Heat capacities at constant volume, free volumes, and rotational freedom in some liquids

Abstract

This paper presents the experimental results on the velocity of sound, densities, and heat capacities of eight organic liquids at 25°, 35°, and 45°C. Using Eyring's equation, the free volumes have been calculated from the sound velocity data. For pure liquids, a quantity C_v* = (C_v)_L- (C_v)_g- C_str called the residual heat capacity is found to be linearly dependent on free volume. Analysis of the data for 34 liquids shows that a plot of residual heat capacity against the free volume gives a series of straight lines differing in slopes for different groups of liquids such as hydrocarbons, halogen-substituted hydrocarbons, alcohols, etc. This behaviour is ascribed as being due to different degrees of rotational freedom of molecules in these liquids.