Entropy of melting 188

A parallel thermodynamic study of 2-methylthia2ole was performed by Goursot and Westnim (299). The stable form was characterized by a temperature of melting of 248.42°K, an enthalpy of melting of 2907 cal mole and an entropy of melting of 11.70 cal mole ... 

It can be seen that a high mp implies either a high enthalpy of melting, or a low entropy of melting, or both. Similar arguments apply to vaporization and the bp, and indicate the difficulties in quantifying the discussion. 

This relationship does not include the correction recommended by Yalkowsky (19) for including differences between solid and liquid compounds because Yalkowsky s work was not available at the beginning of the study. Yalkowsky showed that solids and liquids do not fit well into the same correlation unless the solid solubilities are corrected for the entropy of melting. The error introduced by not including the correction is not significant for this screening analysis. 

Miller, M. M., Ghodbane, S., Wasik, S. R, Tewari, Y. B., Martire, D. E. (1984) Aqueous solubilities, octanol/water partition coefficients and entropies of melting of chlorinated benzenes and biphenyls. J. Chem. Eng. Data 29, 184-190. 

This kind of conformational disorder has also been suggested for poly(methyl sorbate).105 It is therefore not unusual and also explains some properties of these polymers, as, for instance, the low entropy of melting.114... 

In addition to the melting point of the P phase and the a/P allotropic transfonnation temperature in Fig. 6.1(b), there is a fluther intersection between the Gibbs energy of a and liquid phases. This corresponds to the metastable melting point of the a phase. A linear model will then dictate that the entropy of melting for a is defined by the entropies of melting and transformation at the two other critical points (Ardell 1963),... 

Consider a molecule with a smaller symmetry number of ctj and an isomer molecule with a larger symmetry number of a2, the liquid entropy of the less symmetrical molecule is higher than the more symmetrical molecule by F ln(cT2/cri). As a consequence, the more symmetric molecule has a smaller entropy of melting, A5m,2 = A5m,i - R ln(a2/ai), which would lead to a higher melting temperature—if symmetry does not have a big effect on the enthalpy of melting or the entropy of the solid. 

The phenomenon of melting point also involves the entropy of melting, which has a much smaller value for a symmetrical molecule (such as methane, with a symmetry number of 12) than with an asymmetrical molecule (such as CH2CIF, with a symmetry number of 1). When we introduce the fourth parameter of the entropy of rotational symmetry given by R In(cr), the regression results is... 

When groups are inserted into the main chain, its regularity is disturbed and chain mobility increases (reduction of the enthalpy of melting as well as increase of the entropy of melting). Thus, polymers are formed that can be further processed from solution or in bulk. 

The conducting ion sublattice in FICs is generally considered molten . The molten sublattice model for fast ion conduction was first proposed by Strock (1936) on the basis of structural and thermodynamic data for Agl. In most FICs, the entropy of the phase transition to the FIC state is larger than the entropy of melting. For example, in Agl the entropy of the transition at 420 K from the -form to the a-form (FIC state) is 14.7 J deg mol , whereas the entropy of melting at 861 K is only 11 J deg mol . ... 

Dannenfelser, R. M., and S. H. Yalkowsky, Estimation of entropy of melting from molecular structure A non-group contribution method , Ind. Eng. Chem. Res., 35, 1483-1486 (1996). 

The heat of melting and the entropy of melting can be calculated, from the depression of the crystalline melting point by the plasticizer. The high melting point of the polycarbonate is due essentially to a low entropy of fusion, undoubtedly caused by the high rigidity of the polycarbonate chain. 

Table 10.1. Entropy of melting and vaporization of several metals...

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