Gibbs free energy This is a featured page
Available energy (free energy) (f)
Gibbs spontaneity chart
Left: an annotated version (by Ronald Kriz) of Willard Gibbs original 1873 graphical thermodynamics depiction of "available energy" (section AB), as he called it; latter to be named "free energy", by Hermann Helmholtz (1882); which eventually diverged into two types: Helmholtz free energy (isochoric-isobaric processes) and Gibbs free energy (isothermal-isobaric processes) names assigned by Edward Guggenheim (1933). [9] Right: an so-called Gibbs spontaneity chart (V) , showing the basic criterion for spontaneous and nonspontaneous reactions. (Ѻ)
In chemical thermodynamics, the Gibbs free energy, or Gibbs energy, is the free energy of an isothermal-isobaric system. Gibbs free energy, in more detail, is a state function of a closed isothermal-isobaric system in chemical equilibrium. The status of Gibbs free energy is summarized as follows: [8]
“The single most important concept in chemical thermodynamics is that of the Gibbs free energy G a function of state which provides the criterion for deciding whether or not a change of any kind will occur.”
Etymology
In the 1910s and 1920s, the isothermal isobaric potential went by the name of free enthalpy, among German writers. [5] The symbol of G was assigned to Gibbs, in great predominance, in 1933 by English thermodynamicist Edward Guggenheim who stated: [4]
“The function G is due to Gibbs, and is often referred to by modern writers as ‘free energy’. We shall call G the ‘Gibbs free energy’.”
There has been some effort, by the IUPAC, to expunge the middle clarifying term 'free' so to call G simply 'Gibbs energy', but this, for most purposes, has been a failed effort, and it is puzzling why the effort was even made in the first place?
Overview
Changes in the value of free energy can be used to determine if a reaction is thermodynamically favorable. In an equilibrium system at constant temperature and pressure, the the Gibbs free energy is quantified by what is called the Gibbs function:
G = H - T S\,
where H is the enthalpy, T the temperature, and S the entropy of the body; an expression representing the part of the energy content of a system that is available to do external work, also known as the free energy G. [6] In an equilibrium system at constant temperature and pressure, G = H–TS, where H is the enthalpy (heat content), T the temperature, and S the entropy (decrease in energy availability). [6] The function was named after American engineer Willard Gibbs. In expanded form, showing enthalpy as a function of internal energy U and pressure volume work pV, the Gibbs function becomes:
Gibbs free energy (criteria)
Top: The relationships between the equilibrium constant K, Gibbs free energy change ΔG, and the direction of a chemical reaction. Bottom: the spontaneity criterion rules for reaction direction. [7]
G = U + p V - T S\,
The formula was introduced by American mathematical physicist Willard Gibbs in 1873 and called by him "available energy". [1] In chemistry, for simple reactions, the change in Gibbs free energy, between the initial stage and final stage of a reaction, for a system of reacting species, takes the form:
\Delta G = \Delta H - T \Delta S \,
where ∆H is the change in enthalpy, T the temperature, and ∆S the change in entropy for the reaction.
Affinity | Spontaneity criterion
A spontaneous or energetically favored reaction will satisfy the following condition (spontaneity criterion):
DG lz c
To determine the feasibility or reaction possibility for a given reaction, the value of ∆G must be determined. To do this, precalculated values of enthalpy change and entropy (per species) are generally listed in standard thermodynamic tables, as shown below (left).
Prior to 1882, wherein German physicist Hermann von Helmholtz showed that free energy was a measure of the chemical "affinity" between the reactants, these reaction calculation tables took the form of affinity tables, the first of which was constructed in 1718 by French physician and chemist Étienne Geoffroy, as shown below (right).
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