Endothermic
Definition and meaning of Endothermic in chemistry.
Endothermic describes a chemical reaction or physical process that absorbs heat energy from its surroundings, resulting in a positive enthalpy change (ΔH > 0).
In more detail
In an endothermic process, more energy is required to break the bonds of the reactants than is released when new bonds form in the products, so the reaction pulls thermal energy from its surroundings, causing them to cool. This is why endothermic reactions often feel cold to the touch. On a reaction energy diagram, the products end up at a higher energy level than the reactants, and the missing energy comes from the environment. Some endothermic reactions proceed spontaneously despite absorbing heat, provided they are driven by a sufficiently large increase in entropy.
Key facts
| Field | Physical Chemistry |
|---|---|
| Enthalpy change | ΔH > 0 |
| Example compound | NH4NO3 (ammonium nitrate) |
| Opposite of | Exothermic |
Dissolving solid ammonium nitrate in water is endothermic: the surrounding water cools noticeably as the salt dissociates, which is the principle behind instant cold packs.
Frequently asked questions
How can you tell a reaction is endothermic without instruments?
The reaction vessel feels cold, since the reaction is absorbing heat from its surroundings, including your hand.
Can an endothermic reaction still be spontaneous?
Yes. Spontaneity depends on Gibbs free energy, ΔG = ΔH − TΔS. If the entropy increase (ΔS) is large enough at a given temperature, ΔG can be negative even when ΔH is positive.