficient and sustainable process

The effect of increased concentration or pressure

Graph showing the effect of concentration 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 increasing surface area

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

Surface area and particle size

Surface area increases as particle size decreases. A 2 cm³ cube has a surface area of 24 cm² and the same cube cut up into 8 cubes has a surface area of 48 cm²

The effect of increasing temperature

Graph showing the effect of temperature on rate of reaction

Increasing the temperature increases the rate of reaction

Explanation:

• 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 

The effect of using a catalyst

• Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction
• The mass of a catalyst at the beginning and end of a reaction is the same and they do not form part of the equation

Graph showing the effect of using a catalyst on rate of reaction

Graph showing the effect of using a catalyst on the rate of reaction

Explanation:

• Compared to a reaction without a catalyst, the graph line for the same reaction but with a catalyst has a steeper gradient at the start and becomes horizontal sooner
• This shows that with a catalyst, the rate of reaction will increase