Enthalpy crystal lattices

Exothermic events, such as crystallization processes (or recrystallization processes) are characterized by their enthalpies of crystallization (AHc). This is depicted as the integrated area bounded by the interpolated baseline and the intersections with the curve. The onset is calculated as the intersection between the baseline and a tangent line drawn on the front slope of the curve. Endothermic events, such as the melting transition in Fig. 4.9, are characterized by their enthalpies of fusion (AHj), and are integrated in a similar manner as an exothermic event. The result is expressed as an enthalpy value (AH) with units of J/g and is the physical expression of the crystal lattice energy needed to break down the unit cell forming the crystal. 

From this the crystal lattice energy, U, can be calculated from the loljowing enthalpies ... 

The thermochemical characteristics of l,3,5-trinitro-l,3,5-triazepane, such as energies of dissociation of N-NOz bonds, enthalpies of formation, vaporization, and combustion, as well as enthalpy of formation of amine radicals, have been determined <2004MI92>. The rate constants of initial monomolecular stages of thermal decomposition in the solid phase were measured for its furazano-fused analog 20 <1999RCB1250> and the ratio of the rate constants of decomposition in the melt and solid states, characterizing the reaction retardation in the crystal lattice, was determined. The kinetics of the thermal decomposition of 20 has also been studied <1995MI885>. 

Crystal formation depends not only on the interaction energy of a particular synthon but on a wide variety of other factors, particular crystal nucleation and growth kinetics and nucleus-solution interfacial energy. Other important factors are lattice enthalpy and lattice entropy, long range interactions... 

AH is the enthalpy difference between crystal and liquid, the heat of melting. AH is mainly governed by the difference in interaction forces in the solid and the liquid condition it is the energy required to disturb the crystal lattice. 

It is very likely that during the polymerization under the strict control of the monomer crystal lattice, a strain energy caused by molecular movements accumulates on the reacting crystals resulting in the endothermal formation of polymer crystals. The enthalpy difference between the crystalline and amorphous polymers (3.1 kcal/mol) corresponds to the heat of crystallization of as-polymerized poly-DSP crystals. 

The heat capacity of thiazole was determined by adiabatic calorimetry from 5 to 340°K by Goursot and Westrum (295,296). A glass-type transition occurs between 145 and 175 K. Melting occurs at 239.53°K (-33.62°C) with an enthalpy increment of 2292 cal mole" and an entropy increment of 9.57 cal mole" -"K". Table 1-44 summarizes the variations as a function of temperature of the most important thermodynamic properties of thiazole molar heat capacity Cp, standard entropy S°, and Gibbs function -(G°-The variation of Cp for crystalline thiazole between 145 and 175°K reveals a marked inflection that has been attributed to a gain in molecular freedom within the crystal lattice. The heat capacity of the liquid phase varies nearly linearly with temperature to 310°K, at which temperature it rises more rapidly. This thermal behavior, which is not uncommon for nitrogen compounds, has been attributed to weak intermolecular association. The remarkable agreement of the third-law ideal-gas entropy at... 

Conceptually, the solid salt (sodium chloride, for instance) is converted to the gaseous (g) state, Na+(g) -n Cl (g), and each unit is then hydrated to form the species Na (aq) and Cl (aq). If the heat of hydration is sufficient to provide the energy needed to overcome the lattice forces, the salt will be freely soluble at a given temperature and the ions will readily dislodge from the crystal lattice. If the partial molal enthalpy of solution of the substance is positive, the solubility will increase with increasing temperature if it is negative the solubility will decrease, in agreement with Le Chatelier s principle. 

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