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National Advisory Committee for Aeronautics, Technical Notes - Effect of Dissociation on Thermodynamic Properties of Pure Diatomic Gases

A graphical method for obtaining charts for the thermodynamic prop-
erties enthalpy and entropy for the equilibrium mixture of atoms and dia-
tomic molecules for pure gaseous elements is described. The procedure
is equivalent in principle to numerical calculation based on the same
fundamental data, but gives directly the location of points of intersec—
tion of curves for constant pressure and curves for constant values of
the compressibility factor PV/RT. In combination, the resulting graphs
for enthalpy and entropy are equivalent to Mollier diagrams for the gas-
eous fluid in the dissociation region. Results are given for hydrogen Hg,
oxygen 02, and nitrogen N2.

A table of equilibrium constants is included for the reac-
tions H2->2H, 02-€>20, and N2-—>2N. The effect of dissociation on the
heat capacity is also discussed briefly.

This report is the first of a series of reports on the thermodynamic
properties of gases of aeronautical interest which have been prepared at
the National Bureau of Standards under the sponsorship and with the finan-
cial assistance of the National Advisory Committee for Aeronautics.

The thermodynamic properties for gases are usually given without
the effect of dissociation. At high temperatures, a considerable pro-
portion of the gas is not in the form of pure molecular species of lower
temperatures, but instead is broken up into atoms and, in the case of
polyatomic molecules, into molecules of intermediate size. This process
of dissociation when the temperature is raised involves a large energy
if the original molecules are relatively stable, so that large,thermal
effects occur over a considerable range of elevated temperature. The
magnitude of the effect at a given temperature depends very markedly on
the total pressure of the gas. Since the effect becomes much larger than
the ordinary effects of equation of state involving interactions between
stable molecules, it will be appropriate in the following discussions to
treat each constituent as an ideal gas.