Standard free energy change and equilibrium constant
Similar to standard enthalpy of formation of compounds, the standard free energy of formation, ∆Gf0 of compounds have been defined as the change in free energy when one mole of the compound is formed from its elements in their states. The standard free energy of formation of elements in their stable state are assigned the value zero as in the case of standard enthalpies of formation. ∆Gf0 values of different compounds have been determined experimentally. Table gives some values of ∆Gf0 of compounds.
Table: Standard free energy of formation of some substances
By using the values of ∆Gf0 for substances in Table, the value of ∆Gf0 of any reaction involving the reactants and products may he calculated. For this purpose use is made of the following relationship:
∆G0 = ∑ ∆Gf0 (products) – ∆Gf0 (reactants).
The standard free energy of a substance represents the free energy change associated with the formation of the substance from the elements in their most stable forms as they exist under standard conditions. Combine the standard enthalpy of formation and the standard entropy of a substance to get its standard free energy of formation.