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Gibbs free Energy


In thermodynamics, the Gibbs free energy (IUPAC recommended name: Gibbs energy or Gibbs function) is a thermodynamic potential which measures the "useful" work obtainable from an isothermal, isobaric thermodynamic transformation. Technically, the Gibbs free energy is the maximum amount of non-expansion work which can be extracted from
a closed system. When a system changes from a well-defined initial state to a well-defined final state, the Gibbs energy variation ΔG equals the work exchanged by the system with its surroundings, less the work of the pressure forces. This latter is equal to the product of pressure by volume variation (P * ΔV). If we know ΔF, the maximum 'useful' work including expansion work, we can write hence ΔF = ΔG - PΔV.

Hence ΔG is the maximum amont of work (excluding expansion work) which can be extracted from a closed system. The transformation must be isothermal in both cases and proceed in reversible single steps.

Gibbs energy is also the chemical potential that is minimised when a system reaches equilibrium at constant pressure. As such, it is a convenient criterion of spontaineity for isobaric (constant pressure and variable volume) processes.
The Gibbs free energy, originally called available energy, was developed in the 1870s by the American mathematical physicist Willard Gibbs.

The change in Gibbs free energy, ΔG, in a reaction is a very useful parameter. It can be thought of as the maximum amount of work obtainable from a reaction at constant pressure (usually reactions occur at ambient pressure, 1 atm). For example, in the oxidation of glucose, the change in Gibbs free energy is ΔG = 686 kcal = 2870 kJ. This reaction is the main energy reaction in living cells.

The change in Gibbs free energy associated with a chemical reaction is a useful indicator of whether the reaction will proceed spontaneously. Since the change in free energy is equal to the maximum useful work which can be accomplished by the reaction then a negative ΔG associated with a reaction indicates that it can happen spontaneously. ΔG values are listed for a number of important reactions.These values refer to the same number of moles of the stoichiometric coefficients of reacting compounds and of products, as shown in examples

Knowing the ΔG value of a reaction, the value of its equilibrium constant can be calculated. For a gaseous reaction like A + B = C + D it can be demostrated that when equilibrium is reached :

              PC * PD
KP = ____________     and Kp = exp [-ΔG/(RT)]
              PA * PB

where PA , PB, PC , PD are the partial pressure of the reagent and products.

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