Thermodynamic and Kinetic Considerations

Unfortunately thermodynamics, for the most part, is made confusing and very abstract. In reality, thermodynamics, while not being the easiest of subjects, is not as difficult as generally perceived. As somebody once noted, some people use thermodynamics as a drunk uses a lamppost — not so much for illumination as for support. The purpose of this chapter is to dispel some of the mystery surrounding thermodynamics and hopefully illuminate and expose some of its beauty. It should be emphasized, however, that one chapter cannot, by any stretch of the imagination, cover a subject as complex and subtle as thermodynamics. This chapter, as noted in the Preface, is included more for the sake of completion and a reminder of what the reader should already be familiar with, than an attempt to cover the subject in any but a cursory manner. 

This chapter is structured as follows. In the next three subsections, enthalpy, entropy, and free energy are defined and explained. Sec. 5.3 deals with the conditions of equilibrium and the corresponding mass action expression. The chemical stability of ceramics is discussed in Sec. 5.4. In Sec. 5.5 the concept of electrochemical potentials is presented, which is followed by the closely related notion of charged interfaces and Debye length. In Sec. 5.7 

These studies taken together show that, from a thermodynamic perspective, three factors can be utilized to predict the bond dissociation energy of a homobenzylic ether radical cation (BDE(RC)) and thereby its propensity to fragment (1) the bond dissociation energy of the benzylic carbon-carbon bond in the neutral substrate (BDE(S)), (2) the oxidation potential of the substrate (Ep (S)), and (3) the oxidation 

RADICAL CATION FRAGMENTATION REACTIONS IN ORGANIC SYNTHESIS 

This work and related studies provided the basis for the stereoelectronic model for homobenzylic ether cleavage in Fig. 3.1. This model includes overlap of the benzylic carbon-carbon bond with the SOMO of the aromatic ring (structure 13), thereby stabilizing the benzylic radical upon cleavage. Additionally, overlap of a heteroatom lone pair and the benzylic o orbital was shown to be necessary for cleavage (structure 14). 

EIGURE 3.1 Preferred conformation for radical cation carbon-carbon bond cleavage. 

The stability of aromatics in comparison with other hydrocarbons increases with temperature (Fig. XI), so operations are conducted above 300°C However, the kinetic competition between the desired and side reactions gives rise to the selection of specific operating conditions for the required conversions  

These considerations are illustrated by thermodynamic calculations, of which some results are given in Table 221. 

In theory, favorable conditions correspond to a pressure of 0.1. 10 Pa and temperatures not exceeding 350 0 However, cracking reactions (coke formation) are excessive in this case, and the selectivity of the operation is reduced. Hence the reactions producing aromatics must be activated selectively, and operations conducted at a sufficiently high partial pressure of hydrogen. 

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For all three halates (in the absence of disproportionation) the preferred mode of decomposition depends, again, on both thermodynamic and kinetic considerations. Oxide formation tends to be favoured by the presence of a strongly polarizing cation (e.g. magnesium, transition-metal and lanthanide halates), whereas halide formation is observed for alkali-metal, alkaline- earth and silver halates. 

The effects of concentration, velocity and temperature are complex and it will become evident that these factors can frequently outweigh the thermodynamic and kinetic considerations detailed in Section 1.4. Thus it has been demonstrated in Chapter 1 that an increase in hydrogen ion concentration will raise the redox potential of the aqueous solution with a consequent increase in rate. On the other hand, an increase in the rate of the cathodic process may cause a decrease in rate when the metal shows an active/passive transition. However, in complex environmental situations these considerations do not always apply, particularly when the metals are subjected to certain conditions of high velocity and temperature. 

The following section deals with the crystallization and interconversion of polymorphic forms of polymers, presenting some thermodynamic and kinetic considerations together with a description of some experimental conditions for the occurrence of solid-solid phase transitions. 

Crystallizations and Interconversions of Polymorphic F orms 3.1 Thermodynamic and Kinetic Considerations... 

In the previous sections, it was shown how thermodynamic and kinetic considerations govern a CVD reaction. In this section, the nature of the deposit, i.e., its microstructure and how it is controlled by the deposition conditions, is examined. 

A more sophisticated understanding is linked to an appreciation of the interaction of thermodynamic and kinetic considerations and is likely to be dependent upon the ability to visualise some form of mental model involving molecular collisions and interactions (Gilbert, 2005). This allows the student to see that two reactions are occurring simultaneously ... 

Most of the applications so far focus on the production of the chiral amino acid as the end product. Conversion of the chiral amino acid into the prochiral oxoacid as the end product is less common, although, for instance, Odman etal describe the use of GDH to convert L-glutamate into the higher-value 2-oxoglutarate. Similarly, Findrik et al describe in some detail the kinetics of quantitative conversion of L-methionine into 2-oxo-4-methylthiobutyric acid. In view of the relatively unfavorable equilibrium for amino acid oxidation, thermodynamic and kinetic considerations have to be carefully balanced. A high pH favors oxidative deamination, and fortunately also the PheDH has an unusually high pH optimum, above 10. However, this in itself will not secure... 

The suitability of using solvent extraction for a given separation is determined by thermodynamic and kinetic considerations. The main thermodynamic parameter is the solute distribution ratio, Dyi, between the organic and the aqueous phase. This is given by [Eq. (4.3), Chapter 4] ... 

Whether a particular monomer can be converted to polymer depends on both thermodynamic and kinetic considerations. The polymerization will be impossible under any and all reaction conditions if it does not pass the test of thermodynamic feasibility. Polymerization is possible only if the free-energy difference AG between monomer and polymer is negative (Sec. 3-9b). A negative AG does not, however, mean that polymerization will be observed under a particular set of reaction conditions (type of initiation, temperature, etc.). The ability to carry out a thermodynamically feasible polymerization depends on its kinetic feasibility—on whether the process proceeds at a reasonable rate under a proposed set of reaction conditions. Thus, whereas the polymerization of a wide variety of unsaturated monomers is thermodynamically feasible, very specific reaction conditions are often required to achieve kinetic feasibility in order to accomplish a particular polymerization. 

An investigation of both thermodynamic and kinetic considerations is necessary in the understanding of atomization in graphite atomizers. 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

Before we proceed, however, it is important to review briefly the roles thermodynamic and kinetic considerations play in determining the structure. In some cases, the distinction is easy to establish. In the case of the association colloids we discussed in Chapter 8, thermodynamics determined the formation and the structure of the colloidal particles and their subsequent transformations to more complex structures at higher concentrations of the particles. In... 

In this chapter we try to classify the more important types of reactions encountered in inorganic chemistry, and describe some of their mechanisms. The emphasis is placed upon the principles which determine the stability or instability, existence and nonexistence of inorganic substances from the viewpoint of the ease or otherwise of preparing a compound, and the tendency a compound - once prepared - may have to react spontaneously to give other products. Both thermodynamic and kinetic considerations are obviously involved here. The division of material between this chapter and the next has not been easy, and there is inevitably a good deal of overlap. Coupling reactions, which might have deserved a section in this chapter, are discussed in Sections 10.5 and 10.6. 

The intrinsic properties of the lanthanide elements guarantee promising applications in the fields of catalysis and material science. How to cope with these features on a molecular level and under anaerobic conditions is discussed, including thermodynamic and kinetic considerations. Due to the importance of a prolific metal/ligand synergism, current developments in ligand design are emphasized. 

The comparison of energy conversions in linear and nonlinear regions requires a combination of thermodynamic and kinetic considerations to express the exergetic efficiencies of nonlinear 17n and linear 77 modes... 

Fung, K.Z. et al., Thermodynamic and kinetic considerations for BijOj-based electrolytes, Solid State Ionics, 52, 199-211 (1992). 

Nesbitt H. W. and Young G. M. (1984) Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations. Geochim. Cosmochim. Acta 48, 1523-1534. 

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