Enthalpy of dehydration

The obtained value AHf(PUF3,c) = - 1585.7 + 2.9 kJ.mol-1 cannot be considered as entirely satisfactory as the reliability of the adopted value for the enthalpy of dehydration is not demonstrated. The only experimentally known enthalpies of formation for the actinide trifluorides are AHf(PuF3,c) and AHf(UF3,c) accuracy is therefore essential if these two data are used to estimate the enthalpies of formation of the other trifluorides. The low temperature heat capacity measurements of Osborne et al. (22) using 242PuF3(c) yield S°(PuF3,c 126.11+0.38 J.K"l.mol"i. 

Johnson et al. [143] also studied the dehydrated form of mordenite by reaction-solution calorimetry. Their results and the foregoing enthalpy of formation data lead to Af//°(Cao.289Nao.36iAlo.94oSi5.06oOi2.ooo, cr) = —5661.7 4.8 kJ mol-1. From the enthalpies of formation of both forms of mordenite and the enthalpy of formation of liquid water already quoted (—285.830 0.040 kJ mol-1), it is possible to conclude that at 298.15 K, the enthalpy of dehydration of mordenite, which corresponds to the reaction... 

The quantum energy is very similar to the enthalpy of dehydration of the hydrated electron, and the absorption of the 715 nm radiation by the hydrated electron probably causes its dehydration, i.e. its reversion to the gaseous state, e"(g). 

Experimental activation energies calculated for many reversible endothermic dehydrations are close to values of the enthalpy of dehydration. These reactions have been termed "normal" and the energy is identified as that required to rupture the water-cation bond. For such "normal" reactions, the rate of dehydration is expressed [2] by the Polanyi-Wigner [2,35] equation ... 

Kinetic studies [37,40] of the dehydration of chrome alum (260 to 270 K) yielded Arrhenius parameters of greater magnitude than predicted by the Polanyi-Wigner equation ( , = 125 kJ mol, whereas the enthalpy of dehydration is 42 kJ (mol HjO) ). This was ascribed to systematic changes in the thickness of the transition layer and thus on impedance of flie escape of water vapour. At higher temperatures (288 to 308 K) a lower value of was found (96 kJ mol" ) and the Arrhenius... 

These authors studied the enthalpy of dehydration of zirconium phosphate and arsenate, in the temperature range 95 to 140°C and 90 to 110°C, respectively, by measuring the partial pressure of water vapour over the solid phases using a tensimetric method. The reaction studied was ... 

In some instances, a compound of a given hydration state may crystallize in more than one form, so that the hydrates themselves exhibit polymorphism. One such example is nitrofurantoin, which forms two monohydrates that have distinctly different temperatures and enthalpies of dehydration. The monohydrates have quite different packing arrangements, with Form I possessing a layer structure and Form II exhibiting a herringbone motif The included water molecules play a major role in stabilizing the crystal structures. Whereas water molecules are contained in isolated cavities in Form II, in Form I they are located in continuous channels, and this apparently facilitates the escape of water when these crystals are heated [60]. 

Results and Discussion The conditions of measuring and results of determining the enthalpy of dehydration of kaolinite, muscovite, and talc are given in Tables 16.29-16.31, and the averaged enthalpies and molar enthalpies in various decomposition modes, in Table 16.32. These data allow the following conclusions. 

Table 16.34 Experimental conditions and the results of determination of the molar enthalpy of dehydration of Li2S04 H2O by the third-law...

The molar enthalpies of dehydration of Li2S04 H2O in a vacuum, found... 

The thermal transformation fix)m lanthanum hydroxide (La(OH)3) to lanthanum oxide (La203) was studied by Neumann and Walter [188]. They reported two successive endothermic effects, caused by loss of water. Thermal analysis (DTA-TG, DSC), and high-temperature powder XRD were used to characterize this process. Lanthanum hydroxide oxide (LaOOH) was obtained as a temporary product at about 330 C. The structure of lanthanum hydroxide oxide was characterized by powder XRD and subsequent Rietveld refinement. The enthalpies of dehydration were calculated by DSC to be 87 kJ mof for the transformation La(OH)3 to LaOOH, and 54 kJ mof for the transformation LaOOH to La203. The activation energy of the transformation of lanthanum hydroxide to lanthanum hydroxide oxide was estimated by isothermal TG studies. 

Alternatively the polymers may be exposed to different relative humidities and the samples scanned after storage. This requires careful treatment of samples since adsorbed water may be easily lost on transfer of samples from the humidity cabinet to the encapsulation device. Weight gain may be assessed by weight loss on TG analysis or by the enthalpy of dehydration during the DSC scan [125]. The technique has been used to study water distribution for HPMC [128] and in gelatin, pectin and sodium carboxymethylcellulose samples [125]. 

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