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Standard Gibbs energy of a reaction

The standard Gibbs energy of a reaction, DrG0, (or 'standard reaction Gibbs energy') may be obtained from the standard entropies and enthalpies of reaction as follows:

The proper definition of the standard Gibbs energy of reaction is the difference in standard molar Gibbs energies the products and reactants in their standard states at the temperature specified for the reaction as written.

However, as in the case of standard reaction enthalpies, it is convenient to re-express the standard reaction Gibbs energy, DrG0, in terms of the standard Gibbs energies of formation, DfG0,where The standard Gibbs energy of formation is the standard reaction Gibbs energy for the formation of a compound from its elements in their reference states. (Standard Gibbs energies formation of the elements in their reference states are zero, because their formation is 'null' reaction.), i.e.:

The experimental values of DrG0 are obtained either through calorimetric methods (finding DH directly, and DS from the heat capacities), or through equilibrium constants (discussed at a later stage). Other methods are also possible.