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{{Technical|date=April 2018}}{{Thermodynamics|expanded=Potentials}}

In [[thermodynamics]], the '''Gibbs free energy''' ([[International Union of Pure and Applied Chemistry|IUPAC]] recommended name: '''Gibbs energy''' or '''Gibbs function'''; also known as '''free enthalpy'''<ref>{{cite book | last1=Greiner | first1=Walter | last2=Neise | first2=Ludwig | last3=Stöcker | first3=Horst | title=Thermodynamics and statistical mechanics
 | publisher=Springer-Verlag | year=1995 | page=101 }}</ref> to distinguish it from [[Helmholtz free energy]]) is a [[thermodynamic potential]] that can be used to calculate the [[maximum]] of reversible [[work (physics)|work]] that may be performed by a [[thermodynamic system]] at a constant [[temperature]] and [[pressure]] ([[isothermal]], [[Isobaric process|isobaric]]).  The Gibbs free energy (<math>\Delta G^{\circ} = \Delta H^{\circ} - T \Delta S^{\circ}</math>; [[Joule|J]] in [[International System of Units|SI units]]) is the ''maximum'' amount of non-expansion work that can be extracted from a [[closed system|thermodynamically closed system]] (one that can exchange heat and work with its surroundings, but not matter); this maximum can be attained only in a completely [[reversible process (thermodynamics)|reversible process]]. When a system transforms reversibly from an initial state to a final state, the decrease in Gibbs free energy equals the work done by the system to its surroundings, minus the work of the [[pressure]] forces.<ref name="Perrot" >{{cite book | last = Perrot | first = Pierre | title = A to Z of Thermodynamics | publisher = Oxford University Press | year = 1998 | isbn = 0-19-856552-6}}</ref>

The Gibbs energy (also referred to as <math>G</math>) is also the thermodynamic potential that is minimized when a system reaches [[chemical equilibrium]] at constant pressure and temperature. Its derivative with respect to the reaction coordinate of the system vanishes at the equilibrium point. As such, a reduction in <math>G</math> is a necessary condition for the spontaneity of processes at constant [[pressure]] and [[temperature]].

The Gibbs free energy, originally called ''available energy'', was developed in the 1870s by the American scientist [[Josiah Willard Gibbs]]. In 1873, Gibbs described this "available energy" as
{{quote|the greatest amount of mechanical work which can be obtained from a given quantity of a certain substance in a given initial state, without increasing its total [[Volume (thermodynamics)|volume]] or allowing heat to pass to or from external bodies, except such as at the close of the processes are left in their initial condition.<ref>J.W. Gibbs, "A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces," ''[[Transactions of the Connecticut Academy of Arts and Sciences]]'' 2, Dec. 1873, pp. 382–404 (quotation on p. 400).</ref>}}

The initial state of the body, according to Gibbs, is supposed to be such that "the body can be made to pass from it to states of [[dissipated energy]] by [[Reversible process (thermodynamics)|reversible processes]]".  In his 1876 [[Masterpiece|magnum opus]] ''[[On the Equilibrium of Heterogeneous Substances]]'', a graphical analysis of multi-phase chemical systems, he engaged his thoughts on chemical free energy in full.

==Overview==
[[File:Diamond.jpg|thumb|right|The reaction C<sub>(s)</sub><sup>diamond</sup>&nbsp;→&nbsp;C<sub>(s)</sub><sup>graphite</sup> has a negative change in Gibbs free energy and is therefore thermodynamically favorable at 25&nbsp;°C and 1 atm. However, even though favorable, it is so slow that it is not observed.  Whether a reaction is thermodynamically favorable does not determine its rate.]]
According to the [[second law of thermodynamics]], for systems reacting at [[standard temperature and pressure|STP]] (or any other fixed temperature and pressure), there is a general natural tendency to achieve a minimum of the Gibbs free energy.

A quantitative measure of the favorability of a given reaction at constant temperature and pressure is the change Δ''G'' (sometimes written "delta ''G''" or "d''G''") in Gibbs free energy that is (or would be) caused by the reaction. As a necessary condition for the reaction to occur at constant temperature and pressure, Δ''G'' must be smaller than the non-[[pressure-volume work|PV]] (e.g. electrical) work, which is often equal to zero (hence Δ''G'' must be negative).  Δ''G'' equals the maximum amount of non-''PV'' work that can be performed as a result of the chemical reaction for the case of reversible process. If the analysis indicated a positive Δ''G'' for the reaction, then energy — in the form of electrical or other non-''PV'' work — would have to be added to the reacting system for Δ''G'' to be smaller than the non-''PV'' work and make it possible for the reaction to occur.<ref name="AtkinsJones2007">{{cite book|author1=Peter Atkins|author2=Loretta Jones|title=Chemical Principles: The Quest for Insight|date=1 August 2007|publisher=W. H. Freeman|isbn=978-1-4292-0965-6}}</ref>{{rp|298–299}}

We can think of ∆G as the amount of "free" or "useful" energy available to do work. The equation can be also seen from the perspective of the system taken together with its surroundings (the rest of the universe). First, assume that the given reaction at constant temperature and pressure is the only one that is occurring. Then the [[entropy]] released or absorbed by the system equals the entropy that the environment must absorb or release, respectively. The reaction will only be allowed if the total entropy change of the universe is zero or positive. This is reflected in a negative Δ''G'', and the reaction is called [[exergonic]].

If we couple reactions, then an otherwise [[endergonic]] chemical reaction (one with positive Δ''G'') can be made to happen. The input of heat into an inherently endergonic reaction, such as the [[elimination reaction|elimination]] of [[cyclohexanol]] to [[cyclohexene]], can be seen as coupling an unfavourable reaction (elimination) to a favourable one (burning of coal or other provision of heat) such that the total entropy change of the universe is greater than or equal to zero, making the ''total'' Gibbs free energy difference of the coupled reactions negative.

In traditional use, the term "free" was included in "Gibbs free energy" to mean "available in the form of useful work".<ref name="Perrot" /> The characterization becomes more precise if we add the qualification that it is the energy available for ''non-volume'' work{{clarify|date=October 2016}}.<ref>{{cite book | last = Reiss | first = Howard | title = Methods of Thermodynamics | publisher = Dover Publications | year = 1965 | isbn = 0-486-69445-3}}</ref> (An analogous, but slightly different, meaning of "free" applies in conjunction with the Helmholtz free energy, for systems at constant temperature).  However, an increasing number of books and journal articles do not include the attachment "free", referring to ''G'' as simply "Gibbs energy".  This is the result of a 1988 [[IUPAC]] meeting to set unified terminologies for the international scientific community, in which the adjective "free" was supposedly banished.<ref>{{cite journal|title=Glossary of Atmospheric Chemistry Terms (Recommendations 1990) |url=http://goldbook.iupac.org/publications/pac/1990/pdf/6211x2167.html |journal=[[Pure and Applied Chemistry|Pure Appl. Chem.]] |volume=62 |pages=2167–2219 |year=1990 |last=[[International Union of Pure and Applied Chemistry]] Commission on Atmospheric Chemistry |accessdate=2006-12-28 |doi=10.1351/pac199062112167 |first1=J. G. |issue=11 }}{{dead link|date=January 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite book|last=[[International Union of Pure and Applied Chemistry]] Commission on Physicochemical Symbols Terminology and Units|title=Quantities, Units and Symbols in Physical Chemistry (2nd Edition)|publisher=Blackwell Scientific Publications|location=Oxford|year=1993|url=http://goldbook.iupac.org/home/publications/e-resources/nomenclature-and-terminology/quantities-units-and-symbols-in-physical-chemistry-green-book.html|isbn=0-632-03583-8|accessdate=2013-12-20|pages=251}}{{Dead link|date=August 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite journal|title=Glossary of Terms in Quantities and Units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996) |url=http://goldbook.iupac.org/publications/pac/1996/pdf/6804x0957.html |journal=[[Pure and Applied Chemistry|Pure Appl. Chem.]] |volume=68 |pages=957–1000 |year=1996 |last=[[International Union of Pure and Applied Chemistry]] Commission on Quantities and Units in Clinical Chemistry |author2=[[International Federation of Clinical Chemistry Laboratory Medicine]] Committee on Quantities and Units |accessdate=2006-12-28 |doi=10.1351/pac199668040957 |first1=H. P. |issue=4 }}{{dead link|date=January 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> This standard, however, has not yet been universally adopted.

==History==
{{see also|Thermodynamic free energy#History|l1=Thermodynamic free energy}}
The quantity called "free energy" is a more advanced and accurate replacement for the outdated term ''affinity'', which was used by chemists in the earlier years of physical chemistry to describe the ''force'' that caused [[chemical reaction]]s.

In 1873, [[Willard Gibbs]] published ''A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces'', in which he sketched the principles of his new equation that was able to predict or estimate the tendencies of various natural processes to ensue when bodies or systems are brought into contact.  By studying the interactions of homogeneous substances in contact, i.e., bodies composed of part solid, part liquid, and part vapor, and by using a three-dimensional [[Volume (thermodynamics)|volume]]-[[entropy]]-[[internal energy]] graph, Gibbs was able to determine three states of equilibrium, i.e., "necessarily stable", "neutral", and "unstable", and whether or not changes would ensue.  Further, Gibbs stated:<ref name=Gibbs1873>J. W. Gibbs, "A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces", ''Transactions of the Connecticut Academy of Arts and Sciences'' 2, Dec. 1873, pp. 382–404.</ref>

{| class="messagebox"
|-
|align="left"|
If we wish to express in a single equation the necessary and sufficient condition of [[thermodynamic equilibrium]] for a substance when surrounded by a medium of constant [[pressure]] ''p'' and [[temperature]] ''T'', this equation may be written:

:{{nowrap|1=''δ''(''ε'' − ''Tη'' + ''pν'') = 0}}

when ''δ'' refers to the variation produced by any variations in the [[thermodynamic state|state]] of the parts of the body, and (when different parts of the body are in different states) in the proportion in which the body is divided between the different states.  The condition of stable equilibrium is that the value of the expression in the parenthesis shall be a minimum. 
|}

In this description, as used by Gibbs, ''ε'' refers to the [[internal energy]] of the body, ''η'' refers to the [[entropy]] of the body, and ''ν'' is the [[Volume (thermodynamics)|volume]] of the body.

Thereafter, in 1882, the German scientist [[Hermann von Helmholtz]] characterized the affinity as the largest quantity of work which can be gained when the reaction is carried out in a reversible manner, e.g., electrical work in a reversible cell.  The maximum work is thus regarded as the diminution of the free, or available, energy of the system (''Gibbs free energy'' ''G'' at ''T'' = constant, ''P'' = constant or ''Helmholtz free energy'' ''F'' at ''T'' = constant, ''V'' = constant), whilst the heat given out is usually a measure of the diminution of the total energy of the system ([[internal energy]]).  Thus, ''G'' or ''F'' is the amount of energy "free" for work under the given conditions.

Until this point, the general view had been such that: "all chemical reactions drive the system to a state of equilibrium in which the affinities of the reactions vanish".  Over the next 60 years, the term affinity came to be replaced with the term free energy.  According to chemistry historian Henry Leicester, the influential 1923 textbook ''Thermodynamics and the Free Energy of Chemical Substances'' by [[Gilbert N. Lewis]] and [[Merle Randall]] led to the replacement of the term "affinity" by the term "free energy" in much of the English-speaking world.<ref name="Leicester1971">{{cite book|author=Henry Marshall Leicester|title=The Historical Background of Chemistry|year=1971|publisher=Courier Corporation|isbn=978-0-486-61053-5}}</ref>{{rp|206}}

==Graphical interpretation==
Gibbs free energy was originally defined graphically.  In 1873, American scientist [[Willard Gibbs]] published his first thermodynamics paper, "Graphical Methods in the Thermodynamics of Fluids", in which Gibbs used the two coordinates of the entropy and volume to represent the state of the body. In his second follow-up paper, "A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces", published later that year, Gibbs added in the third coordinate of the energy of the body, defined on three figures. In 1874, Scottish physicist [[James Clerk Maxwell]] used Gibbs' figures to make a 3D energy-entropy-volume [[Maxwell's thermodynamic surface|thermodynamic surface]] of a fictitious water-like substance.<ref>James Clerk Maxwell, Elizabeth Garber, Stephen G. Brush, and C. W. Francis Everitt (1995), ''[https://books.google.com/books?id=hA-oIDR0eXkC&pg=PA248 Maxwell on heat and statistical mechanics: on "avoiding all personal enquiries" of molecules]'', Lehigh University Press, {{ISBN|0-934223-34-3}}, p. 248.</ref>  Thus, in order to understand the very difficult concept of Gibbs free energy one must be able to understand its interpretation as Gibbs defined originally by section AB on his figure 3 and as Maxwell sculpted that section on his [[Maxwell's thermodynamic surface|3D surface figure]].

[[File:Gibbs-Maxwell surfaces.png|750px|center|thumb|American scientist [[Willard Gibbs]]' 1873 figures two and three (above left and middle) used by Scottish physicist [[James Clerk Maxwell]] in 1874 to create a three-dimensional [[entropy]], [[Volume (thermodynamics)|volume]], [[energy]] '''[[Maxwell's thermodynamic surface|thermodynamic surface]]''' diagram for a fictitious water-like substance, transposed the two figures of Gibbs (above right) onto the volume-entropy coordinates (transposed to bottom of cube) and energy-entropy coordinates (flipped upside down and transposed to back of cube), respectively, of a three-dimensional [[Cartesian coordinates]]; the region AB being the first-ever three-dimensional representation of Gibbs free energy, or what Gibbs called "available energy"; the region AC being its capacity for [[entropy]], what Gibbs defined as "the amount by which the entropy of the body can be increased without changing the energy of the body or increasing its volume.]]

==Definitions==
[[File:Wykres Gibbsa.svg|thumb|[[Willard Gibbs]]’ 1873 '''available energy''' (free energy) graph, which shows a plane perpendicular to the axis of ''v'' ([[Volume (thermodynamics)|volume]]) and passing through point A, which represents the initial state of the body.  MN is the section of the surface of [[dissipated energy]]. Q''ε'' and Q''η'' are sections of the planes ''η'' = 0 and ''ε'' = 0, and therefore parallel to the axes of ''ε'' ([[internal energy]]) and ''η'' ([[entropy]]), respectively.  AD and AE are the energy and entropy of the body in its initial state, AB and AC its ''available energy'' (Gibbs free energy) and its ''capacity for entropy'' (the amount by which the entropy of the body can be increased without changing the energy of the body or increasing its volume) respectively.]]
The Gibbs free energy is defined as
:<math>G(p,T) = U + pV - TS,</math>

which is the same as
:<math>G(p,T) = H - TS,</math>

where:
: ''U'' is the [[internal energy]] (SI unit: [[joule]]),
: ''p'' is [[pressure]] (SI unit: [[Pascal (unit)|pascal]]),
: ''V'' is [[Volume (thermodynamics)|volume]] (SI unit: m<sup>3</sup>),
: ''T'' is the [[temperature]] (SI unit: [[kelvin]]),
: ''S'' is the [[entropy]] (SI unit: joule per kelvin),
: ''H'' is the [[enthalpy]] (SI unit: joule).

The expression for the infinitesimal reversible change in the Gibbs free energy as a function of its "natural variables" ''p'' and ''T'', for an [[Thermodynamic system#Open system|open system]], subjected to the operation of external forces (for instance, electrical or magnetic) ''X<sub>i</sub>'', which cause the external parameters of the system ''a<sub>i</sub>'' to change by an amount d''a<sub>i</sub>'', can be derived as follows from the first law for reversible processes:

:<math>T\,\mathrm{d}S= \mathrm{d}U + p\,\mathrm{d}V - \sum_{i=1}^k \mu_i \,\mathrm{d}N_i + \sum_{i=1}^n X_i \,\mathrm{d}a_i + \cdots</math>
:<math>\mathrm{d}(TS) - S\,\mathrm{d}T = \mathrm{d}U + \mathrm{d}(pV) - V\,\mathrm{d}p - \sum_{i=1}^k \mu_i \,\mathrm{d}N_i + \sum_{i=1}^n X_i \,\mathrm{d}a_i + \cdots</math>
:<math>\mathrm{d}(U - TS + pV) = V\,\mathrm{d}p - S\,\mathrm{d}T + \sum_{i=1}^k \mu_i \,\mathrm{d}N_i - \sum_{i=1}^n X_i \,\mathrm{d}a_i + \cdots</math>
:<math>\mathrm{d}G = V\,\mathrm{d}p - S\,\mathrm{d}T + \sum_{i=1}^k \mu_i \,\mathrm{d}N_i - \sum_{i=1}^n X_i \,\mathrm{d}a_i + \cdots</math>

where:
: ''μ''<sub>''i''</sub> is the [[chemical potential]] of the ''i''th [[chemical species|chemical component]]. (SI unit: joules per particle<ref>[http://goldbook.iupac.org/C01032.html Chemical Potential], IUPAC Gold Book.</ref> or joules per mole<ref name="Perrot" />)
: ''N''<sub>''i''</sub> is the [[particle number|number of particles]] (or number of moles) composing the ''i''th chemical component.

This is one form of '''Gibbs fundamental equation'''.<ref>{{cite book | last = Müller | first = Ingo | title = A History of Thermodynamics – the Doctrine of Energy and Entropy | publisher = Springer | year = 2007 | isbn = 978-3-540-46226-2}}</ref>  In the infinitesimal expression, the term involving the chemical potential accounts for changes in Gibbs free energy resulting from an influx or outflux of particles. In other words, it holds for an [[Open system (systems theory)|open system]] or for a [[Closed system|closed]], chemically reacting system where the ''N<sub>i</sub>'' are changing. For a closed, non-reacting system, this term may be dropped.

Any number of extra terms may be added, depending on the particular system being considered. Aside from [[mechanical work]], a system may, in addition, perform numerous other types of work.  For example, in the infinitesimal expression, the contractile work energy associated with a thermodynamic system that is a contractile fiber that shortens by an amount −d''l'' under a force ''f'' would result in a term ''f'' d''l'' being added.  If a quantity of charge −d''e'' is acquired by a system at an electrical potential Ψ, the electrical work associated with this is −Ψ d''e'', which would be included in the infinitesimal expression.  Other work terms are added on per system requirements.<ref>{{cite book | last = Katchalsky | first = A.  |author2=Curran, Peter F.| title = Nonequilibrium Thermodynamics in Biophysics | publisher = [[Harvard University Press]] | year = 1965 | id = CCN 65-22045}}</ref>

Each quantity in the equations above can be divided by the amount of substance, measured in [[Mole (unit)|moles]], to form ''molar Gibbs free energy''. The Gibbs free energy is one of the most important thermodynamic functions for the characterization of a system. It is a factor in determining outcomes such as the [[voltage]] of an [[electrochemical cell]], and the [[equilibrium constant]] for a [[reversible reaction]]. In isothermal, isobaric systems, Gibbs free energy can be thought of as a "dynamic" quantity, in that it is a representative measure of the competing effects of the enthalpic{{clarify|reason=What is enthalpic force? No reference to it in Wikipedia's enthalpy article.|date=March 2015}} and entropic driving forces involved in a thermodynamic process.

[[File:Thermodynamics_Gibbs_E0_Ecell_pH.png|thumb|upright=1.9|Relation to other relevant parameters]]

The temperature dependence of the Gibbs energy for an [[ideal gas]] is given by the [[Gibbs–Helmholtz equation]], and its pressure dependence is given by
:<math>\frac{G}{N} = \frac{G^\circ}{N} + kT\ln \frac{p}{p^\circ}.</math>

If the volume is known rather than pressure, then it becomes
:<math>\frac{G}{N} = \frac{G^\circ}{N} + kT\ln \frac{V^\circ}{V},</math>

or more conveniently as its [[chemical potential]]:
:<math>\frac{G}{N} = \mu = \mu^\circ + kT\ln \frac{p}{p^\circ}.</math>

In non-ideal systems, [[fugacity]] comes into play.

==Derivation==
The Gibbs free energy [[total differential]] [[Thermodynamic potential#Natural variables|natural variables]] may be derived by [[Legendre transformation|Legendre transforms]] of the [[internal energy]].
:<math>\mathrm{d}U = T\,\mathrm{d}S - p \,\mathrm{d}V + \sum_i \mu_i \,\mathrm{d} N_i.</math>

The definition of ''G'' from above is
:<math>G = U + p V - T S</math>.

Taking the total differential, we have
:<math>\mathrm{d}G = \mathrm{d}U + p\,\mathrm{d}V + V\,\mathrm{d}p - T\,\mathrm{d}S - S\,\mathrm{d}T.</math>

Replacing d''U'' with the result from the first law gives<ref name="Salzman2001"/>
:<math>\begin{align}
\mathrm{d}G &= T\,\mathrm{d}S - p\,\mathrm{d}V + \sum_i \mu_i \,\mathrm{d}N_i + p \,\mathrm{d}V + V\,\mathrm{d}p - T\,\mathrm{d}S - S\,\mathrm{d}T\\
&= V\,\mathrm{d}p - S\,\mathrm{d}T + \sum_i \mu_i \,\mathrm{d} N_i.
\end{align}</math>

The natural variables of ''G'' are then ''p'', ''T'', and {''N''<sub>''i''</sub>}.

===Homogeneous systems===

Because ''S'', ''V'', and ''N''<sub>''i''</sub> are [[Intensive and extensive properties|extensive variables]], an [[Thermodynamic potentials#Euler integrals|Euler integral]] allows easy integration of d''U'':<ref name="Salzman2001">{{cite web |url=http://www.chem.arizona.edu/~salzmanr/480a/480ants/opensys/opensys.html |title=Open Systems |accessdate=2007-10-11 |last=Salzman |first=William R. |date=2001-08-21 |work=Chemical Thermodynamics |publisher=[[University of Arizona]]  |archiveurl=https://web.archive.org/web/20070707224025/http://www.chem.arizona.edu/~salzmanr/480a/480ants/opensys/opensys.html |archivedate=2007-07-07}}</ref>
:<math>U = T S - p V + \sum_i \mu_i N_i.</math>

Because some of the natural variables of ''G'' are intensive, d''G'' may not be integrated using Euler integrals as is the case with internal energy. However, simply substituting the above integrated result for ''U'' into the definition of ''G'' gives a standard expression for ''G'':<ref name="Salzman2001"/>
:<math>\begin{align}
G &= U + p V - TS\\
  &= (T S - p V + \sum_i \mu_i N_i) + p V - T S\\
  &= \sum_i \mu_i N_i.
\end{align}</math>

This result applies to homogeneous, macroscopic systems, but not to all thermodynamic systems.<ref>{{Cite journal | doi = 10.1063/1.1740312| last = Brachman | first = M. K.| title = Fermi Level, Chemical Potential, and Gibbs Free Energy | journal = The Journal of Chemical Physics | volume = 22 | issue = 6 | page = 1152| year = 1954 | pmid =  | pmc = |bibcode = 1954JChPh..22.1152B }}</ref>

==Gibbs free energy of reactions==
The system under consideration is held at constant temperature and pressure, and is closed (no matter can come in or out). The Gibbs energy of any system is {{tmath|1=G=U+PV-TS}} and an infinitesimal change in ''G'', at constant temperature and pressure yields:

:<math>dG=dU+PdV-TdS</math>

By the [[first law of thermodynamics]], a change in the internal energy ''U'' is given by

:<math>dU=\delta Q+\delta W</math> 

where ''&delta;Q'' is energy added as heat, and ''&delta;W'' is energy added as work. The work done on the system may be written as ''&delta;W'' = −''PdV'' + ''&delta;W<sub>x</sub>'', where −''PdV'' is the mechanical work of compression/expansion done on the system and ''&delta;W<sub>x</sub>'' is all other forms of work, which may include electrical, magnetic, etc. Assuming that only mechanical work is done,

:<math>dU=\delta Q-PdV</math>

and the infinitesimal change in ''G'' is:

:<math>dG=\delta Q-TdS</math>

The [[second law of thermodynamics]] states that for a closed system, <math>TdS \ge \delta Q</math>, and so it follows that:

:<math>dG \le 0</math>

This means that for a system which is not in equilibrium, its Gibbs energy will always be decreasing, and when it is in equilibrium (i.e. no longer changing), the infinitesimal change ''dG'' will be zero. In particular, this will be true if the system is experiencing any number of internal chemical reactions on its path to equilibrium.

===In electrochemical thermodynamics===
When electric charged ''dQ'' is passed in an electrochemical cell the emf ℰ yields a thermodynamic work term that appears in the expression for the change in [[Gibbs free energy|Gibbs energy]]:

:: <math>dG = -SdT + VdP + \mathcal{E}dQ\ , </math>

where ''G'' is the Gibb's free energy, ''S'' is the [[entropy]], ''V'' is the system volume, ''P'' is its pressure and ''T'' is its [[absolute temperature]].

The combination  ( ℰ, ''Q'' ) is an example of a [[conjugate variables (thermodynamics)|conjugate pair of variables]]. At constant pressure the above relationship produces a [[Maxwell relation]] that links the change in open cell voltage with temperature ''T'' (a measurable quantity) to the change in entropy ''S'' when charge is passed [[isothermally]] and [[isobarically]]. The latter is closely related to the reaction [[entropy]] of the electrochemical reaction that lends the battery its power. This Maxwell relation is:<ref>H. S. Harned, B. B. Owen, The Physical Chemistry of Electrolytic Solutions, third edition, Reinhold Publishing Corporation, N.Y.,1958, p. 2-6</ref>{{cn|reason=incomplete reference|date=April 2019}}

:<math>
\left(\frac{\partial \mathcal{E}}{\partial T}\right)_Q=
-\left(\frac{\partial S}{\partial Q}\right)_T
</math>

If a mole of ions goes into solution (for example, in a Daniell cell, as discussed below) the charge through the external circuit is:

:<math> \Delta Q = -n_0F_0 \ , </math>

where ''n''<sub>0</sub> is the number of electrons/ion, and ''F''<sub>0</sub> is the [[Faraday constant]] and the minus sign indicates discharge of the cell. Assuming constant pressure and volume, the thermodynamic properties of the cell are related strictly to the behavior of its emf by:</ref>

:<math>\Delta H = -n_0 F_0 \left( \mathcal{E} - T \frac {d\mathcal{E}}{dT}\right) \ , </math>

where Δ''H'' is the [[standard enthalpy of reaction|enthalpy of reaction]]. The quantities on the right are all directly measurable.

==Useful identities to derive the Nernst equation==
{{confusing section|small=no|reason=the physical situation is not explained. Also, the circle notation is not well explained (even in the one case where it is attempted). It's just bare equations|date=March 2015}}

During a reversible electrochemical reaction at constant temperature and pressure, the following equations involving the Gibbs free energy hold:
:<math>\Delta_\text{r} G = \Delta_\text{r} G^\circ + R T \ln Q_\text{r}</math> (see [[chemical equilibrium]]),
:<math>\Delta_\text{r} G^\circ = -R T \ln K_\text{eq}</math> (for a system at chemical equilibrium),
:<math>\Delta_\text{r} G = w_\text{elec,rev} = -nFE</math> (for a reversible electrochemical process at constant temperature and pressure),
:<math>\Delta_\text{r} G^\circ = -nFE^\circ</math> (definition of ''E''°),

and rearranging gives
:<math>nFE^\circ = RT \ln K_\text{eq},</math>
:<math>nFE = nFE^\circ - R T \ln Q_\text{r}r,</math>
:<math>E = E^\circ - \frac{R T}{n F} \ln Q_\text{r},</math>

which relates the cell potential resulting from the reaction to the equilibrium constant and [[reaction quotient]] for that reaction ([[Nernst equation]]),

where
: Δ<sub>r</sub>''G'' = Gibbs free energy change per mole of reaction,
: Δ<sub>r</sub>''G°'' = Gibbs free energy change per mole of reaction for unmixed reactants and products at standard conditions (i.e. 298K, 100kPa, 1M of each reactant and product),
: ''R'' = [[gas constant]],
: ''T'' = absolute [[temperature]],
: ln = [[natural logarithm]],
: ''Q<sub>r</sub>'' = [[reaction quotient]] (unitless),
: ''K<sub>eq</sub>'' = [[equilibrium constant]] (unitless),
: ''w''<sub>elec,rev</sub> = [[Work (electrical)|electrical work]] in a reversible process (chemistry sign convention),
: ''n'' = number of [[Mole (unit)|moles]] of [[electrons]] transferred in the reaction,
: ''F'' = [[Faraday constant]] = 96485 C/mol (charge per [[Mole (unit)|mole]] of electrons),
: ''E'' = [[Electrode potential|cell potential]],
: ''E°'' = [[Standard electrode potential|standard cell potential]].

Moreover, we also have:
:<math>K_\text{eq} = e^{-\frac{\Delta_\text{r} G^\circ}{RT}},</math>
:<math>\Delta_\text{r} G^\circ = -RT(\ln K_\text{eq}) = -2.303\,RT(\log_{10} K_\text{eq}),</math>

which relates the equilibrium constant with Gibbs free energy.  This implies that at equilibrium
:<math>Q_\text{r} = K_\text{eq}</math> and <math>\Delta_\text{r} G=0</math>

==Standard energy change of formation==
{| class="wikitable sortable floatright" style="text-align:right"
|+ Table of selected substances<ref>CRC Handbook of Chemistry and Physics, 2009, pp. 5-4–5-42, 90th ed., Lide.</ref>
|-
! Substance
(State)
! Δ<sub>''f''</sub>''G°''
([[Joule (unit)|kJ]]/[[Mole (unit)|mol]])
! Δ<sub>''f''</sub>''G°''
([[Kilocalorie|kcal]]/[[Mole (unit)|mol]])
|-
| NO(g)
| 87.6
| 20.9
|-
| NO<sub>2</sub>(g)
| 51.3
| 12.3
|-
| N<sub>2</sub>O(g)
| 103.7
| 24.78
|-
| H<sub>2</sub>O(g)
| −228.6
| −54.64
|-
| H<sub>2</sub>O(l)
| −237.1
| −56.67
|-
| CO<sub>2</sub>(g)
| −394.4
| −94.26
|-
| CO(g)
| −137.2
| −32.79
|-
| CH<sub>4</sub>(g)
| −50.5
| −12.1
|-
| C<sub>2</sub>H<sub>6</sub>(g)
| −32.0
| −7.65
|-
| C<sub>3</sub>H<sub>8</sub>(g)
| −23.4
| −5.59
|-
| C<sub>6</sub>H<sub>6</sub>(g)
| 129.7
| 29.76
|-
| C<sub>6</sub>H<sub>6</sub>(l)
| 124.5
| 31.00
|}
The [[standard Gibbs free energy of formation]] of a compound is the change of Gibbs free energy that accompanies the formation of 1 [[mole (unit)|mole]] of that substance from its component elements, at their [[standard state]]s (the most stable form of the element at 25&nbsp;°C and 100 [[kilopascal|kPa]]). Its symbol is Δ<sub>''f''</sub>''G''˚.

All elements in their standard states (diatomic [[oxygen]] gas, [[graphite]], etc.) have standard Gibbs free energy change of formation equal to zero, as there is no change involved.
:{{math|1= Δ<sub>f</sub>''G'' = Δ<sub>''f''</sub>''G''˚ + ''RT'' ln ''Q<sub>f</sub>''}}&nbsp;; 
::''Q<sub>f</sub>'' is the [[reaction quotient]].
:At equilibrium, Δ<sub>''f''</sub>''G'' = 0, and ''Q<sub>f</sub>'' = ''K'', so the equation becomes Δ<sub>''f''</sub>''G''˚ = −''RT'' ln ''K'' (where ''K'' is the [[equilibrium constant]]).

==See also==
* [[Calphad]]
* [[Electron equivalent]]
* [[Enthalpy-entropy compensation]]
* [[Free entropy]]
* [[Grand potential]]
* [[Thermodynamic free energy]]
* [[Spinodal]] – Spinodal Curves (Hessian matrix)
* [[Critical point (thermodynamics)]]
* [[Non-random two-liquid model]] (NRTL model) – Gibbs energy of excess and mixing calculation and activity coeffients
* [[UNIQUAC]] model – Gibbs energy of excess and mixing calculation and activity coeffients

==Notes and references==
{{Reflist|2}}

==External links==
* [http://goldbook.iupac.org/goldbook/G02629.html IUPAC definition (Gibbs energy)]
* [http://www.fxsolver.com/solve/share/DMJI1n0mVCIyv_6uYuRIig==/ Gibbs free energy calculator]
* [https://web.archive.org/web/20060710183429/http://wine1.sb.fsu.edu/chm1046/notes/Thermody/Gibbs/Gibbs.htm Gibbs energy] – Florida State University
* [http://scienceworld.wolfram.com/physics/GibbsFreeEnergy.html Gibbs Free Energy] – Eric Weissteins World of Physics
* [http://2ndlaw.oxy.edu/gibbs.html Entropy and Gibbs Free Energy] – www.2ndlaw.oxy.edu
* [http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/helmholtz.html#c2 Gibbs Free Energy] – Georgia State University
* [https://web.archive.org/web/20060621115139/http://mc2.cchem.berkeley.edu/Java/Gibbs/Gibbs.html Gibbs Free Energy Java Applet] – University of California, Berkeley
* [http://www.composite-agency.com/product.htm Using Gibbs Free Energy for prediction of chemical driven material ageing]
* [http://hdl.handle.net/10045/51725 Topological Analysis of the Gibbs Energy Function (Liquid-Liquid Equilibrium Correlation Data). Including a Thermodinamic Review and a Graphical User Interface (GUI) for Surfaces/Tie-lines/Hessian matrix analysis] – University of Alicante (Reyes-Labarta et al. 2015–18)

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