Freezing point curve curvature

The dependence of the curvature of the freezing point curve at the origin upon the entropy of fusion of the solvent was first pointed out by van Laar. The most common case corresponds to (22.17) and fig. 22.1 (c/. chap. XIV, 5, table 14.6). On the other hand, for spherical solvent molecules (c/. table 14.5) for which the entropy of fusion is abnormally small, the case (22.18) is realized. The form of the freezing point curve at the origin is thus a useful criterion, in the absence of calorimetric data, for the identification of those compounds which have a low entropy of fusion, f... 

This discussion of addition compounds covers only those compounds which are completely dissociated on melting, and the change in shape of the freezing point curve near a maximum is attributed to deviations of the liquid phase from ideality ( 5). In many instances there is good evidence that addition compounds exist also in the hquid phase when this occurs the curvature of the freezing point curve near the maximum is related to the equilibrium constant for the dissociation of the addition compound in the hquid. One example of this behaviour which has... 

Phase diagrams from freezing point depressions show true compound formations for simpler amides—e.g., water-N-methylacetamide forms a compound at a mole ratio of 2 to 1, water-N,N-dimethylacetamide at 3 to 2 and 3 to 1, and water-N-methylpyrrolidone at 2 to 1. The heats of mixing and heat capacities at 25°C. of a number of water-amide systems were determined. All mixing curves were exothermic and possess maxima at definite mole ratios, while the heat capacities for the most part show distinct curvature changes at the characteristic mole ratios. Both experimental results point to the stability of the particular complexes even at room temperature. This is further supported by absolute viscosity studies over the whole concentration range where large maxima occur at these same mole ratios for disubsti-tuted amides and N-substituted pyrrolidones. 

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