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" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false.
The connective is bi ...
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 (physics), work that may be performed by a closed system, thermodynamically closed system a ...
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") 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 (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
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 ...
, 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 chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...
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 thermodyna ...
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. ...
where the entropy is not included. The Gibbs free energy is calculated with the
Gibbs–Helmholtz equation:
:
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 ph ...
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:
:
exergon
:
endergon
where:
:
equals the change in the Gibbs free energy after completion of a chemical reaction.
See also
*
Endergonic
*
Endergonic reaction
*
Exothermic process
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electri ...
*
Endothermic process
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. ...
*
Exergonic reaction
In chemical thermodynamics, an exergonic reaction is a chemical reaction where the change in the free energy is negative (there is a net release of free energy). This indicates a spontaneous reaction if the system is closed and initial and fina ...
*
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 ...
*
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. ...
*
Endotherm
An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
*
Ectotherm
An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life a ...
References
{{Reflist
Thermodynamic processes
Chemical thermodynamics