Endothermic transformations

The barium sulfate in lithopone can be identified thermoanalytically by a reversible endothermic transformation at 1150°C. Both Sachtolith and lithopone are thermally stable up to ca. 550 °C in the presence of air. Due to their low Mohs hardness, they are less abrasive than other white pigments. Barium sulfate is practically inert toward acids, bases, and organic solvents. Zinc sulfide is stable in aqueous media between pH 4 and 10, and is largely inert toward organic media. In the presence of water and oxygen, it can be oxidatively decomposed by the action of UV radiation. 

If the photodriven half-reactions are physically separated, large scale ground state-endothermic transformations can be observed in which visible light absorption by the anion drives the electron... 

In a number of other cases, however, notably in the exothermic decompositions of solids which can become explosive and in the endothermic transformation of hydrates of salts and of carbonates (to oxides), the slow processes seem to be nucleation. The rate laws for such nucleation-con-trolled processes can be very complex, and the rate studies are difficult to make and to reproduce. In many of these cases smj amounts of impurities play an important role in governing the de fed t nff and growth of nuclei, and there are a number of instances of sm bA ounts of water vapor having a significant catalytic effect. ... 

Early transition states are frequently typical of fast, exothermic reactions in which there is no substantial bond making or breaking. Hence, early transition states resemble starting materials in structure. Late transition states are frequently typical of relatively slow, endothermic transformations. There is substantial bond formation in late transition states consequently, late transition states resemble products. 

Scheme 2.2 Reaction profile for exothermic and endothermic transformations.

Other important endothermic transformations are those involving C02 (reactions 10 and 11). The Boudouard reaction occurs between carbon and carbon dioxide to increase the yield of carbon monoxide (reaction 10). Finally, some methanation reactions by hydrogenation of carbon oxides also take place (reactions 12 and 13), which may lead to a significant decrease in the H2 concentration of the final synthesis gas. The overall energy requirement of gasification can be balanced by a suitable combination of exothermic and endothermic reactions, mainly through the control of the 02/H20 ratio in the reaction medium. 

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. 

I. Cryolite—563.7 °C endotherm, transforms from monocline to cubic. 

The activation energy, corresponding to the thermal decomposition, is characterised by negative values, which are specific for the endothermal transformations, being of about 1.8 times higher for the stressed sample at 14.3 MPa as compared to the unstressed one. Table 3.112. In all cases to reaction order is about equal to 1. 

Usually, transformations involve heat exchanges with the outside environment, which may be due to heat being released (exothermic transformations) or absorbed (in the case of endothermic transformations). We shall express those heat exchanges in two important cases. 

Figure 3-8 Potential-energy diagrams (left) the reaction of a fluorine atom with CH4, an exothermic process with an early transition state and (right) the reaction of an iodine atom with CH4, an endothermic transformation with a late transition state. Both are thus in accord with the Hammond postulate.

If dh/dt eorresponds to an absorbed thermal power due to an endothermic transformation or reaction, the dh/dt value is positive. 

The energy of the triplet state of the sensitizer ( Sens ) must be greater than that of the reactant. If this condition is not met, the energy transfer becomes endothermic and cannot compete with other transformations of Sens. ... 

The irradiation of 3-carbomethoxyisoxazole (47) gave the corresponding oxazole (48) in very low yields (5-8%) without the isolation of the corresponding azirine (Scheme 22) [71JCS(C)1196]. Also in this case calculations show that the energy of the triplet state allows the formation of the biradical intermediate and then of the azirine. However, the low yields of the conversion can be explained considering that the transformation of the biradical intermediate into the azirine is an endothermic reaction (Fig. 10) [99H(50)1115]. 

In the case of monotropic behavior, the isotropiza-tion endotherm and the corresponding thermodynamic parameters for the mesophase-isotropic transition can be obtained by isolating the mesophase when cooling from the melt and holding the temperature in a region where the transformation into the crystal is very slow... 

Focusing attention on PTEB, it has been found that, similar to the case of PDTMB, the mesophase experiences a very slow transformation into the crystal. Thus, only the isotropization is observed in a sample freshly cooled from the melt [27]. However, after a long time at room temperature, the transformation mesophase-crystal is produced, owing to a glass transition temperature of about 14°C. Moreover, several endotherms were obtained before the final isotropization for a sample of PTEB annealed at 85°C for 12 days, i.e., PTEB shows enantiotropic behavior. The different endotherms may arise from polymorphism or melting-recrystallization phenomena [30]. 

An endothermic compound, which is hardiy stable thermodynamically. However, it only decomposes under extreme conditions (very violent shock, high temperature). When it is involved in a reaction, this compound can combust or detonate easily because of the exothermicity of the transformation. Since it is hardly ever handled in the pure state (it forms when compounds containing arsenic are handled) it gives rise to a limited number of accidents. 

CO2 in Figure 225(c) induces also non-equilibrium state and enhances CO2 production, then H2 productivity and purity are also enhanced. These separation processes would realize not only high-yield of H2, but also decrease of temperature of the endothermic reforming. It means that the separation process is important methodology for energy media transformation and chemical energy conversion. 

The oxidative coupling of methane has been studied by several authors. The most elusive transformation has been the oxidative coupling of methane into C2 hydrocarbons (ethene, ethane), because the reaction is more endothermic than other transformations [2]. The application of fast and efficient microwave heating to endothermic reactions is particular interest. 

Although there are other ways, one of the most convenient and rapid ways to measure AH is by differential scanning calorimetry. When the temperature is reached at which a phase transition occurs, heat is absorbed, so more heat must flow to the sample in order to keep the temperature equal to that of the reference. This produces a peak in the endothermic direction. If the transition is readily reversible, cooling the sample will result in heat being liberated as the sample is transformed into the original phase, and a peak in the exothermic direction will be observed. The area of the peak is proportional to the enthalpy change for transformation of the sample into the new phase. Before the sample is completely transformed into the new phase, the fraction transformed at a specific temperature can be determined by comparing the partial peak area up to that temperature to the total area. That fraction, a, determined as a function of temperature can be used as the variable for kinetic analysis of the transformation. 

It is seen that this difference ranges from 137 to 204 kJ mol-1, which is very significant. The high chemical reactivity of free atoms and radicals in various chemical reactions is one of the reasons for which the radical chain reactions occur much more rapidly than the direct molecular transformation of reactants into products. Although the radical formation is an endothermic reaction but, when appearing in the system, radicals rapidly enter into the reaction, and each radical induces the chain of transformations. 

Any transformation of material accompanied by a change of enthalpy which may be endothermic or exothermic. 

---