One also often needs mixture-averaged thermodynamic properties. As with the pure-species properties, the Gas-phase Kinetics thermodynamics subroutines return properties in either mass or molar units. Since, for a perfect gas mixture, the standard-state specific heats, enthalpies, and internal energies are also the actual values, we drop the superscript ^o for these quantities.

The mixture-averaged specific heats are

(2–43)

(2–44)

(2–45)

and

(2–46)

the enthalpies are

(2–47)

and

(2–48)

and the internal energies are

(2–49)

and

(2–50)

The mixture properties are more complex for the entropies and the Gibb’s and Helmholtz free energies. Here the actual values are not the same as the standard-state values and we must account for the appropriate pressure and entropy-of-mixing terms. The entropy is then

(2–51)

Thus the mixture-averaged entropies are

(2–52)

and

(2–53)

Similarly, the mixture-averaged Gibb’s and Helmholtz free energies are

(2–54)

(2–55)

(2–56)

and

(2–57)