Phase change fusion

If, in going from 0 K to T, a substance undergoes phase changes (fusion, vaporization, etc) at and Tg with molar enthalpies of transition AHy, one can write... 

Tantalum Oxides. Tantalum pentoxide [1314-61 -0] Ta20, (mp = 1880°C, density = 8.73 g/cm ) is a white powder existing in two thermodynamically stable modifications. The orthorombic P-phase changes at 1360°C into the tetragonal a-modiftcation. The existence of an S-modiftcation has also been reported (70). Tantalum pentoxide reacts slowly with hot hydrofluoric acid but is insoluble in water and in most solutions of acids and alkalies. For analytical purposes, it can be dissolved by fusion with alkali hydroxides, alkali carbonates, and potassium pyrosulfate. 

The heat accompanying the phase change (2) is 1.44 kcal/mole. This is much less than the molar heat of vaporization of water, 10 kcal/mole. Table 5-II contrasts the melting points and the heats of melting per mole (the molar heat of melting, or the molar heat of fusion) of the same pure substances listed in Table 5-1. 

Show that the ratio of the molar heat of formation of gaseous water from the elements (a chemical reaction) to the molar heat of the fusion of water (a phase change) is of the order of 50. 

Fusion and vaporization are the most familiar phase changes, but sublimation is also common. Sublimation is a phase change in which a solid converts directly to a vapor without passing through the liquid phase. Dry ice (solid CO2) sublimes at 195 K with A ii/subl — 25.2 kJ/mol. Mothballs contain naphthalene (Cio Hg,... 

The basic idea of such systems is that snow/ice is stored in a more or less water tight pond where a cold carrier is cooled by the snow, to utilize the large latent heat of fusion. For comfort cooling about 90% of the extracted energy is in the phase change, i.e., the melting. The cold carrier is either circulated between the load and the snow or rejected after it has been used for cooling. 

The metastable liquid form boils at 44°C, the more stable solid alpha form melts at 62°C and has 25-30 J/g heat of fusion. This permits pressure generation when proceeding in either direction through the phase change. 

Our current view of the mechanism of membrane fusion in the case of PS vesicles in media containing divalent cations is along the lines of Papahadjopoulos et al. (15), where it was proposed that the key event leading to vesicle membrane fusion is the isothermic phase change... 

Behme, R.J. and Brooke, D. (1991). Heat of fusion measurement of a low melting polymorph of car-bamazepine that undergoes multiple-phase changes during differential scanning calQfirRfebiy(n. 

For latent heat, we look up the corresponding entry in the tables for either the latent heat of vapourisation (or simply the heat of vapourisation) or the heat of fusion, depending on the type of phase change encountered (liquid to vapour and solid to liquid, respectively). These quantities are in units of energy per unit mass and are given for a specific reference state (often the 1 atm boiling point or melting point of the substance). 

The entropy change, A. 3, for the process (s) -> (1) at Tm and hence the magnitude of the entropy rise at temperature brought about by the phase change (CD) can be calculated using equation (15.10) (Frame 15), applying to the fusion process ... 

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