Phase transition sublimation

Identify and explain the sign on A ansW in equation 6.5 if it is used for (a) a solid-to-gas phase transition (sublimation),... 

As with other crystalline substances, on heating coordination compounds may melt, sublime, decompose, or undergo a solid phase transition. The greater complexity of the constituents present increases the number of types of bond redistribution processes which are, in principle, possible within and between the coordination spheres. The following solid-state transitions may be distinguished (i) changes in relative dispositions... 

The sample temperature is increased in a linear fashion, while the property in question is evaluated on a continuous basis. These methods are used to characterize compound purity, polymorphism, solvation, degradation, and excipient compatibility [41], Thermal analysis methods are normally used to monitor endothermic processes (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, and chemical degradation) as well as exothermic processes (crystallization and oxidative decomposition). Thermal methods can be extremely useful in preformulation studies, since the carefully planned studies can be used to indicate the existence of possible drug-excipient interactions in a prototype formulation [7]. 

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

The best-known examples of phase transition are the liquid-vapour transition (evaporation), the solid-liquid transition (melting) and the solid-vapour transition (sublimation). The relationships between the phases, expressed as a function of P, V and T consitute an equation of state that may be represented graphically in the form of a phase diagram. An idealized example, shown in figure 1, is based on the phase relationships of argon [126]. 

The temperature at which a phase transition occurs is dependent on pressure (Figure 7). At atmospheric pressure (1 atm) the solid-to-liquid phase transition occurs at 0 °C and the liquid-to-gas phase transition occurs at 100 °C. If we increase the pressure, say to 100 atm, the solid-to-liquid phase transition occurs at a temperature slightly less than 0°C (—0.74°C) however, the liquid-to-gas phase transition occurs at a much greater temperature (312°C). If we decrease the pressure, say to 0.1 atm, the solid-to-liquid phase transition occurs at a temperature slightly greater than 0°C (0.004 °C) and the liquid-to-gas phase transition occurs at a lower temperature (46 °C). If we decrease the pressure further to below the triple point, there is no solid-to-liquid phase transition rather, the solid-to-gas phase transition occurs directly. At a pressure of 0.001 atm, the sublimation temperature is — 20.16°C. 

For each of the phase transitions, there is an associated enthalpy change or heat of transition. For example, there are heats of vaporization, fusion, sublimation, and so on. 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

The types of values reported in the database standard enthalpies of formation at 298.15 K and 0 K, bond dissociation energies or enthalpies (D) at any temperature, standard enthalpy of phase transition—fusion, vaporization, or sublimation—at 298.15 K, standard entropy at 298.15 K, standard heat capacity at 298.15 K, standard enthalpy differences between T and 298.15 K, proton affinity, ionization energy, appearance energy, and electron affinity. The absence of a check mark indicates that the data are not provided. However, that does not necessarily mean that they cannot be calculated from other quantities tabulated in the database. 

If the phase transition is a vaporization or sublimation, and if the vapor can be assumed to be an ideal gas, an alternative approximation applies ... 

Knowledge of the mode of crystn of TNT is essential because it underlies the widespread practice of melt-pouring employed in the preparation of the commonly-used composite expls, such as Composition B. Samples of TNT obtained by sublimation onto a condensing surface held at a temp (78°) close to the mp, or by freezing melts at temps close to the mp, consist solely of the simple monoclinic form (Ref 26). Crystn from solvents at room temp, or from strongly supercooled melts, yields primarily monoclinic variant forms. Orthorhombic TNT is formed by crystn from solvents at low temps. At least seven morphological types of TNT have been identified (Ref 48). Two types have been identified by nuclear quadripole resonance (NQR) (Ref 66) a phase transition was noted at... 

Ammonium azide is a colorless, crystalline substance. Some of its properties are given in Table 19. No phase transitions are found between 90 and 348 K. Sublimation of ammonium azide begins at 406 K, recondensation of the vapors at 418 K. Ammonium azide vapors decompose, similar to other ammonium salts, to ammonia and HN3 at 418 K (A// = -1-67.7 kJmol ). In the solid state, all ammonium ions are tetrahedrally connected to azide ions via hydrogen bonds with an NH -N distance of 297.5(4) and 296.7(3) pm. ... 

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