Vaporization endothermic nature

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

Cinnabar is a natural mercury(II) sulfide, HgS, found near volcanic rocks and hot springs. It is the only important source of mercury, which has many uses, including dental amalgams, thermometers, and mercury vapor lamps. Mercury is formed in the following endothermic reaction when mercury(II) sulfide is heated. Consider a system in which a sample of HgS in a closed container is heated with a Bunsen burner flame. 

Thermal reactions can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

DSC is often used in conjunction with TA to determine if a reaction is endothermic, such as melting, vaporization and sublimation, or exothermic, such as oxidative degradation. It is also used to determine the glass transition temperature of polymers. Liquids and solids can be analyzed by both methods of thermal analysis. The sample size is usually limited to 10-20 mg. Thermal analysis can be used to characterize the physical and chemical properties of a system under conditions that simulate real world applications. It is not simply a sample composition technique. Much of the data interpretation is empirical in nature and more than one thermal method may be required to fully understand the chemical and physical reactions occurring in a sample. Condensation of volatile reaction products on the sample support system of a TA can give rise to anomalous weight changes. 

The product composition is controlled mainly by thermodynamics, which favor the formation of methane at lower temperatures of about 623 K, and of hydrogen at higher temperatures of about 1223 K [113]. The required heat for endothermic SR can be provided by the total oxidation of hydrocarbons. An increase in the molar ratio of water vapor to carbon content also causes a decrease in carbon monoxide content following Eq. (15.9), a decrease in methane content, and an increase in carbon monoxide content according to Eq. (15.10). More details about thermodynamic considerations [116] and more information about the elementary steps of the reaction of aliphatic hydrocarbons [113] are available in the literature. The SR process has been well examined, since the SR of natural gas is the dominant process for industrial hydrogen production with technical availability for large-scale application and cost-effectiveness [118]. 

The reactions shown in Eqs. 3a and 3c are endothermic in nature, whereas the water-gas shift (WGS) reaction shown in Eq. 3b is moderately exothermic. If these reactions are carried out externally (external reformaticm), the efficiency and operation of the cell are significantly affected. Hence, internal reforming is preferred however, Ni acts as an excellent coking catalyst. As a consequence, in the presence of carbonaceous fuels (and in the absence of sufficient water vapor), there is always a possibility of deposition of carbon filament on the surface of Ni. The mechanism involves carbon formation on the metal surface followed by dissolution of the carbon into the bulk of the metal and finally precipitation of graphitic carbon at some surface of the metal particles after it becomes supersaturated with carbon [6]. It not only reduces the active sites for reactions mentioned in Eqs. 2c-j and 3a-c but also destroys the whole anode over a period of time. The following three reactions are the most probable catalytic reactions that lead to carbon formation in high-temperature systems ... 

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