Effects of Constitutive Properties on Pressure and Extent of Reaction

The paper applies the theory of mixtures to the chemical reaction rate. Concerning the time dependence of pressure, it is shown that pressure increases, is constant or decreases depending on the analogous behaviour of mole numbers. The results are established analytically and then numerically for the ideal gas, the van der Waals and the truncated virial equations. Next, in connection with the ideal gas model, Denbigh assumption is established by starting from the thermodynamic relation between (partial) pressure and Helmholtz free energy. Moreover, it is pointed out that the chemical potential does not exactly equal the partial derivative of the Gibbs free energy with respect to the corresponding mole number. This in turn is shown to imply that the evolution of a reaction is provided by the chemical potentials rather than by the derivative of the Gibbs free energy. Subject to the assumption of ideal gas for the constituents, as a thermodynamic requirement it is shown that if the number of moles increases the reaction is favoured by low pressures, and viceversa, and explicit estimates are established.