AQA GCSE Chemistry
Rates of reaction and energy changes — AQA GCSE Chemistry revision
Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 4 minutes to read.
What speeds a reaction up
- Higher temperature: particles move faster and collide more often with more energy.
- Higher concentration or pressure: more particles in the same space, so more frequent collisions.
- Smaller pieces (bigger surface area): more particles exposed to collide.
- Catalyst: gives a different pathway with lower activation energy and is not used up.
Energy changes
- Exothermic reactions release energy and heat the surroundings (combustion, neutralisation).
- Endothermic reactions take energy in and cool the surroundings (thermal decomposition).
- Energy change = energy to break bonds − energy released making bonds. Negative = exothermic.
Key terms
- Activation energy
- The minimum energy particles need to react on collision.
- Catalyst
- Speeds up a reaction without being used up.
- Exothermic
- Releases energy to the surroundings — temperature rises.
- Collision theory
- The idea that reactions only happen when particles collide with enough energy and the correct orientation.
- Exothermic reaction
- A reaction that transfers energy to the surroundings, so temperature increases.
- Endothermic reaction
- A reaction that takes in energy from the surroundings, so temperature decreases.
Common exam traps
- Saying a catalyst lowers the temperature — it lowers activation energy.
- Saying particles 'collide harder' without mentioning frequency of collisions.
- Mixing bond breaking (endothermic) with bond making (exothermic).
Practice questions with answers
1. Which change always increases the rate of reaction?
- • Cooling the mixture
- • Using larger lumps
- • Increasing concentration
- • Adding water
Answer: Increasing concentration
More particles per volume means more frequent successful collisions.
2. Define activation energy.
Answer: The minimum energy particles must have for a collision to result in a reaction.
Say 'minimum energy' for the mark.
3. Bond breaking is:
- • Exothermic
- • Endothermic
- • Neither
- • Both
Answer: Endothermic
Energy must be supplied to break bonds.
4. A reaction's temperature drops. What type of reaction is it?
Answer: Endothermic — energy is taken in from the surroundings.
Surroundings cool because energy moves into the reaction.
5. Which factor does NOT usually increase the rate of a reaction?
- • Decreasing temperature
- • Increasing concentration
- • Adding a catalyst
- • Increasing surface area
Answer: Decreasing temperature
Lower temperature means less kinetic energy and fewer successful collisions, slowing the reaction.
Keep going in the app
This topic has 58 questions in total, plus flashcards, cram mode and Grade 9 Push challenges.
More free Chemistry revision
- All AQA Chemistry revision pages
- All GCSE Chemistry topics
- AQA Chemistry: Atoms, elements and the periodic table
- AQA Chemistry: Ionic, covalent and metallic bonding
- AQA Chemistry: Moles, masses and conservation
- AQA Chemistry: Chemical changes: reactivity, acids and electrolysis
- AQA Chemistry: Energy changes: exothermic, endothermic and bond energies
- AQA Chemistry: Reversible reactions and equilibrium
- Every subject on ReviseRush
ReviseRush is an independent educational revision aid. It is not affiliated with, endorsed by or connected to AQA, Edexcel, OCR, WJEC, Pearson or any exam board. All notes and questions are original. Always check content against your official course and specification. No grade outcome is guaranteed.