Endothermic

Details

Whether a process can occur spontaneously depends not only on the enthalpy change but also on the entropy change (∆S) and absolute temperatureT. If a process is a spontaneous process at a certain temperature, the products have a lower Gibbs free energyG = H – TS than the reactants (an exergonic process), even if the enthalpy of the products is higher. Thus, an endothermic process usually requires a favorable entropy increase (∆S > 0) in the system that overcomes the unfavorable increase in enthalpy so that still ∆G < 0. While endothermic phase transitions into more disordered states of higher entropy, e.g. melting and vaporization, are common, spontaneous chemical processes at moderate temperatures are rarely endothermic. 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