Exothermic transformation

To confirm that the matrix is amorphous following primary solidification, isothermal dsc experiments can be performed. The character of the isothermal transformation kinetics makes it possible to distinguish a microcrystalline stmcture from an amorphous stmcture assuming that the rate of heat released, dH/dt in an exothermic transformation is proportional to the transformation rate, dxjdt where H is the enthalpy and x(t) is the transformed volume fraction at time t. If microcrystals do exist in a grain growth process, the isothermal calorimetric signal dUldt s proportional to, where ris... 

Materials or substances, which, by themselves, are unstable and easily cause a violent chemical reaction without causing an explosion. This includes all substances that can easily engage in very exothermic transformations at high temperature and pressure. It also includes all substances that react violently with water or that can produce mixtures potentially explosive to water. 

The rearrangements 67 —> 70, 71 —> 72 and 74 —> 75 include the transformation of conjugated dienes to cumulenes. Nevertheless, these reactions take place with very high yields in some cases, because either an irreversible step of hydrolysis such as 69 —> 70 is involved or the very exothermic transformation from cyanates to isocyanates is used. Comparison of the energies, calculated by ab initio methods [121], shows that, for example, the energy of methyl isocyanate is lower than that of methyl cyanate by 26.8 kcal mol-1 and that of vinyl isocyanate is lower than that of vinyl cyanate by 28.1 kcal mol-1. 

Frank-Kamenetskii first considered the nonsteady heat conduction equation. However, since the gaseous mixture, liquid, or solid energetic fuel can undergo exothermic transformations, a chemical reaction rate term is included. This term specifies a continuously distributed source of heat throughout the containing vessel boundaries. The heat conduction equation for the vessel is then... 

More recent developments exploit the energy content of readily accessible cycloalkynes based on phenyl-alkynyl structural motives, albeit not always with fullerene formation in mind. For example, the strained dehydrobenzoannulene 4 [19] could be converted by light, heat (145 °C), or pressure (20000 psi) in a topochemical polymerization reaction typical for butadiynes to a deeply coloured polymer. A similar thermochemical behaviour (strongly exothermic transformation around 200 °C) was observed for compounds 5 and 6 [20]. However, none of the systems 4-6 shows any tendency to produce spherical forms of carbon under the conditions investigated. 

The crystallization process is a highly exothermic transformation (or glow phenomenon ) that normally occurs at remarkably low temperatures (350-400°C) this... 

Orthorhombic Q -Se2Br2 is thermodynamically stable compared to the monoclinic jS-Se2Br2 modification. Monofropic exothermic transformation of the metastable into the stable form occurs in the range —40 to —5 °C. 

The enthalpy values of dehydriding AIH3 have been determined. The a-phase dehydriding reaction was found to be endothermic with 6.0 1.5kJ (molH2) consistent with values reported elsewhere of 7.6kJ (molH2) [19]. Similar to early reports, the y-phase showed exothermic transformation to the a-phase followed by dehydriding to aluminum with a net endothermic heat of reaction of 1.0 0.5kJ (molH2). The P-phase, however, was found to dehydride directly to aluminum, which is a different result from results reported elsewhere [15, 16, 19]. 

At temperatures attainable in the laboratory we may observe, as has been, said, the exothermic transformation of liquid white phosphorus into the red solid form, but not the inverse transformation the region where the observations are made is, therefore, according to Moutier s rule, to the left of the curve of tensions of transformation from red phosphorus into liquid white phosphorus if the transformation of red into liquid white phosphorus is possible, it is at temperatures much higher than those realizable in our laboratories. 

As the content of nonfatty materials in sunflower seeds decreases as the oil content increases, the moisture content corresponding to one critical moisture value is inversely related to the oil content, as shown in Table 16 [based on Muller (50)]. For critical moisture levels above 15%, the rate of respiration of seeds increases. Respiration is accompanied by an exothermic transformation of organic substances of seeds, creating conditions that may lead to spontaneous combustion (50). 

Thermal analysis of the unsupported acid shows two endothermic (347 and 504 K) and one exothermic transformations, the latter proceeds at 812 K without mass loss (Figure 2). Transformations are expressed by eqs (l)-(3) [10]... 

The exothermic effects associated with some dehydrations [18] may result from delayed reciystallization, or the oxidation of a product of dehydration. Dehydration of magnesium carbonate trihydrate, followed by heating in carbon dioxide, resulted in the exothermic transformation of an anhydrous pseudomorph of the reactant into magnesite, MgCOj. 

The DTA curves for both ot- and P-picoline (pi) platinum(Il) coordination compounds, -[PtlpOjCy, showed [36] exothermic peaks at 478 K which are caused by isomerization to the more stable trms isomer. At higher temperatures the organic ligand was released to yield RClj. The pyridine (py) compound cis-[Pt(py)2BrJ behaves similarly undergoing an exothermic transformation to the tram form at 528 K, prior to dissociation at 553 K. 

In 1805, Gough studied the elasticity of natural rubber and proposed two laws which were confirmed by Joule in 1859 (1) a piece of rubber, stretched under constant constraint, contracts in a reversible way when its temperature increases (2) the stretching of rubber is a reversible exothermic transformation. 

The clinker heat of formation only depends on the physical-chemical properties of the raw meal fed to the kiln. For such calculation, it is conventional to use the isothermal analytical method, which consists in quantifying step-by-step the heat required by each chemical and physical transformation of the material during the burning process. The reference temperature to which all heat quantities are referred is 273.16 K. The final heat of reaction is obtained subtracting from the total heat required for endothermic transformations the heat released by the exothermic transformations. 

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