Note on the inconsistent definition assigned in the literature to the heat capacity of the so-called “equilibrium hydrogen” mixture

Abstract In classical chemical engineering thermodynamics, constant-pressure heat capacity has a rigorous – non-negotiable – definition. With this communication we aim to make researchers aware of a thermodynamically inconsistent definition assigned, in the literature, to the heat capacity of equilibrium gaseous mixtures of reacting ortho-hydrogen and para-hydrogen (the so-called “equilibrium-hydrogen mixtures”). Along this note we show that in order to define its heat capacity, the equilibrium reacting mixture is inconsistently assimilated to a pseudo pure component having the same enthalpy as the reacting mixture of p-H2 and o-H2 at chemical equilibrium. In so doing, the pseudo-component heat capacity aggregates not only sensible-heat effects but also thermal effects due to compositional changes.