An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an

endergonic In chemical thermodynamics, an endergonic reaction (; also called a heat absorbing nonspontaneous reaction or an unfavorable reaction) is a chemical reaction in which the standard change in free energy is positive, and an additional driving fo ...

process. Constant pressure, constant temperature reactions are exergonic

if and only if In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false. The connective is b ...

the

Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and ...

change is negative (∆''G'' < 0). "Exergonic" (from the prefix exo-, derived for the Greek word ἔξω ''exō'', "outside" and the suffix -ergonic, derived from the Greek word ἔργον ''ergon'', "

work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an animal t ...

") means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from the system (the internal region) to the

surroundings Surroundings are the area around a given physical or geographical point or place. The exact definition depends on the field. Surroundings can also be used in geography (when it is more precisely known as vicinity, or vicinage) and mathematics, ...

(the external region) during a given process. All physical and chemical systems in the universe follow the

second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unles ...

and proceed in a downhill, i.e., ''exergonic'', direction. Thus, left to itself, any physical or chemical system will proceed, according to the

, in a direction that tends to lower the free energy of the system, and thus to expend energy in the form of work. These reactions occur spontaneously. A

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

is also exergonic when spontaneous. Thus in this type of reactions the Gibbs free energy decreases. The

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

is included in any change of the Gibbs free energy. This differs from an

exothermic reaction In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...

or an

endothermic reaction In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. p ...

where the entropy is not included. The Gibbs free energy is calculated with the

Gibbs–Helmholtz equation The Gibbs–Helmholtz equation is a thermodynamic equation used for calculating changes in the Gibbs free energy of a system as a function of temperature. It was originally presented in an 1882 paper entitled " Die Thermodynamik chemischer Vorgang ...

: :

\Delta G = \Delta H- T \cdot \Delta S

where: : ''T'' = temperature in

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...

s (K) : Δ''G'' = change in the Gibbs free energy : Δ''S'' = change in entropy (at 298 K) as Δ''S'' = Σ − Σ : Δ''H'' = change in enthalpy (at 298 K) as Δ''H'' = Σ − Σ A chemical reaction progresses spontaneously only when the Gibbs free energy decreases, in that case the Δ''G'' is negative. In exergonic reactions the Δ''G'' is negative and in

reactions the Δ''G'' is positive: :

\Delta_\mathrm G < 0

exergon :

\Delta_\mathrm G > 0

endergon where: :

\Delta_\mathrm G

equals the change in the Gibbs free energy after completion of a chemical reaction.

References

{{Reflist Thermodynamic processes Chemical thermodynamics

See also

References