The Calculation of Thermodynamic Quantities from Spectroscopic Data for Polyatomic Molecules; the Free Energy, Entropy and Heat Capacity of Steam

The methods already developed by Gordon and Barnes for computing thermodynamic quantities from the band spectrum of a diatomic molecule are extended to deal with the case of the triatomic unsymmetrical rotator. The free energy, entropy and heat capacity of steam are computed for the temperature range 298.1-1500 degrees K from the spectroscopic data of Mecke and his associates. From the free energy so calculated, the known spectroscopic free energies of carbon monoxide, carbon dioxide and hydrogen, and the thermal data of Rossini, revised values of the equilibrium constant for the water-gas reaction are computed. The numbers so obtained are in close agreement with the experimental results of Neumann and Koehler and of Emmett and Schultz.