Absorption or evolution of heat

The thermal efficiency of the process (QE) should be compared with a thermodynamically ideal Carnot cycle, which can be done by comparing the respective indicator diagrams. These show the variation of temperamre, volume and pressure in the combustion chamber during the operating cycle. In the Carnot cycle one mole of gas is subjected to alternate isothermal and adiabatic compression or expansion at two temperatures. By die first law of thermodynamics the isothermal work done on (compression) or by the gas (expansion) is accompanied by the absorption or evolution of heat (Figure 2.2). 

The techniques referred to above (Sects. 1—3) may be operated for a sample heated in a constant temperature environment or under conditions of programmed temperature change. Very similar equipment can often be used differences normally reside in the temperature control of the reactant cell. Non-isothermal measurements of mass loss are termed thermogravimetry (TG), absorption or evolution of heat is differential scanning calorimetry (DSC), and measurement of the temperature difference between the sample and an inert reference substance is termed differential thermal analysis (DTA). These techniques can be used singly [33,76,174] or in combination and may include provision for EGA. Applications of non-isothermal measurements have ranged from the rapid qualitative estimation of reaction temperature to the quantitative determination of kinetic parameters [175—177]. The evaluation of kinetic parameters from non-isothermal data is dealt with in detail in Chap. 3.6. 

In practice, every chemical reaction carried out on a commercial scale involves the transfer of reactants and products of reaction, and the absorption or evolution of heat. Physical design of the reactor depends on the required structure and dimensions of the reactor, which must take into account the temperature and pressure distribution and the rate of chemical reaction. In this chapter, after describing the methods of formulating optimization problems for reactors and the tools for their solution, we will illustrate the techniques involved for several different processes. 

When an adsorbing surface is exposed to a gas or vapour adsorption will take place, being accompanied by the absorption or evolution of heat. Such thermal changes have already been noted in the extension and contraction of surface films of liquids. Although the direct determination of the surface energy of solid surfaces presents many experimental difficulties yet of its existence there is no doubt. On the adsorption of a gas or vapour a diminution in the free surface energy of the system likewise occurs. From the Gibbs-Helmholtz relationship dcr... 

The phenomena of heat require explanation, however, and he expresses himself in favor of the material theory of heat—as an imponderable fluid pervading all space, which condensing in the pores of a substance accounts for the various phenomena of absorption or evolution of heat. The physicists, in fact, were divided for a long time after Lavoisier upon the nature of heat—whether it were a mode of motion or an imponderable fluid. An English writer, Metcalfe, in a two volume work on caloric, 1837, presents the material theory about as strongly as possible. 

SUBLIMATION. The direct transition, under suitable conditions, bet ween the vapor and the solid state of a substance. If solid iodine is placed in a tube and slightly warmed, it vaporizes and the vapor reforms into crystals on the cooler parts of the tube. Many crystalline substances, both metallic and nonmetallic, may be similarly sublimated in a vacuum fairly large crystals of selenium have been thus prepared. The most familiar sublimates are frost and snow, As in the case of other changes of state, sublimation is accompanied by the absorption or evolution of heat, the quantity of which... 

Thus the mixing of two components to form a perfect solution takes place at constant enthalpy. This means that if the components are mixed at constant T and p, no absorption or evolution of heat occurs. For since p is constant, the first law gives, cf. (2.15),... 

Most every chemical reaction is accompanied by the absorption or evolution of heat. Those reactions that absorb heat as they take place are called endothermic reactions. If the source of heat is removed from an endothermic reaction, it stops. Many decomposition reactions are endothermic processes, and for each individual decomposition reaction the amount of heat needed to decompose 1 mole of that specific compound is always the same. For example, it requires 178 kj (kilojoules) of heat energy to decompose 1 mole of CaC03 to CaO and C02. The heat quantity is added to the reactant side of the equation to indicate it is absorbed (consumed) as 1 mole of calcium carbonate decomposes. 

Mass transfer may ocur simultaneously with the transfer of heat, either as a result of an externally imposed temperature difference or because of the absorption or evolution of heat, which generally ocurs when a substance is transferred from one phase to another. In such cases, within one phase, the heat transferred is the result not only of the conduction or convection by virtue of the temperature difference which would happen in the absence of mass transfer, but also includes the sensible heat carried by the diffusing matter. 

When a solute is dissolved in a solvent to form a solution, there is almost always absorption or evolution of heat. According to the principle of Le Chatelier, substances that absorb heat as they dissolve must show an increase in solubility with an increase in temperature. Those which evolve heat upon dissolution must become less soluble at higher temperatures. 

Less useful for degradation studies than TG are differential thermal analysis (DTA) and differential scanning calorimetry (DSC), both of which measure effects due to heat evolution or absorption by the polymer as its temperature is raised. DTA and DSC indicate the temperature regions of occurrence of decomposition processes, but do not distinguish these clearly from physical changes in the sample which also involve absorption or evolution of heat. Product analysis is not possible. 

When a chemical reaction occurs, it is, in general, accompanied by a measurable heat eflfect— absorption or evolution of heat— and the amount of heat absorbed or evolved depends (r) on tbe nature of the reaction (2) on the condition (temperature and physical state) of the reacting substances (3) on the amounts of the substances. When, therefore, the last two factors remain the same, the heat eflfect accompanying a chemical reaction is a constant for that reaction. 

The absorption or evolution of heat and the performance of work require changes in fhe energy of a sysfem and ifs surroundings. When considering the energy of a sysfem, we use fhe concepf of internal energy and how heat and work are related to it. 

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