Whether a process can occur spontaneously depends not only on the enthalpy change but also on the entropy change () and absolute temperature . If a process is a spontaneous process at a certain temperature, the products have a lower Gibbs free energy  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 () in the system that overcomes the unfavorable increase in enthalpy so that still . 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 in a hypothetical strongly endothermic process usually results in , which means that the process will not occur (unless driven by electrical or photon energy). An example of an endothermic and exergonic process is