Diffusion at strictly uniform pressure in a mixture of a dissociating gas and an inert gas

Abstract

In the light of recent revived interest in Graham's law of diffusion, attention is given to the diffusive fluxes in a mixture of a dissociating and an inert gas, in conditions in which the pressure is strictly uniform. Explicit expressions are given for the continuum limit, in the case in which there is local chemical equilibrium. The quotient of the fluxes of the dissociating gas and the inert gas (expressed in terms of masses, rather than in terms of numbers of molecules) can be interpreted as the square root of the ratio of an effective molecular weight of the dissociating gas to the molecular weight of the inert gas. This effective molecular weight depends not only on the extent of dissociation but also on diffusion coefficients. For an isomerizing gas, however, the effective molecular weight is simply equal to the true molecular weight.