Enthalpy, of fusion

The properties of the solids most commonly encountered are tabulated. An important problem arises for petroleum fractions because data for the freezing point and enthalpy of fusion are very scarce. The MEK (methyl ethyl ketone) process utilizes the solvent s property that increases the partial fugacity of the paraffins in the liquid phase and thus favors their crystallization. The calculations for crystallization are sensitive and it is usually necessary to revert to experimental measurement. 

The fact that water is a liquid at room temperature with high enthalpies of fusion and vaporisation can be attributed to hydrogen bond formation. The water molecule is shown in Figure 10.3. 

The normal boiling point of 2-methylthiazole is 17 0= 128.488 0.005°C. The purity of various thiazoles was determined cryometrically by Handley et al. (292), who measured the precise melting point of thiazole and its monomethyl derivatives. Meyer et al. (293, 294) extended this study and, from the experimental diagrams of crystallization (temperature/degree of crystallization), obtained the true temperatures of crystallization and molar enthalpies of fusion of ideally pure thiazoles (Table 1-43). 

Molar enthalpy of fusion of ideally pure sample. 

Other crystallization parameters have been determined for some of the polymers. The dependence of the melting temperature on the crystallization temperature for the orthorhombic form of POX (T = 323K) and both monoclinic (T = 348K) and orthorhombic (T = 329K) modifications of PDMOX has been determined (284). The enthalpy of fusion, Aff, for the same polymers has been determined by the polymer diluent method and by calorimetry at different levels of crystallinity (284). for POX was found to be 150.9 J/g (36.1 cal/g) for the dimethyl derivative, it ranged from 85.6 to 107.0 J/g (20.5—25.6 cal/g). Numerous crystal stmcture studies have been made (285—292). Isothermal crystallization rates of POX from the melt have been determined from 19 to —50 C (293,294). Similar studies have been made for PDMOX from 22 to 44°C (295,296). 

Enthalpy of Fusion The enthalpy (heat) of fusion AiTfus is defined as the difference of the enthalpies of a unit mole or mass of a solid and hquid at its melting temperature and one atmosphere pressure of a pure component. There are no generally apphcable estimation techniques that are very accurate. However, if the melting temperature is known, the atomic group contribution method of Chickos et al. " yields approximate results ... 

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. 

Table 2.2 Enthalpies and temperatures of fusion and vaporization. Normal melting and boiling points and enthalpies of fusion and vaporization are tabulated by type of...

The melting temperature of Si is 1683 K where the enthalpy of fusion is 46.4 kJ mol-1. Calculate the amount of heat that must be added to isobarically raise the temperature of one mole of silicon from 298.15 K to 3000 K. 

When the enthalpy of fusion can be assumed to be constant, equation (8.28) can be integrated to give... 

In Chapter 6, we considered the more general behavior of the activity with temperature and showed with equations (6.163) to (6,166) how to integrate the right hand of equation (8,28) when the enthalpy of fusion is not constant over the temperature range of interest. Those same considerations apply directly here and they will not be repeated, except to give the final result ... 

Benson and coworkers15, and the values differ somewhat. The differences probably reflect the small differences in the enthalpies of formation of the parent compounds used in the two cases (i.e., the differences between enthalpies of formation compiled by Benson and coworkers15 and those compiled by Pedley, Naylor and Kirby17) as well as different approaches to deriving the group contributions. The major differences probably arise from the different values used for the relevant enthalpies of fusion and vaporization. It should be emphasized that the derivation of group contributions is not purely quantitative in nature, and that subjective elements enter into the selection of molecules to be used to... 

Enthalpy of fusion, Pu-oxygen systems 129 Enthalpy of stabilization, alkali... 

The molar enthalpy change that accompanies melting (fusion) is called the enthalpy of fusion, AHfus, of the substance ... 

Melting is, with only one known exception (helium), endothermic, and so all enthalpies of fusion (with the exception of that special case) are positive and are reported without their sign (see Table 6.3). The enthalpy of fusion of water at 0°C is 6.0 kj-mol to melt 1.0 mol H20(s) (18 g of ice) at 0°C, we have to supply 6.0 kj of heat. Vaporizing the same amount of water takes much more energy (more than 40 kj) because, when water is vaporized to a gas, its molecules must be separated completely. In melting, the molecules stay close together, and so the forces of attraction and repulsion are nearly as strong as those experienced in the solid (Fig. 6.22). 

Because enthalpy is a state function, the enthalpy of sublimation of a substance is the same whether the transition takes place in one step, directly from solid to gas, or in two steps, first from solid to liquid and then from liquid to gas. The enthalpy of sublimation of a substance must therefore be equal to the sum of the enthalpies of fusion and vaporization, provided that they are measured at the same temperature (Fig. 6.25) ... 

Self-Test 6.10A The enthalpy of fusion of sodium metal is 2.6 kj-mol 1 at 25°C, and the enthalpy of sublimation of solid sodium at that temperature is 101 kj-mol 1. What is the enthalpy of vaporization of sodium at 25°C ... 

FIGURE 6.25 Because enthalpy is a state property, the enthalpy of sublimation can be expressed as the sum of the enthalpies of fusion and vaporization measured at the same temperature. 

The enthalpies of fusion and vaporization affect the appearance of the heating curve of a substance. A heating curve is the graph showing the variation in the temperature of a sample as it is heated at a constant rate at constant pressure and therefore at a constant rate of increase in enthalpy (Box 6.1). 

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