Endothermic: definition in chemistry
Endothermic: Describes a process or reaction that absorbs energy, usually as heat, from its surroundings, so the surroundings get colder. Its enthalpy change (ΔH) is positive.
In an endothermic reaction, more energy is needed to break the bonds in the reactants than is released when bonds form in the products. The extra energy comes from the surroundings, which is why the container often feels cold. Melting, boiling and photosynthesis are all endothermic, and instant cold packs use the endothermic dissolving of ammonium nitrate in water.
Examples
- Ice melting in a drink
- Photosynthesis, which absorbs energy from sunlight
- Instant cold packs containing ammonium nitrate and water
- Heating limestone: CaCO₃ → CaO + CO₂
On the periodic table
Removing an electron from an atom is always endothermic, and the energy needed, the ionization energy, generally rises from left to right across each period.