Energy ChangesTopic Summary
Knowledge Organiser: Exothermic Reactions
This topic summary covers Knowledge Organiser: Exothermic Reactions within Exothermic Reactions for GCSE Chemistry. Revise Exothermic Reactions in Energy Changes for GCSE Chemistry with 20 exam-style questions and 14 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 12 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 12 of 12
Practice
20 questions
Recall
14 flashcards
Knowledge Organiser: Exothermic Reactions
Key Terms
- Exothermic — releases energy to surroundings
- ΔH — enthalpy (energy change) — negative for exothermic
- Activation energy (Ea) — minimum energy to react
- Energy profile — diagram of energy changes
- Surroundings — everything outside the reaction
Must-Know Facts
- Temperature of surroundings INCREASES
- Products have LESS energy than reactants
- ΔH is NEGATIVE
- Products are LOWER on energy profile
- Examples: combustion, neutralisation, respiration
- Energy made making bonds > energy needed breaking bonds
Key Equations
- ΔH = energy required to break bonds − energy released making bonds
- Exothermic: making bonds releases MORE energy than breaking them costs → ΔH negative
- q = mcΔT (used to calculate energy change from temperature change)
Common Mistakes
- Getting the ΔH formula backwards: ΔH = energy to break bonds (in) − energy released making bonds (out). Exothermic means OUT > IN, so ΔH is negative. The most common error is reversing the subtraction.
- Confusing ΔH sign: Exothermic = negative ΔH; endothermic = positive ΔH — a common sign error loses marks
- Saying products have more energy in exothermic reactions: Products have LESS energy than reactants — the difference is transferred to surroundings as heat
- Forgetting bond breaking is endothermic: ALL bond breaking requires energy input — even in overall exothermic reactions, energy is still needed to break reactant bonds first