Enthalpy of transformation

In the sohd state, uranium metal exists in three aHotropic modifications. The transformation temperatures and the enthalpies of transformation are given in Table 5. The thermodynamic properties of uranium metal have been deterrnined with great accuracy and have been discussed (50). 

The last term in the above equation, AH, refers to the enthalpies of transformation that the reactants and/or products may undergo in the temperature interval 298 to T. Enthalpies of transformations are added (the sign is + ) if products transform and subtracted (the sign is if reactants transform. Molar heat capacities of reactants and products do vary... 

The shaded areas represent the values of the different enthalpies of transformation. 

MnAs exhibits this behavior. It has the NiAs structure at temperatures exceeding 125 °C. When cooled, a second-order phase transition takes place at 125 °C, resulting in the MnP type (cf. Fig. 18.4, p. 218). This is a normal behavior, as shown by many other substances. Unusual, however, is the reappearance of the higher symmetrical NiAs structure at lower temperatures after a second phase transition has taken place at 45 °C. This second transformation is of first order, with a discontinuous volume change AV and with enthalpy of transformation AH. In addition, a reorientation of the electronic spins occurs from a low-spin to a high-spin state. The high-spin structure (< 45°C) is ferromagnetic,... 

The isothermal and isoperibol calorimeters are well suited to measuring heat contents from which heat capacities may be subsequently derived, while the adiabatic and heat-flow calorimeters are best suited to the direct measurement of heat capacities and enthalpies of transformation. 

Enthalpies of transformation, melting and evaporation can simply be calculated from these values. 

Based on highly accurate adiabatic-shield calorimetry measurements, Gronvold and Westrura ( ) reported that the enthalpy of transformation of marcasite to pyrite is -1.05 0.05 kcal raol" at 700 K. The adopted value of AjH (298.15 K) is selected to reproduce this enthalpy of reaction within the reported uncertainty. Lipin et al. (2), based on combustion calorimetry, reported a value of -5.6 kcal mol" for the marcasite-pyrite transformation at 298.15 K. Due to the state of the art in combustion calorimetry at the time of this measurement and uncertainty in the products (oxides of sulfur), this value must have a high uncertainty and is given no weight in our selection process. 

Navrotsky A, Kleppa OJ (1967) Enthalpy of the anatase-rutile transformation. J Am Ceram Soc 50 626 Navrotsky A, Jamieson JC, Kleppa OJ (1967) Enthalpy of transformation of a high-pressure polymorph of titanium dioxide to the ratile modification. Science 158 388-389 Navrotsky A, Petrovic 1, Hu Y, Chen CY, Davis ME (1995) Little energetic limitation to microporous and mesoporous materials. Microporous Mater 4 95-98... 

The total pressure of the vapour in equilibrium with (3-SnSe was measured in the temperature range 862 to 920 K using the Knudsen effusion technique. The enthalpy and entropy of sublimation according to the reaction a-SnSe SnSe(g) were calculated by the review from the reported vapour pressure expression and the selected heat capacities of SnSe(g) and a-SnSe, the enthalpy of transformation a-SnSe —> (3-SnSe being 1.28 kJ-mof (cf V.7.4.1.2), and a heat capacity expression of (3-SnSe being identical to that of a-SnSe, yielding (SnSe, a, 298.15 K) = (188.2 + 20.0) kJ-mol and... 

The specific heat of Ag2Se(cr) was measured in the temperature range 120 to 520 K. The experimental heat capacity is shown only in figures and a value at 298.15 K was derived from Figure 1 of the paper, C° (Ag2Se, a, 298.15 K) = (83.68 3.00) J-K" -mol. The uncertainty has been estimated by this review because no error analysis was made by the authors nor were any estimated experimental errors given. The entropy change for the a to (3 transition was found to be 16.48 J-K -mol and corresponds to an enthalpy of transformation of (6.69 1.00) kJ-mol". The uncertainty has been assessed by this review. 

While the large-aliquot effect was explored using measurements on LaNis-H2, it is just as relevant to materials of current interest based on Li complexes, as these exhibit pressure hysteresis, have high enthalpies of transformation, poor thermal conductivity owing to small particle size and sloping pressure plateaux (e.g. Luo and Ronnebro, 2005). 

Crystallization (Chap. 9) AHAT/T 3000.0 AH is enthalpy of transformation. AT is undercooling, and T , is melting point... 

The enthalpy of transformation has been taken from the work of Levinson [1966LEV], who investigated the enthalpy of thorium metal to just beyond the melting point and found (Th, cr) = (3.6 + 0.1) kJ-mol . This value has been confirmed... 

LEV]. As for the enthalpy of transformation, this has been confirmed by the recent work of Boivineau et al. [1996B01/C0L]. No other recent studies of the melting point of thorium have been found. 

The enthalpies of transformation and fusion of ThCU (and several elements and alloys) were measured with an adiabatic calorimeter. Adequate adiabatic conditions were obtained by using a small external power source to compensate for a lack of temperature uniformity on the surface of the platinum radiation shield. 

The thermodynamic properties of some solid ternary alloys were measured by [1973Mal] and [1987Che]. The enthalpies of formation of the a8 solid solution as well as enthalpies of transformation fiom the a phase to the bcc structure were determined. The activities of Fe and V in hquid Cr-Fe-V alloys were reported by [1975Fur]. 

Fig. 3 presents the state diagram of the system PEO-water. It is of an eutectic type exhibiting mixed crystal formation and glass transition phenomena like the system mentioned. The graph of the experimentally determined eutectic enthalpy of transformation versus mass fraction of PEO verifies the eutectic composition. 

Transition temperatures thus obtained differ considerably from those determined for the bulk of the substance because the enthalpies of transformation are much smaller. In many instances the decrease of transition temperature is due to impurities which cause a heterogeneous pre-transformation this may not easily be distinguished from a homogeneous pre-transition of a monophasic system because the depression of the anisotropic-isotropic transformation temperature also depends strongly on the enthalpy of transition [195]. 

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