exothermicendothermic

Exothermic and endothermic describe two classes of processes based on heat transfer between a system and its surroundings. An exothermic process releases heat to the surroundings, often causing the surroundings to warm. In thermodynamic terms, the enthalpy change ΔH is negative for exothermic processes when the reaction is considered at constant pressure. An endothermic process absorbs heat from the surroundings, causing the surroundings to cool; its enthalpy change ΔH is positive.

Common exothermic examples include combustion of fuels, oxidation reactions such as rusting, and many condensation or freezing processes that release latent heat. Common endothermic examples include melting of ice, boiling or vaporization of water, sublimation of solid substances, and photosynthesis in plants, which requires an input of energy. Some chemical reactions may have different energy profiles depending on conditions or stage; the same system can exhibit exothermic and endothermic steps in a sequence.

Because heat transfer governs observed temperature changes, calorimetry is used to measure ΔH for chemical reactions. Standard enthalpies of formation and Hess’s law enable prediction of whether a reaction is exothermic or endothermic and how much heat is released or absorbed under specified conditions.