Exothermic reactions Definition

The term lime also has a broad coimotation and frequently is used in referring to limestone. According to precise definition, lime can only be a burned form quicklime, hydrated lime, or hydraiflic lime. These products are oxides or hydroxides of calcium and magnesium, except hydraiflic types in which the CaO and MgO are chemically combined with impurities. The oxide is converted to a hydroxide by slaking, an exothermic reaction in which the water combines chemically with the lime. These reversible reactions for both high calcium and dolomitic types are Quicklime... 

The arguments advanced in Sect. 3.2.3 apply equally well to a continuous stirred tank reactor. With a reversible exothermic reaction and a fixed mean residence time, t, there is an optimum temperature for operation of a continuous stirred tank reactor. Since the conditions in an ideal stirred tank are, by definition, uniform, there is no opportunity to employ a temperature gradient, as with the plug-flow reactor, to achieve an even better performance. 

Exothermic Reaction. A reaction in which heat is liberated. It usually proceeds rapidly sometimes explosively. In an endothermic reaction a chemical change proceeds slowly with absorption of a definite number of calories Ref Hackh s (1944), 328-R (Exothermic) 305 R (Endothermic)... 

Erdey-Gruz, 1048, 1306 1474 Erschler, 1133, 1134, 1425 Ethylene oxidation, anodic, 1052 1258 Exchange current density, 1049, 1066 correction of, 1069 definition, 1053 electrocatalysis and, 1278 impedance and, 1136 interfacial reaction, 1047 and partly polarizable interface, 1056 Excited states, lifetime, 1478 Exothermic reaction, 1041 Ex situ techniques, 785, 788, 1146... 

This definition excludes those exothermic reactions which shown and increase in rate with time (like explosions caused by the rapidly rising temperature. 

A combustion process is an exothermic reaction initiated by a source of ignition that produces more energy than it consumes. The speed at which the reaction proceeds through the mixture of reactants depends on the concentration of the flammable gas or vapor. This speed is lower at higher ( rich ) as well as a lower ( lean ) concentrations of the flammable gas than at the stoichiometric mixture. There are lower and upper limits beyond which the reaction cannot propagate through the gas mixture on its own. Some definitions follow ... 

Heat accompanying a chemical change. Thermochemistry, exothermic reaction, endothermic reaction. Definition of heat of reaction. Heat content. Heat of formation. Heat of combustion. Heat values of foods. Heat of neutralization. Heats of formation and relative electronegativity of atoms. The production of high temperatures and low temperatures. 

The definition for an exothermic reaction is one where energy is given off. An endothermic reaction is where energy is required. 

Early studies of acids and bases were restricted to aqueous solutions and were made with an inadequate understanding of the peculiarities of water. The compounds were defined as substances which dissociated in water, the acids to give free hydrogen ions and the bases free hydroxide ions (Arrhenius, Ostwald, 1887). This definition does not express present views about aqueous solutions and cannot be applied to solutions in other solvents. Thermodynamics shows that free H+ ions cannot exist in appreciable concentration in water itself, and spectroscopy that the hydration of the proton is a strongly exothermic reaction. Accordingly the dissociation of an acid in water leads to often represented by H3O+ and referred to as the... 

Both the routes mentioned earlier pertain to cell metabolism, which broadly speaking cormotes any biologically inspired reaction sequence. Most usually, however, the term applies to oxidative or exothermic reaction sequences involving energy sources such as glucose or glycogen, and the expression of primary metabolism is more definitive, as distinguished from secondary metabolism. 

Catalyst supports such as silica and alumina have low thermal conductivities so that temperature gradients within catalyst particles are likely in all but the finely ground powders used for infrinsic kinetic studies. There may also be a film resisfance fo heaf fransfer af fhe exfemal surface of the catalyst. Thus the internal temperatures in a catalyst pellet may be substantially different than the bulk gas temperature. The definition of the effectiveness factor, Equation 10.23, is unchanged, but an exothermic reaction can have reaction rates inside the pellet that are higher than would be predicted using the bulk gas temperature. In the absence of a diffusion limitation, rj > 1 would be expected for an exothermic reaction. (The case > 1 is also possible for some isothermal reactions with weird kinetics.) Mass transfer limitations may have a larger... 

The relative importance of vibrational and translational energy in promoting chemical reactions is of both theoretical and practical interest. In reactions of diatomic molecules with atoms it has been substantiated both experimentally and theoretically that for endothermic reactions vibrational energy is more important, while for exothermic reactions the opposite is true. For polyatomic molecules, however, there is insufficient experimental and theoretical evidence to draw conclusions. The major work on laser-excited polyatomic reactions has involved the vibrational excitation of ozone in its exothermic reaction with nitric oxide. Although the vibrational energy increased the reaction rate, comparison with statistical models and the temperature dependence of the thermal reaction indicate about equal importance for vibrational and translational energy. On the other hand, a molecular beam study of the temperature dependence of the reaction of potassium with sulfur hexafluoride" has shown a definite preference for vibrational energy of the SF. ... 

From the definition of the periodic steady state it follows, that on one side the oxygen removed from the solid by the reduction must be replaced during the oxidation step. On the other side the heat released by the exothermic reaction must be used by the endothermic reaction or leave the reactor by convection. Since equation 8 is a combination of heat and mass balance and temperature is a measure for heat it can be used for the... 

The reactants are mixed at time t seconds and rapidly stirred so that reaction is complete within 30 s of mixing. As the reaction mixture can never be truly insulated, the container and its contents immediately start to lose heat (points C to D in an exothermic reaction. Fig. 13.5(a)) or gain heat (points E to F in an endothermic reaction. Fig. 13.5(b)). The temperature of the mixture changes too rapidly for a single definitive measurement to be taken immediately after mixing. Instead, tempera-... 

With other very exothermic reactions, such as air oxidation of aromatic hydrocarbons, the number of beds would have to be uneconomically large to limit the temperature increase per bed, so that the multitubular reactor is definitely preferred Cooling the reactor with. he incoming reactant would be insufficient, however, and require too much heat exchanging surface. Such reactors are therefore cooled by means of circulating molten salts which in turn give off their heat to a boiler. The phthalic anhydride synthesis reactor shown in Fig. 11.3-6 [6] may contain up to 10,000 tubes of 2.5 cm inside diameter. The tube diameter has to limited to such a small value to avoid excessive overtemperatures on the axis, a feature that is discussed later in this chapter. 

During paste preparation, exothermic reactions proceed which cause the temperature in the paste mixer to rise. In order to obtain reproducibly a paste with definite phase composition and crystal morphology, which would ensure high and stable battery performance, the temperature in the mixer should be monitored and controlled continuously throughout the process of paste preparation. 

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