Standard Enthalpy of Formation and Absolute Entropy Terms
Formation terms are not included in the enthalpy, entropy, or internal energy. If the enthalpy of formation is specified for all species, EnthalpyF (HF) represent the enthalpy including the formation terms. If the absolute entropy of all species are specified then the EntropyF (SF) includes the absolute entropy.
At reference conditions, the heat of any reaction relates to the heat of formation as
(2-127)
Note that for reacting flow or a heat balance in a reactor when the heat of reaction is explicitly taken into account, the enthalpy should not include the heat of formation.
The following enthalpy expression should be used:
(2-128)
Hf,i,ref is calculated such that it matches the reference state of the package, even if the species have a different reference state. Similarly, the absolute entropy of each species is taken into account according to
(2-129)
The entropy balance over a process that includes reactions should include either the entropy of reaction and use S, or use SF without entropy of reaction.
In database, a set of parameters introduced to define Absolute entropy and Standard enthalpy of formation for ideal gas and different phases (Vapor, liquid, Solid, and Aqueous infinite dilution). One can use these parameters to calculate Gibbs free energy of reaction when different phases are available (See Electrode reaction).
Species Gibbs Free Energy of Formation
As the COMSOL ThermoDB does not include entropy of formation of species, entropy Equation 2-129 and in consequence Gibbs free energy Equation 2-110 are estimated from absolute entropy’s value. Therefore, the absolute value of entropy of formation and Gibbs free energy of formation of the species are not correct. However, for our applications these absolute values are not required while the value of entropy and Gibbs free energy change of reaction (the difference value) are correct.
If user needs the absolute value of entropy of formation, it is possible to estimate it by:
(2-130)
where the entropy of formation of species i is calculated from its elemental constituent j. For example, entropy of formation of ammonia at 298 K is
(2-131)
(2-132)
The values from RHS of Equation 2-132 can be extracted from COMSOLThermoDB. This value should be used in Equation 2-128 to estimate the absolute value of Gibbs free energy of species.