Gibbs free energy Totally Explained

Everything about Gibbs Free Energy totally explained

In thermodynamics, the Gibbs free energy (IUPAC recommended name: Gibbs energy or Gibbs function) is a thermodynamic potential which measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system. Technically, the Gibbs free energy is the maximum amount of non-expansion work which can be extracted from a closed system or this maximum can be attained only in a completely reversible process. When a system changes from a well-defined initial state to a well-defined final state, the Gibbs free energy ΔG equals the work exchanged by the system with its surroundings, less the work of the pressure forces, during a reversible transformation of the system from the same initial state to the same final state.
Gibbs energy is also the chemical potential that's minimized when a system reaches equilibrium at constant pressure and temperature. As such, it's a convenient criterion of spontaneity for processes with constant pressure and temperature.
The Gibbs free energy, originally called available energy, was developed in the 1870s by the American mathematical physicist Willard Gibbs. In 1873, in a footnote, Gibbs defined what he called the “available energy” of a body as such:
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.

Definitions

The Gibbs free energy is defined as:
ť

G = U+pV-TS ,

which is the same as:
ť

G = H-TS ,

where:

U is the internal energy (SI unit: joule)
p is pressure (SI unit: pascal)
V is volume (SI unit: m³)
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, for an open system, subjected to the operation of external forces X_i, which cause the external parameters of the system a_i to change by an amount da_i, is given by:

ť ) which relates the equilibrium constant with Gibbs free energy.

Standard change of formation

The standard Gibbs free energy of formation of a compound is the change of Gibbs free energy that accompanies the formation of 1 mole of that substance from its component elements, at their standard states (the most stable form of the element at 25 degrees Celsius and 100 kilopascals). Its symbol is ΔG_f^O.
All elements in their standard states (oxygen gas, graphite, etc.) have 0 standard Gibbs free energy change of formation, as there's no change involved. ť ΔG = ΔG˚ + RT ln Q

At equilibrium, ΔG=0 and Q = K so the equation becomes ΔG˚= −RT ln K

Table of Selected Substances

Substance	State	ΔG˚ (cal/mol)
NH₃	g	-3.976
H₂O	lq	-56.69
H₂O	g	-54.64
CO₂	g	-94.26
CO	g	-32.81
CH₄	g	-12.14
C₂H₆	g	-7.86
C₃H₈	g	-5.614
C₈H₁₈	g	4.14
C₁₀H₂₂	g	8.23

Further Information

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