THERMODYNAMIC PROPERTIES OF NONAQUEOUS SOLUTIONS .V. IONIC ENTROPIES - THEIR ESTIMATION AND RELATIONSHIP TO STRUCTURE OF ELECTROLYTIC SOLUTIONS

Thermodynamic data obtained in this laboratory and from the literature have led to a compilation of partial molal entropies of 1:1 electrolytes in seven nonaqueous solvents. For each solvent the absolute entropies of the ions can be expressed by the equation S̄2° (X) = a + bS̄2°(H2O), where a and b are constants characteristic of the solvent and S̄2° (H2O) is the absolute entropy of the corresponding ions in water. The partial molal entropy of any particular electrolyte in the various solvents increases in the order NH3 < N,N-dimethylformamide ≈ EtOH < MeOH < N-methylformamide < formamide < H2O < D2O, indicating that entropies are most positive for those solvents having the highest degree of internal order. It is suggested that the absolute entropy of any given ion in a solvent is given by S̄2° = kSstr + C, where Sstr is that part of the entropy of a solvent which arises because of its internal order and C is a constant characteristic of the ion. This equation is tested by relating Sstr to the deviation from the ideal boiling point of the solvent, ΔTbp, and plotting S̄2° vs. ΔTbp. Linear relationships are observed. Both equations are useful for estimating ionic entropies for species for which there are no data.