Nonspontaneous

Nonspontaneous is a term used in thermodynamics and chemistry to describe a process or reaction that will not proceed by itself under a given set of conditions. Whether a process is nonspontaneous depends on the environment (temperature, pressure, concentrations) and on the definition of spontaneity for those conditions. In thermodynamics, spontaneity is determined by the Gibbs free energy change ΔG. A process is spontaneous if ΔG < 0; it is nonspontaneous if ΔG > 0, and it is at equilibrium when ΔG = 0.

Under constant temperature and pressure, ΔG = ΔH − TΔS. This means that both enthalpy changes (ΔH) and entropy changes (ΔS) influence spontaneity, and the sign can depend on temperature. For example, some reactions are nonspontaneous at low temperatures but become spontaneous at higher temperatures as the entropy term becomes more influential.

Nonspontaneity does not imply that a process cannot happen; it means it will not occur without a continuous input of energy or external driving force. A nonspontaneous reaction can be made to proceed by coupling it to a spontaneous process or by supplying energy, as in electrolysis or photochemical reactions. Conversely, a reaction can be thermodynamically spontaneous (ΔG < 0) but kinetically hindered, proceeding very slowly without a catalyst.

Examples include the electrolysis of water, which is nonspontaneous under standard conditions; photosynthesis, which uses light energy to drive endergonic steps; and high-pressure, high-temperature conditions required for certain transformations such as graphite-to-diamond formation.