酸化還元自由エネルギー｜ざいつ内科クリニック｜山口市小郡の一般内科、血液内科、アレルギー科

酸化還元自由エネルギー

物質Xが酸化されると

Ｘ　　＋　　e- >>>> X- となる

このときの自由エネルギーの変化　ΔＧは

ΔＧ＝-nFΔＥ

Ｅ＝E0 + RT/nF ln(X-/X)

酸化還元の場合のエネルギ－はギプスの自由エネルギ－より電位差を用いるほうが一般的

In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol $G$ ) is a thermodynamic potential that can be used to calculate the maximum amount of work, other than pressure-volume work, that may be performed by a thermodynamically closed system at constant temperature and pressure. It also provides a necessary condition for processes such as chemical reactions that may occur under these conditions. The Gibbs free energy is expressed as $G(p,T)=U+pV-TS=H-TS$ Where:

${\textstyle U}$ is the internal energy of the system
${\textstyle H}$ is the enthalpy of the system
${\textstyle S}$ is the entropy of the system
${\textstyle T}$ is the temperature of the system
${\textstyle V}$ is the volume of the system
${\textstyle p}$ is the pressure of the system (which must be equal to that of the surroundings for mechanical equilibrium).

The Gibbs free energy change ( $\Delta G=\Delta H-T\Delta S$ , measured in joules in SI) is the maximum amount of non-volume expansion work that can be extracted from a closed system (one that can exchange heat and work with its surroundings, but not matter) at fixed temperature and pressure. This maximum can be attained only in a completely reversible process. When a system transforms reversibly from an initial state to a final state under these conditions, the decrease in Gibbs free energy equals the work done by the system to its surroundings, minus the work of the pressure forces.^[1]

The Gibbs energy is the thermodynamic potential that is minimized when a system reaches chemical equilibrium at constant pressure and temperature when not driven by an applied electrolytic voltage. Its derivative with respect to the reaction coordinate of the system then vanishes at the equilibrium point. As such, a reduction in $G$ is necessary for a reaction to be spontaneous under these conditions.

The concept of 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^[2]^: 400

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 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.

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 processes". In his 1876 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.

If the reactants and products are all in their thermodynamic standard states, then the defining equation is written as $\Delta G^{\circ }=\Delta H^{\circ }-T\Delta S^{\circ }$ , where $H$ is enthalpy, $T$ is absolute temperature, and $S$ is entropy.

自由エネルギー（じゆうエネルギー、英: free energy）とは、熱力学における状態量の1つであり、化学変化を含めた熱力学的系の等温過程において、系の最大仕事(潜在的な仕事能力)、自発的変化の方向、平衡条件などを表す指標となる^[1]^[2]。

自由エネルギーは1882年にヘルマン・フォン・ヘルムホルツが提唱した熱力学上の概念で、呼称は彼の命名による。一方、等温等圧過程の自由エネルギーと化学ポテンシャルとの研究はウィラード・ギブズにより理論展開された。等温等積過程の自由エネルギーはヘルムホルツの自由エネルギー（Helmholtz free energy）と呼ばれ、等温等圧過程の自由エネルギーはギブズの自由エネルギー（Gibbs free energy）と呼びわけられる。ヘルムホルツ自由エネルギーは F で表記され、ギブズ自由エネルギーは G で表記されることが多い。両者の間には G = F + pV の関係にあり、体積変化が系外に為す仕事 pV の分だけ異なる。

熱力学第二法則より、系は自由エネルギーが減少する方向に進行する。また、閉じた系における熱力学的平衡条件は自由エネルギーが極小値をとることである。

ヘルムホルツの自由エネルギー

ヘルムホルツの自由エネルギー（英語: Helmholtz free energy）は、等温条件の下で仕事として取り出し可能なエネルギーを表す示量性状態量である。なお、IUPACでは「自由」を付けずにヘルムホルツエネルギー（英語: Helmholtz energy）とすることが推奨されている^[3]。記号 $F$ や $A$ で表されることが多い。