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 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" (often shortened as "iff") is paraphrased by the biconditional, a logical connective between statements. The biconditional is true in two cases, where either bo ...

the

Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol is a thermodynamic potential that can be used to calculate the maximum amount of Work (thermodynamics), work, other than Work (thermodynamics)#Pressure–v ...

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 ani ...

") means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from the

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...

(the internal region) to the

surroundings Surroundings, or environs is an 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 ...

(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 (metaphysics), universal empirical observation concerning heat and Energy transformation, energy interconversions. A simple statement of the law is that heat always flows spont ...

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 chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

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

entropy Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...

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 define ...

or an endothermic reaction 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 to calculate 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 Vorgänge" ...

: :

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

where: : ''T'' = temperature in

kelvin The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...

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 endergonic 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