GCSE Chemistry Rates of Reaction Notes
Rate of reaction is how fast reactants are used up or products are formed. Collision theory explains every factor that changes it.
Key concepts
- Collision theory
- Reactions happen when particles collide with enough energy (≥ activation energy) and correct orientation.
- Rate
- Change in amount of reactant or product per unit time (g/s, cm³/s or mol/s).
- Catalyst
- Substance that speeds up a reaction by providing a lower activation-energy pathway without being used up.
- Increasing concentration or pressure increases frequency of collisions.
- Increasing temperature increases both frequency and energy of collisions.
- Increasing surface area (smaller pieces / powder) increases collision frequency.
Key formulas
Mean rate of reaction
rate = quantity of reactant used or product formed ÷ time
Worked example
50 cm³ of gas is collected in 25 s. Calculate the mean rate of reaction.
- 1rate = volume ÷ time.
- 2rate = 50 ÷ 25.
- 3rate = 2.0 cm³/s.
Answer: 2.0 cm³/s
Common mistakes
✗ Saying a catalyst is used up in the reaction.
✓ A catalyst is chemically unchanged at the end and can be reused.
✗ Explaining higher temperature only as 'more collisions'.
✓ State more frequent AND more energetic collisions.
✗ Reading the wrong axis when finding rate from a graph.
✓ Rate = gradient. Use tangent at a point or (y₂−y₁)/(x₂−x₁) for a straight line.
Quick check
Try these, then reveal the answers.
How does increasing temperature affect rate?Reveal answer
Increases it — particles collide more often and with more energy.
How does a catalyst speed up a reaction?Reveal answer
By lowering the activation energy.
Units for rate when measuring volume of gas per second?Reveal answer
cm³/s
Exam tips
- Always link a factor to collision theory in your explanation.
- Find rate from a graph using the gradient — draw a tangent if the curve is not linear.
- State the units in your final answer.
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Frequently asked questions
What is collision theory?+
The idea that reactions occur when particles collide with sufficient energy (≥ activation energy) and the correct orientation.
Why does a powder react faster than a lump?+
Powder has a larger surface area, so more particles are exposed and collisions happen more often.
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