Reaction, exo- and endothermic

The potential energy profile and also the location of saddle point appears different for exo- and endothermic reactions. In general, the saddle point is located in the entry channel for an exothermic reaction as shown in Fig. 9.16. For example, reaction F + H2 —> FH + H. The shift in location of the saddle point from symmetric position has also been related to the magnitude of the exo (endo) thermicity. 

Coriolis coupling (p. 906 and 912) critical points (p. 888) cross section (p. 901) curvature coupling (p. 906 and 914) cycloaddition reaction (p. 944) democratic coordinates (p. 898) diabatic and adiabatic states (p. 949) donating mode (p. 914) early and late reaction barriers (p. 895) electrophilic attack (p. 938) entrance and exit channels (p. 895) exo- and endothermic reactions (p. 909) femtosecond spectroscopy (p. 889) Franck-Condon factors (p. 962) intrinsic reaction coordinate (IRC) (p. 902) inverse Marcus region (p. 954) mass-weighted coordinates (p. 903)... 

Chemical transformations can be performed in a gas, liquid, or solid phase, but, with good reasons, the majority of such reactions is carried out in the liquid phase in solution. At the macroscopic level, a liquid is the ideal medium to transport heat to and from exo- and endothermic reactions. From the molecular-microscopic point of view, solvents break the crystal lattice of solid reactants, dissolve gaseous or liquid reactants, and they may exert a considerable influence over reaction rates and the positions of chemical equilibria. Because of nonspecific and specific intermolecular forces acting between the ions or molecules of dissolved reactants, activated complexes as well as produets and solvent molecules (leading to differential solvation of all solutes), the rates, equilibria, and the selectivity of chemical reactions can be strongly influenced by the solvent. Other than the fact that the liquid medium should dissolve the reactants and should be easily separated from the reaction products afterwards, the solvent can have a decisive influence on the outcome (i.e., yield and product distribution) of the chemical reaction under study. Therefore, whenever a chemist wishes to perform a certain chemical reaction, she/he has to take into account not only suitable reaction partners and their concentrations, the proper reaction vessel, the appropriate reaction temperature, and, if necessary, the selection of flic right reaction catalyst but also, if the planned reaction is to be successful, flic selection of an appropriate solvent or solvent mixture. 

The potential benefits of foam-based catal ts have been adequately demonstrated. The most important attributes are decreased diffusion limitations, lower pressure drop, increased heat transfer, improved mixing, and prefabrication of i dal shapes. Ideal processes are highly exo- and endothermic reactions and those requuing good selectivity control. Other novel applications, e.g. in trickle bed reactors, will no doubt appear. 

As expected, heat exchanged per unit of volume in the Shimtec reactor is better than the one in batch reactors (15-200 times higher) and operation periods are much smaller than in a semibatch reactor. These characteristics allow the implementation of exo- or endothermic reactions at extreme operating temperatures or concentrations while reducing needs in purifying and separating processes and thus in raw materials. Indeed, since supply or removal of heat is enhanced, semibatch mode or dilutions become useless and therefore, there is an increase in selectivity and yield. 

If the AH listed for a reaction is negative, then that reaction releases heat as it proceeds — the reaction is exothermic (exo- = out). If the A//listed for the reaction is positive, then that reaction absorbs heat as it proceeds — the reaction is endothermic (endo- = in). In other words, exothermic reactions release heat as a product, and endothermic reactions consume heat as a reactant. 

Figure 17.23. Representative temperature profiles in reaction systems (see also Figs. 17.20, 17.21(d), 17.22(d), 17.30(c), 17.34, and 17.35). (a) A jacketed tubular reactor, (b) Burner and reactor for high temperature pyrolysis of hydrocarbons (Ullmann, 1973, Vol. 3, p. 355) (c) A catalytic reactor system in which the feed is preheated to starting temperature and product is properly adjusted exo- and endothermic profiles, (d) Reactor with built-in heat exchange between feed and product and with external temperature adjustment exo- and endothermic profiles.

Recycle reactors are often able to cope with fast, complex, exo- or endothermic reactions, and are frequently used in research practice. A great number of different constructions (with external and internal recycling, with stationary and movable catalyst) are described in the literature [32],... 

Distillation columns with multiple conventional side reactors were first suggested by Schoenmakers and Buehler German Chem. Eng., 5, 292 (1982)] and have the potential to accommodate gas-phase reactions, highly exo- or endothermic reactions, catalyst deactivation, and operating conditions ontside the normal range snitable for distillation (e.g., short contact times, high temperatnre and pressure, etc). Krishna (chap. 7 in Sundmacher and Kienle, eds.. Reactive Distillation, Wiley-VCH, 2003). 

Industrially, selectivity is often as important as conversion in considering the efficiency of the reactor. In isothermal reactions, the dilute phase and transition zone may cause better selectivity due to better contact in that region. But in nonisothermal reactions, the effect will be different because of the temperature effect. Mixing of gas and solids in the dilute phase is not sufficient, and this may cause a temperature distribution for exo- or endothermic reactions. 

Among the main usual reasons for implementing microstructured reactors, improved heat management and control of residence time distributions are considered for fast and highly exo- or endothermic reactions. 

For strong exo- or endothermic reactions it is often difficult to operate the reactor isothermally. This makes modeling of the laboratory-type reactor more difficult. To keep the temperature gradients in the reactor as low as possible, the diameter of the reactor has to be minimized and the flow rate in the reactor maximized to achieve good heat transfer. 

Fixed bed reactors are the most common reactors for the study of gas-phase heterogeneous catalytic reactions. They are easy to use and catalysts in powder form can be readily used (usually after sieving the proper particle size fraction). Semi-empirical correlations are used to describe the physical phenomena occurring in these types of reactors [21,22]. However, in the case of very last and for exo- or endothermic reactions, it becomes difficult to measure the intrinsic kinetics with these reactors. To avoid external heat and mass transfer limitations as well as internal diffusion limitations, high flow rates and small particles are necessary. This quickly will lead to an excessive pressure drop over the reactor. 

The TAP reactor is very well suited for kinetic studies. At low pressure, all transport of gas-phase species is by (Knudsen) diffusion, thus ruling out any external mass transfer limitations. The diffusion as a random movement also eliminates all radial concentration gradients. Very low amounts of reactants are pulsed into the reactor, which are on the order of a few nanomoles. Thus, the amount of heat generated is very small even in the case of strongly exo- or endothermic reactions. Therefore, the reactor is operated isothermally and no heat transfer limitations occur. Concentration profiles inside the pores for transient experiments might arise even in the absence of chemical reaction. If significant diffusion of reactants and products inside the catalyst pores occurs, it will be revealed by the transient response and then needs to be addressed correctly by a modeling approach. This is often the case for microporous materials [26,27,72]. 

The number of phenomena which can be directly studied by thermal analysis (DSC (DTA), TG, TMA, DMTA, TOA and DETA) is impressive. Typical of these methods is that only small amounts of sample (a few milligrams) are required for the analysis. Calorimetric methods record exo- and endothermic processes, e.g. melting, crystallization, liquid-crystalline phase transitions, and chemical reactions, e.g. polymerization, curing, depolymerization and degradation. Second-order transitions, e.g. glass transitions, are readily revealed by the calorimetric methods. Thermodynamic quantities, e.g. specific heat, are sensitively determined. TG is a valuable tool for the determination of the content of volatile species and fillers in polymeric materials and also for studies of polymer degradation. The majority of the aforementioned physical transitions can also be monitored by TMA (dilatometry). DMTA and DETA... 

Mass spectrometric studies of the ionic species which arrive at the cathode of both glow and corona discharges yield useful information regarding ion-molecule reactions which occur within these systems. Glow discharges have been used to study endothermic reactions, and their usefulness and limitations have been demonstrated by studies of the dissociative charge transfer reactions Ar+ + N2 N+ + N + Ar N2+ + N2 N+ + N + N2 N2+ + 02 0+ + O + N2. Exo-... 

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