Endothermicity
Definition and meaning of Endothermicity in chemistry.
Endothermicity is the property of a chemical or physical process by which it absorbs heat from its surroundings, resulting in a positive enthalpy change (ΔH > 0). Endothermic processes leave the surroundings cooler as thermal energy flows into the system.
In more detail
A process is endothermic when the total bond energy of the products is lower than that of the reactants, meaning more energy is required to break existing bonds than is released when new bonds form. This energy deficit is supplied by the surroundings, so the temperature of the surroundings drops as the reaction proceeds. Endothermicity is quantified by a positive ΔH (at constant pressure) and is distinct from spontaneity: an endothermic process can still occur spontaneously if it is accompanied by a sufficiently large increase in entropy (ΔS), since spontaneity depends on the sign of ΔG = ΔH − TΔS.
Key facts
| Field | Physical Chemistry |
|---|---|
| Sign convention | ΔH > 0 (heat absorbed by system) |
| Opposite term | Exothermic (ΔH < 0) |
| Common examples | Photosynthesis, melting ice, thermal decomposition |
Dissolving ammonium nitrate (NH4NO3) in water is endothermic, with ΔH ≈ +25.7 kJ/mol; this is the reaction used in instant cold packs, where breaking the crystal's ionic lattice absorbs more energy than is released upon hydration of the ions.
Frequently asked questions
Does endothermic mean a reaction cannot happen spontaneously?
No. Spontaneity depends on Gibbs free energy, ΔG = ΔH − TΔS. An endothermic reaction (ΔH > 0) can still be spontaneous if the entropy increase (ΔS > 0) is large enough that TΔS exceeds ΔH, making ΔG negative.
How can you tell a reaction is endothermic just by touch?
The container or surrounding solution feels cold, because the reaction is pulling heat energy out of its surroundings to drive the process forward.