Contents - Index



GibbsFreeEnergy [J/kg, J/kmol, kJ/kg, kJ/kmol, Btu/lbm Btu/lbmol] returns the specific Gibbs free energy of a specified substance.  The Gibbs free energy is defined as:


g = h - T*s


The value and units of the returned value depends on the Unit System setting.  The exact form of the GibbsFreeEnergy function depends on the substance and independent variable(s) selected.  Ideal gas substances and real substances will require two inputs.  Substances in the NASA condensed matter database can accept two inputs but only temperature is required since the pressure x volume correction for enthalpy is not used for these substances, as discussed here


The reference state upon which the value of enthalpy is based varies with the substance.  However, all ideal gas substances (which have a chemical symbol name, e.g., N2, CO2, CH4)  have enthalpy values corresponding to JANAF table references.  The JANAF table reference for enthalpy is based on the elements having an enthalpy value of 0 at 298 K (537 R).


The list of property identifiers needed in the calling arguments and instructions are available in the Thermophysical Function help.




$UnitSystem SI C kPa mass

g_air=gibbsfreeenergy(Air,T=25 [C], P=100 [kPa])

g_stm=gibbsfreeenergy(Steam,T=100 [C],P=50 [kPa])

g_sat=gibbsfreeenergy(Steam,T=100 [C],x=1)



g_air=156.1 [kJ/kg]

g_stm=1913 [kJ/kg]

g_sat=1940 [kJ/kg]



Thermophysical functions