Abstract
A flow heat capacity calorimeter and a flow vibrating tube^densimeter have been used to measure the apparent molal heat^capacities and volumes of 40 nonelectrolytes and 3 organic^electrolytes in methanol at 25 C. These quantities have been^extrapolated to infinite dilution to obtain the standard partial^molal heat capacities and volumes. Plots of the Cp(,2)('0) and V(,2)('0) with(' )^the number of carbon atoms in the molecule have been made for^n-alkanes, n-alcohols, n-amines and n-diols. The slopes of these^lines yield the contribution of the -CH(,2)- group which was found to^be constant for all four series. Least square fitting of the data forfourteen alkanes gave group contributions for -CH(,3), -CH(,2)-,-CH and -C-. Using these parameters and the results for theremaining compounds, contributions were evaluated for thefollowing functional groups -OH, -NH(,2), -COOH, -C(,6)H(,5), C=O,-COO-, -CONH-, -O-,-S-,-S(,2)-. The effect of branching was foundto be significant and could be quantitatively predicted for volumes.The group parameters can reproduce the Cp(,2)('0) and V(,2)('0) with(' )standard deviations of 3 J/K-mol and 0.9 ml/mol respectively. The results are discussed in terms of the scaled particle theory and by comparison with group parameters derived in other solvents.From literature data group parameters were also evaluated for aqueous solutions and the group contributions in methanol and water were used to estimate the volumes and heat capacities of neutral compounds of the type R(,4)M (R = Me through n-Hep). These compounds are structurally analogous to the tetraalkylammonium cations and the tetraalkylborate anions. By comparison of the properties of these models to those of the tetraalkylammonium bromides and lithium tetrabutylborate, the effect of charge on the central atom of these ions has been investigated. The charge effect is found to be significant for both volumes and heat capacities in both methanol and water, and is approximately constant except for heat capacities in water. This suggests that solvent molecules penetrate all these ions about equally so that the ion charge-solvent interactions are maintained constant. Model compound volumes and heat capacities have been calculated also for the tetraphenyl cations and anions. The charge effect is much less for the tetraphenyl single ion volumes and heat capacities have been examined. Recommended values for the heat capacities of the bromide and sodium ions in water and methanol have been determined. In both solvents the heat capacity of the bromide ion is much lower than the heat capacity of the sodium ion.