Exothermic/endothermic

Therefore, for a thermoneutral reaction, the intersection point between the bond order profiles for the bond making and the bond breaking processes coincides with the TS the reactivity of the two reacting atoms also equalizes at the TS, as can be seen from the intersection of their IT profiles. These intersection points of the associated bond orders and condensed FFs he toward the left (right) of the TS for an endothermic (exothermic) reaction, in agreement with the Hammond postulate. 

Temperature rule (T-rule). Temperature increase (decrease) favors endothermic (exothermic) reactions. Since the total T-dependence is determined by a combination of formation energies, the final result is not always obvious. However, usually, the defect concentrations rise with increasing temperature. 

When a solute is dissolved in a solvent, heat change generally occurs. A dissolution process may be exothermic or endothermic. Exothermic processes emit energy as heat. Endothermic processes absorb energy as heat. Temperature rises in an exothermic process, but falls in an endothermic one. When lithium chloride (LiCl) dissolves in water, the solution gets warmer and the temperature goes up. We can say that the dissolution of lithium chloride is exothermic. (Figure 6). 

An endothermic transition was observed at 134.6°C, followed by an endothermic-exothermic combination beginning near 181.6°C, and an additional endothermic transition at 263.5°C. The TGA curve exhibited a 7.8% weight loss measured at 150°C, which correlated with the 134.6°C endotherm observed in the DSC. A large weight loss near 250°C confirmed the 263.5°C endotherm as a decomposition. The logical conclusion that may be drawn from this preliminary data set would be that the material possibly melted and volatilized at 134.6°C, possibly melted and recrystallized at 181.6°C, and then melted with decomposition at 263.5°C. However, no visual confirmation of these transitions was available. 

FIGURE 7.37 Compound 5 overlay of DSC curve (bottom) exhibiting endotherm maxima at 134.6 and 263.5°C, and an endotherm-exotherm combination near 181.6°C, and TGA curve (top) exhibiting a 7.8% weight loss at 150°C. 

Table 2-26 permits one to classify chemical reactions into groups with specific probable values of the a-coefficients in each class. Such a classification (Levitsky, Macheret, Fridman, 1983) is presented in Table 2-27. Reactions are divided in this table into endothermic, exothermic, and thermoneutral categories and into simple- and double-exchange elementary processes. The classification also separates reactions with breaking bonds into excited or non-excited molecules. This classification table approach is useful for determining the efficiency of vibrational energy a in elementary reactions if it is not known experimentally or from special detailed modeling.

Chemical energy (qualitative) Endothermic/exothermic reactions... 

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