Factors that Affect the Rate of ReactionsOxford AQA IGCSE Chemistry: Revision notes
Section 1
How does temperature affect rate of reaction?
Increasing the temperature increases the rate of reaction. When reactants are heated, their particles gain kinetic energy and move faster, so they collide more frequently and, crucially, a greater proportion of those collisions have energy greater than or equal to the activation energy, so more collisions are successful.
For temperature, always mention both effects: particles move faster (more frequent collisions) AND more collisions have enough energy to be successful — this is worth more marks than either point alone.
Section 2
How does concentration affect rate of reaction?
Increasing the concentration of reactants in solution increases the rate of reaction. A higher concentration means there are more particles of reactant in the same volume, so particles are more crowded together and collide more frequently, increasing the number of successful collisions per second.
Section 3
How does pressure affect rate of reaction?
For reactions involving gases, increasing the pressure increases the rate of reaction. Higher pressure squeezes the same number of gas particles into a smaller volume, increasing the frequency of collisions between particles.
Section 4
How does surface area affect rate of reaction?
Increasing the surface area of a solid reactant (for example by breaking a large lump into smaller pieces or a powder) increases the rate of reaction. A larger surface area means more particles of the solid are exposed and available to react at any one time, so collisions between the solid and the other reactant happen more frequently.
A large lump of calcium carbonate reacts more slowly with dilute hydrochloric acid than the same mass of calcium carbonate powder, because the powder has a much greater surface area exposed to the acid.
Section 5
What do all four factors have in common?
Temperature, concentration, pressure and surface area all increase the rate of reaction ultimately by increasing the frequency of successful collisions between reacting particles. Temperature is unique among these four because it also increases the energy of each collision, not just the frequency.
Must Know
- Increasing temperature increases rate: particles move faster (more frequent collisions) AND more collisions exceed the activation energy
- Increasing concentration increases rate by increasing collision frequency
- Increasing pressure of gases increases rate by increasing collision frequency
- Increasing surface area of solids increases rate by increasing collision frequency
- All four factors work by increasing the number of successful collisions per second; only temperature also increases the energy of collisions
That's the notes covered.
Carry on to the next subtopic.