Endothermicity, defined

A considerable amount of research has been conducted on the decomposition and deflagration of ammonium perchlorate with and without additives. The normal thermal decomposition of pure ammonium perchlorate involves, simultaneously, an endothermic dissociative sublimation of the mosaic crystals to gaseous perchloric acid and ammonia and an exothermic solid-phase decomposition of the intermosaic material. Although not much is presently known about the nature of the solid-phase reactions, investigations at subatmospheric and atmospheric pressures have provided some information on possible mechanisms. When ammonium perchlorate is heated, there are three competing reactions which can be defined (1) the low-temperature reaction, (2) the high-temperature reaction, and (3) sublimation (B9). 

Heats of Reaction. Chemical reactions absorb or liberate energy, usually in the form of heat. The heat of reaction, h.Hn, is defined as the amount of energy absorbed or liberated if the reaction goes to completion at a fixed temperature and pressure. When > 0, energy is absorbed and the reaction is said to be endothermic. When /sHr < 0, energy is liberated and the reaction is said to be exothermic. The magnitude of Is.Hr depends on the temperature and pressure of the reaction and on the phases (e.g., gas, liquid, solid) of the various components. It also depends on an arbitrary constant multiplier in the stoichiometric equation. 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Worz et al. stress a gain in reaction selectivity as one main chemical benefits of micro-reactor operation [110] (see also [5]). They define criteria that allow one to select particularly suitable reactions for this - fast, exothermic (endothermic), complex and especially multi-phase. They even state that by reaching regimes so far not accessible, maximum selectivity can be obtained [110], Although not explicitly said, maximum refers to the intrinsic possibilities provided by the elemental reactions of a process under conditions defined as ideal this means exhibiting isothermicity and high mass transport. 

Define exothermic and endothermic reactions. What is the sign of AH for an exothermic reaction An endothermic reaction ... 

Figure 15.3 (a) Heat absorption in solutions of native RNase A (trace 1) and RNase A kept in 10% buffered formalin for 2 days (trace 2) and 6 days (trace 3) at pH 7.4 and 23°C. All samples were dialyzed against 75 mM potassium phosphate buffer (pH 7.4) prior to DSC. (b) Dependence of Td of the dialyzed RNase A samples on time of incubation in 10% buffered formalin at pH 7.4 and 23°C. (c) Heat absorption of solutions of formalin-treated RNase A fractions isolated by size-exclusion gel chromatography monomer (trace 1), dimmer (trace 2), and a mixture of oligomers with >5 cross-linked proteins (trace 3). Protein concentrations were 0.5 mg/mL. The thermal denaturation transition temperature (Td) is defined as the temperature of the maximum in the excess heat absorption trace associated with the protein s endothermic denaturation transition. See Rait et al.10 for details. 

A new evaluation standard for the dehydrogenation catalysts in the superheated liquid-film states is introduced here. This standard is called as the "ratio of heat recuperation" [39], being defined as the ratio of endothermic reaction heat to the denominator of heat supplied from the external thermo-reservoir to the catalyst layer shown as follows (Equations 13.10 and 13.11) ... 

Alternatively, the translational energy threshold for endothermic proton transfer from MH+ to R can be measured using a flowing afterglow triple quadrupole instrument.127 These data define the proton affinity of M, relative to that of R. Thus, the PA of cyclopropenylidene was found to exceed that of ammonia by 23.3 1.8 kcal/mol (Table 6).128 In order to obtain absolute proton affinities, the enthalpies of formation of both the base and the conjugate acid must be known from other measurements (Eq. 9). Numerous reference compounds with known absolute PA are available.124... 

To understand the dissolution of ionic solids in water, lattice energies must be considered. The lattice enthalpy, A Hh of a crystalline ionic solid is defined as the energy released when one mole of solid is formed from its constituent ions in the gas phase. The hydration enthalpy, A Hh, of an ion is the energy released when one mole of the gas phase ion is dissolved in water. Comparison of the two values allows one to determine the enthalpy of solution, AHs, and whether an ionic solid will dissolve endothermically or exothermically. Figure 1.4 shows a comparison of AH and A//h, demonstrating that AgF dissolves exothermically. 

Pyrolysis of biomass is defined as the chemical degradation of the biopolymers (cellulose, lignin and hemicellulose) constituting the wood fuel which initially requires heat. As can be seen in Figure 51, all reaction pathways making up the pyrolysis are not endothermic, which implies that some of the pyrolysis reactions generate heat. However, overall the pyrolysis process is endothermic. 

To avoid confusion it should be emphasized that the terms "endothermic and exothermic adsorption are defined here relative to the sign of the energy change (but not the sign of the heat of adsorption). 

It is interesting to note that the exothermic reversible reaction A B is identical to the same endothermic reversible reaction B A. This can be seen by plotting X versus T and noting that the r = 0 equihbrium hue separates these two reactions. In the reaction A B the rate in the upper portion where r < 0 is exactly the reaction B A. We defined all rates as positive quantities, but if we write the reaction as its reverse, both r and AZ/y reverse signs. This is plotted in Figure 5-19. 

The DSC thermogram of procaine hydrochloride was obtained using a DuPont model TA 9900 computer / thermal analyzer system. The analysis was carried out between 60 and 240"C, and the resulting thermogram is found in Figure 2. The compound was found to exhibit a clear and well-defined melting endotherm, whose temperature inflection was observed at 159<>C. 

First, we begin with a brief review of some fundamental physical properties. We defined the glass transition temperature in the usual way— as the midpoint of the transition endotherm. Starting with 1 at.% Sb, our data for Tg show the monotonic increase with increasing Sb concentration. At the same time, we observe a change... 

Generally, the crystalline melting point of a polymer corresponds to a change in state from a solid to a liquid and gives rise to an endothermic peak in the DSC curve [3,9], The equilibrium melting point may be defined as... 

Figure 7.5 shows the relative endotherms of PVDF/PMMA blends as a function of PVDF content. A relative endotherm is defined here as the ratio of a melt endotherm at a certain PVDF blend composition to the endotherm of the pure annealed PVDF in DSC measurements. The DSC measurements are at a constant heating rate of lOC/min. For annealed and solution-cast blends, the relative endotherms are in general greater than those of quenched blends of the same composition. Endotherms can be detected at all compositions when the blends are annealed. The quenched PVDF shows only 75% endotherm relative to the annealed PVDF. No endotherm can be detected when the PVDF content is lower than 50 wt % in quenched PVDF/PMMA blends. Obviously the thermal history of the blends has a tremendous effect on the final performance of the material. 

Attention is also often restricted to the case when both reactions are exothermic, though Balakotaiah and Luss (1982b, 1984), Kahlert et al. (1981) and Jorgensen et al. (1984) allowed one of the reactions to be endothermic. We will not make this restriction either, so the parameters p and (3 are allowed to be of either sign. Since we are only concerned with multiplicity, we can define... 

Note that for a nonisothermal CSTR we will always obtain unique solutions (no bifurcation) for endothermic reactions, defined by [3 < 0, for both the adiabatic and the nonadiabatic cases. 

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