TY - JOUR
T1 - Equilibrium between a condensed phase and a reactive vapour: the heat capacity of a saturated, dissociating vapour and the form of the vapour pressure curve
JF - Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences
JO - Proc R Soc Lond A Math Phys Sci
SP - 516
LP - 539
M3 - 10.1098/rspa.1961.0215
VL - 264
IS - 1319
AU -
Y1 - 1961/12/05
UR - http://rspa.royalsocietypublishing.org/content/264/1319/516.abstract
N2 - At constant pressure, the vapour of a dissociating substance has an anomalously high heat capacity Cgp. The anomalous part is always positive and may be many times the â€˜normalâ€™ value for an otherwise similar, inert vapour. The saturated vapour of the same substance, however, with a heat capacity Cgsat may show either a smaller anomaly in the same sense, or be entirely free from any anomaly, or show an anomaly in the opposite sense to, and even larger than, that in Cgp. The factors governing the occurrence, the size and the sign of this anomaly are the degree of dissociation, and the relative magnitudes of the enthalpies of dissociation and vaporization. Expressions are derived for the heat capacities of saturated vapours for the general dissociation (Am ^mA) and the particularly common case of dimerization (A2 ^2A) and their application to real substances illustrated. The influence of gas imperfections is discussed. Except in the region of the critical point (where valid, explicit equations of state are not available) the existence of non-ideality modifies the conclusions only in degree. Like the heat capacity of saturated vapour, the latent heat of vaporization and its temperature coefficient also contain anomalous, reactive contributions. The influence of these anomalies on the form of the relationship between vapour pressure and temperature is examined. It is found that, despite the existence of such a mobile equilibrium, and even in the presence of an abnormally large heat capacity of saturated vapour, the saturation line (in the form of a graph of In P against T-1) may show no marked curvature. Conversely, linearity of a graph of In P against T-1 is no guarantee that reactions of dimerization or polymerization are absent from a vapour phase.
ER -