Pure and Applied Chemistry

Abstract: In the first part of this work, the focus is on the molar heat capacity C_V at constant volume of fairly simple liquids. This quantity contains valuable information on the type of motion executed by the constituent molecules. Using the residual property formalism, the (hindered) rotational behavior of quasi-rigid molecules in the dense liquid phase will be discussed. To this end, the change of C_V determined for states along the saturation curve has to be resolved into the separated changes with temperature and volume, respectively. In the second part, the focus is on binary liquid mixtures containing one strongly polar component. Polar interactions constitute an important example of orientational forces between molecules, and substantial deviations of the properties of polar fluids from those of comparable nonpolar fluids are to be expected and indeed observed. At sufficiently low temperatures, these directional forces lead to significantly increased local structure. Extension of these ideas to liquid mixtures allows a semi-quantitative discussion of W-shaped excess molar heat capacities C_p^E at constant pressure and M-shaped excess molar enthalpies H^E.