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
Memory Aid
- EXO = EXIT = energy EXITS to surroundings = temperature goes UP
- The "exo" prefix means "outside" or "exit". Energy exits the chemical system and goes into the surroundings, warming them up. Hand warmers, fires, and explosions all release heat — they all EXOT energy OUT.
- ΔH sign rule: EXO = EXtra (negative) — you gain extra heat in the surroundings
- Exothermic → ΔH is NEGATIVE (the system loses energy, so the change is negative)
- On the energy profile: "EXothermic EXits — the products EXIT below the reactants on the diagram."