Exothermic phenomenon

The cloud point, usually between 0 and -10°C, is determined visually (as in NF T 07-105). It is equal to the temperature at which paraffin crystals normally dissolved in the solution of all other components, begin to separate and affect the product clarity. The cloud point can be determined more accurately by differential calorimetry since crystal formation is an exothermic phenomenon, but as of 1993 the methods had not been standardized. 

One essential experimental feature is that pressure resistant closed crucibles are used. This is because, at heating, volatile compounds may evaporate that may mask an exothermal phenomenon occurring in the same temperature range, and so the sample mass is no longer defined (see Section 4.3.2.1). 

It is clear from the results obtained that the core temperature rises above the platen temperature in the presence of phthalic anhydride. Since no unusual differences in the DSC thermogram of whey-based resin in the presence and in the absence of phthalic anhydride were seen, the exothermic phenomenon being observed under conditions of board preparation deserves further investigation. 

It was observed by Leslie in 1802 that heat was produced when liquid was added to a powder. The heat evolved by the immersion of dry sand in water was described by Pouillet in 1822. This exothermic phenomenon became known in France as the Pouillet effect . Gore (1894) recognized that the amount of heat was related to the surface area of the powder, while Gurvich (1915) suggested that it was also dependent on the polarity of the liquid and the nature of the powder. 

Adsorption calorimetry consists of the coupling of a heat flow calorimeter with a system able to monitor the adsorption of a probe molecule by determining the amount of probe gas that has reacted with the solid under study. It is probably the most direct method for describing in detail both the quantitative and energetic features of surface sites. The adsorption of a probe molecule is an exothermic phenomenon (AHj s < 0), while desorption processes are associated with endothermic peaks (AHdes > 0). Heats of reduction are generally associated with an... 

Reduction of the oxide begins with some difficulty, in the absence of metal nuclei, and this accounts for the slow exothermic phenomenon whose intensity is maximum at the beginning of reduction and which results probably in the formation of metal nuclei on the oxide surface. Since the intensity of the fast exothermic phenomenon increases when the extent of reduction is larger, it must be related to a reduction process now occurring at the metal-oxide interface, carbon monoxide being adsorbed on metal crystallites. All carbon monoxide in dose G is adsorbed on the metal and reacts with nickel oxide at the metal-oxide interface since the slow exothermic phenomenon does not appear on curve G (Fig. 34). Calorimetric curves similar to curve G are obtained during the reaction of subsequent doses of carbon monoxide. Finally, it appears from curves B to G (Fig. 34) that desorption of carbon dioxide is a slower process than the adsorption of carbon monoxide and its interaction with nickel oxide. 

The peak 5 at around 470°C, observed in DSC curve, corresponds to an exothermic phenomenon without any loss of weight. It can thus be attributed to the crystallization of the amorphous zirconia. This is confirmed by the X-ray diffraction pattern obtained after the thermogravimetric analysis (Fig. 9). According to del Monte et al. [17] the peaks located at 20 = 28 and 31.5° are characteristic of the monoclinic zirconia whereas those situated at 20 = 30, 34.5 and 50° belong to the tetragonal structure.This study showed that nanostructured porous zirconia has a low thermal stability. In the preparation of the stable and efficient catalysts for the complete oxidation of aromatics, this low thermal stability will be taken into account. 

The adsorption in a catalytic reaction is an exothermic phenomenon. However, either the adsorption of molecules at the surface or desorption from the surface occurs under different strengths, decreasing the degree of freedom that facilitates the reaction. The energy of activation of a catalytic reaction is, therefore, lower than the energy... 

Since A Gads < 0 and AS < 0, then the enthalpy change of the system is less than zero, and thus the adsorption is an exothermal phenomenon. 

Lipid oxidation is an exothermic phenomenon that can be followed, at least at elevated temperatures, by DSC or (preferably) by isothermal calorimetry [41-44], Measurements can be performed under a static air atmosphere or, better, under oxygen flow or oxygen pressure. In the isothermal mode, induction times can be defined according to published procedures using other techniques (see Fig. 4). Figure 5 compares the oxidative stability at 130°C of three very different oils (safflower, blackcurrant seed, and Nujol). Induction time values can be used... 

Figure 6.3 represents the potential energy W) of a gas molecule at a distance r from the surface of the solid. The molecule being more stable in an adsorbed state than in a free state, point A, which represents the energy level of an adsorbed molecule, is lower than the BC stage, which represents the energy level of the free molecule (far from the srrrface). Point S is at the top of the energy barrier, which the molecule must cross to adsorb or desorb because adsorption is an exothermic phenomenon. 

The condensation of vapor into liquid is always an exothermic phenomenon (Hq ( ) < 0), which is thinkable because intermolecular coimections are stronger and more numerous (by molecule) in a liquid that is denser than vapor. 

These three dehydration steps may easily be linked with the first three endothermic peaks of the differential thermal curve. The small inflections at higher temperatures are attributed to impurities, and the exothermic phenomenon arises from recrystallization. 

Fig. 8A. The exothermic energy release of shocked but unreacted Ni-Al mixtures shows a profound change. A preinitiation" phenomenon in which reaction temper ture is reduced by over 200 °C is caused by the shock process. A compact composer of composite particles that inhibit mixing shows no such effect [88H01].

The curves in Figure 5.2 are typical of exothermic reactions in batch or tubular reactors. The temperature overshoots the wall temperature. This phenomenon is called an exotherm. The exotherm is moderate in Example 5.2 but becomes larger and perhaps uncontrollable upon scaleup. Ways of managing an exotherm during scaleup are discussed in Section 5.3. 

In the case of polymer blends, the fractionated crystallization phenomenon that has been widely reported for many polymer systems can not be attributed to simple size effects. For instance, in Fig. 1, one could argue that the different exotherms originated in the crystallization of different droplet populations that have diverse average diameters. This cannot be the case, since the droplet distribution is monomodal and a smooth variation in heat... 

It is obvious, then, that only the H2—Cl2 reaction can be exploded photo-chemically, that is, at low temperatures. The H2—Br2 and H2—12 systems can support only thermal (high-temperature) explosions. A thermal explosion occurs when a chemical system undergoes an exothermic reaction during which insufficient heat is removed from the system so that the reaction process becomes selfheating. Since the rate of reaction, and hence the rate of heat release, increases exponentially with temperature, the reaction rapidly runs away that is, the system explodes. This phenomenon is the same as that involved in ignition processes and is treated in detail in the chapter on thermal ignition (Chapter 7). 

This vork was also particularly important in that it showed that polymerisation ceased in the absence of ultrasound (Fig. 5.36 at T = 40 °C). Although this phenomenon has not been utilised it does offer the possibility of controlling runaway exothermic reactions. 

Nonstoichiometric mixtures were also concocted from BCB and K-353. All were completely compatible, as evidenced by a single initial Tg, except the BCB/dicyanate blends, which showed small exothermic transitions attributable to crystallization phenomenon. The results from the thermal analyses of these blend systems, in particular the K353/BCB system, lend credence to our belief that curing via Diels-Alder cycloaddition may predominate. As such, the blend systems were more stable toward thermo-oxidative degradation than their pure bisdienophile components. 

The ceiling temperature phenomenon is observed because AH is highly exothermic, while AS is mildly exoentropic. The opposite type of phenomenon occurs in rare instances where AS is endoentropic (AS = +) and AH is very small (either + or —) or zero. Under these conditions, there will be a floor temperature Tf below which polymerization is not possible. This behavior has been observed in only three cases—the polymerizations of cyclic sulfur and selenium octamers and octamethylcyclotetrasiloxane to the corresponding linear polymers (Secs. 7-lla). AH is 9.5, 13.5, and 6.4 kJ mol-1, respectively, and AS is 27, 31, and 190 J K-1 mol-1, respectively [Brandrup et al., 1999 Lee and Johannson, 1966, 1976]. 

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