Chemical Equilibrium for Ideal Gases

Abstract

The treatment of ideal gases thus far has referred essentially to the determination of the thermodynamic functions for pure substances. In considering chemical equilibrium, however, we are dealing with homogeneous mixtures of gases in which two chemical changes, one the reverse of the other, are occurring at equal rates at a constant total pressure and temperature. One must enquire to what extent the expressions deduced in the earlier chapters are applicable to ideal gases in ideal gas mixtures. Without deriving the result rigorously we will say immediately that the results are applicable without modification. This is because, in a mixture of ideal gases, each gas is independent of the others and behaves as if they were not present, apart from any chemical interactions among them. Just as the molar thermodynamic properties of a pure ideal gas depend only on its pressure and temperature, so also do those of an ideal gas in an ideal gas mixture depend only on its partial pressure and temperature. This is true for E, H, S, G, and A. Moreover, E and H are even independent of pressure. It follows that the values for E⁰, H⁰, S⁰, G⁰, and A⁰ deduced earlier for pure ideal gases apply to these gases in ideal gas mixtures when they are individually at a partial pressure of one atmosphere.