Factors Affecting Rates
- Factors that can affect the rate of a reaction are:
- The concentration of the reactants in solution or the pressure of reacting gases
- The temperature of the reaction
- Surface area of solid reactants
- The presence of a catalyst
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 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 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
Exam Tip
- You should be able to recall how changing the concentration, pressure, temperature, surface area and catalysts affect the rate of reaction
- The specification talks about the effects of increasing temperature, concentration, pressure and surface area but you could be expected to apply the same knowledge when they are decreasing
- When answering questions on the effect of concentration / pressure on the rate of reaction, you should mention that there are more particles per unit volume (usually cm3) rather than just more particles.
