Enthalpy of melting

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. 

In some cases there is evidence of multiple solid-solid transitions, either crystal-crystal polymorphism (seen for Cl salts [20]) or, more often, formation of plastic crystal phases - indicated by solid-solid transitions that consume a large fraction of the enthalpy of melting [21], which also results in low-energy melting transitions. The overall enthalpy of the salt can be dispersed into a large number of fluxional modes (vibration and rotation) of the organic cation, rather than into enthalpy of fusion. Thus, energetically, crystallization is often not overly favored. 

Why does the enthalpy of melting ice decrease as the temperature decreases ... 

The enthalpy A W((lcUi is the energy required to melt 1 mol of material at constant pressure. We need to be careful when obtaining data from tables, because many books cite the enthalpy of fusion, which is the energy released during the opposite process of solidification. We do not need to worry, though, because we know from Hess s law that AH elt) = — A//( lsion). The molar enthalpy of melting water is +6.0 kJmol-1. 

Several enthalpies of melting and vaporization are shown in Table 5.1. Notice that the same units (kj/mol) are used for the enthalpies of melting, vaporization, condensation, and freezing. Also notice that energy changes associated with phase changes can vary widely. 

The process of melting is also known as fusion. Therefore, you will sometimes see the enthalpy of melting referred to as the enthalpy of fusion. 

Table 5.1 Enthalpies of Melting and Vaporization for Several Substances... 

ISO 11357-3 1999 Plastics - Differential scanning calorimetry (DSC) - Part 3 Determination of temperature and enthalpy of melting and crystallization ISO 11357-7 2002 Plastics - Differential scanning calorimetry (DSC) - Part 7 Determination of crystallization kinetics... 

Golovanova, Yu.G. and Kolesov, V.P. Enthalpies of melting, melting temperatures, and cryoscopic constants of some haloorganic compounds, Vestn. Mosk Univ, Ser. 2 Khlm, 25(3) 244-248, 1984. 

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. 

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. 

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