// FIELD GUIDE 10 — LEVEL L3 (GCSE / NATIONAL 5)
Energetics: where the energy goes
Breaking bonds always costs energy; making bonds always releases it. Whichever wins decides whether a reaction warms your hands or cools them.
// FIG. 01 — REACTION ENERGY PROFILE
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// L3 — EXOTHERMIC
Making the new bonds releases more energy than breaking the old ones cost. The surplus energy escapes as heat — the surroundings warm up. Combustion and neutralisation are both exothermic; that's why a burning fire and a hand-warmer both feel hot.
// L3 — ENDOTHERMIC
Breaking the old bonds costs more energy than making the new ones gives back. The reaction has to pull that shortfall in from its surroundings — which cool down. Instant cold packs and photosynthesis are both endothermic.
// L3 — ACTIVATION ENERGY
Every reaction — exothermic or endothermic — has to climb over a hill first, labelled Eₐ on the diagram. That's why a pile of wood doesn't spontaneously combust: it needs a spark (a little starting energy) to get over the hill, even though burning releases far more energy once it's going.
// REFERENCES & FURTHER READING
Cross-check against your own exam board's specification (AQA, Edexcel, OCR, SQA).