Anendothermic process is a chemical or physical process that absorbs heat from its surroundings.[1] In terms ofthermodynamics, it is athermodynamic process with an increase in theenthalpyH (orinternal energyU) of the system.[2] In an endothermic process, the heat that a system absorbs isthermal energy transfer into the system. Thus, an endothermic reaction generally leads to an increase in thetemperature of the system and a decrease in that of the surroundings.[1]
The term was coined by 19th-century French chemistMarcellin Berthelot.[3] The termendothermic comes from theGreek ἔνδον (endon) meaning 'within' and θερμ- (therm) meaning 'hot' or 'warm'.[4]
An endothermic process may be a chemical process, such as dissolvingammonium nitrate (NH4NO3) inwater (H2O), or a physical process, such as the melting ofice cubes.[5]
The opposite of an endothermic process is anexothermic process, one that releases or "gives out" energy, usually in the form of heat and sometimes aselectrical energy.[1] Thus,endo in endothermic refers to energy or heat going in, andexo in exothermic refers to energy or heat going out. In each term (endothermic and exothermic) the prefix refers to where heat (or electrical energy) goes as the process occurs.[6]
Due to bonds breaking and forming during various processes (changes in state, chemical reactions), there is usually a change in energy. If the energy of the forming bonds is greater than the energy of the breaking bonds, then energy is released. This is known as an exothermic reaction. However, if more energy is needed to break the bonds than the energy being released, energy is taken up. Therefore, it is anendothermic reaction.[7]
Whether a process can occur spontaneously depends not only on theenthalpy change but also on theentropy change (∆S) andabsolute temperatureT. If a process is aspontaneous process at a certain temperature, the products have a lowerGibbs free energyG =H –TS than the reactants (anexergonic process),[2] even if the enthalpy of the products is higher. Thus, an endothermic process usually requires afavorable entropy increase (∆S > 0) in the system that overcomes the unfavorable increase in enthalpy so that still∆G < 0. While endothermicphase transitions into more disordered states of higher entropy, e.g. melting and vaporization, are common, spontaneous chemical processes at moderate temperatures are rarely endothermic.[8] The enthalpy increase ∆H ≫ 0 in a hypothetical strongly endothermic process usually results in∆G = ∆H –T∆S > 0, which means that the process will not occur (unless driven by electrical or photon energy). An example of an endothermic and exergonic process is
The terms "endothermic" and "endotherm" are both derived fromGreekἔνδονendon "within" andθέρμηthermē "heat", but depending on context, they can have very different meanings.
In physics,thermodynamics applies to processes involving a system and its surroundings, and the term "endothermic" is used to describe a reaction where energy is taken "(with)in" by the system (vs. an "exothermic" reaction, which releases energy "outwards").[12][13]
In biology,thermoregulation is the ability of anorganism to maintain its body temperature, and the term "endotherm" refers to an organism that can do so from "within" by using the heat released by its internal bodily functions (vs. an "ectotherm", which relies on external, environmental heat sources) to maintain an adequate temperature.[14]
.Bibcode:1998ApJ...494..285Q.doi:10.1086/305198.