Energy in reactions
Chemistry · Key Stage 3 · The School
Out or in
An EXOTHERMIC reaction releases energy to its surroundings, which get warmer — burning, respiration, neutralisation, and a hand warmer. An ENDOTHERMIC reaction takes energy in, so the surroundings get colder — thermal decomposition, photosynthesis, and the instant cold packs used for sports injuries. Feeling the container is a genuine measurement, not a shortcut.
Bonds explain it
Breaking bonds REQUIRES energy; making bonds RELEASES it. If the bonds formed release more than the bonds broken absorbed, the surplus leaves as heat and the reaction is exothermic. If forming them releases less, the difference is drawn from the surroundings and the reaction is endothermic. The whole distinction is that one comparison.
Where you meet them
Almost every reaction you meet at home is exothermic: a gas hob, a car engine, a fire, rusting, and respiration in your own cells keeping you at 37°C. Endothermic reactions are rarer in daily life, which is why a cold pack feels surprising — most chemistry warms things up rather than cooling them down.
Add magnesium to dilute hydrochloric acid: the thermometer rises from 20 °C to 32 °C. Energy was released to the surroundings — EXOTHERMIC. Now dissolve ammonium nitrate in water: the thermometer FALLS from 20 °C to 12 °C, and the beaker feels cold. No cold was produced; the reaction absorbed energy from the water and from your hand — ENDOTHERMIC. Bond breaking takes energy in; bond making gives it out.
An ENERGY LEVEL DIAGRAM makes it visible. Reactants sit on the left, products on the right, and the height is energy. Exothermic: the products sit LOWER, and the drop is the energy released. Endothermic: the products sit HIGHER, and the rise is the energy taken in. Between them is a hump — the ACTIVATION ENERGY, the push needed to start. Every reaction has one, which is why a fire releases energy and still needs a match.