The cohesive entropy of condensed materials, empirical relations and restrictions

Abstract

Entropy factors of a material are connected through an equation which relates three independent experimental values specific to each material: ΔfS=S-∑inSatoms,i. (Note: our results refer to independently summed ∑1nSatoms,i values rather than calculation by simple difference of the third dependent value from the other two of these values.) We define the cohesive entropy coefficient, αScoh, through ΔfS=α∑inSatoms,i. Having ready access to a large database of thermodynamic data for solid ionic materials from earlier studies, we have investigated the generalisations that may be made among these entropy quantities for this group of materials. We find that the data points of this three-dimensional system are confined to a fan-shaped tilted plane which has rather strict lateral limits, with the upper limits controlled by the entropies of gaseous elements and the lower limits by the entropies of solid elements. This has the consequence of providing insight into the understanding of entropy values and their limits as a check on experimental determinations. In particular, formation and standard entropies for condensed phases are shown to be proportional to one another with a fixed proportionality constant, the cohesive entropy coefficient, αScoh = -0.831. Evidence is provided that the same restrictions apply to condensed organic materials, and we suggest that these entropy relations are applicable to condensed materials in general.