Standard Free Energy (G°):
Like standard enthalpy of formation of substances, standard enthalpy change of a reaction, standard free energy of formation of substances and standard free energy change of reactions are considered. The standard free energy value (G°) of all substances either elements or compounds may be calculated from H° and S° values at standard conditions of temperature (298 K) and pressure (1 atm) and the substance being present in the standard state.
i.e G° = H° – TS°
Standard free energies of formation of elements are taken as zero. Hence, standard free energy change of a reaction which is stoichiometrically balanced, is equal to the difference between the total sum of the standard free energies of products and the total sum of the standard free energies of reactants, at standard conditions.
ΔG°reaction = ΣG°product – ΣG°reactants
ΔG°reaction can also be calculated from ΔH°reaction and ΔS°reaction values. ΔH°reaction and ΔS°reaction can be calculated from H°f and S° values of respective product and reactant molecules at the constant temperature and pressure.