Kinetic factor

During the application of thermal energy to convert the amorphous film into a crystalline film, the kinetics of competing reactions occurring during annealing affects the formation of the desired crystalline phase and the film microstructure. The formation and stabilization of second crystalline phases, volatilization of elements of low melting point from the film surface, or interdifiusion between the film and the substrate are some of the reactions that can occur. A few examples of these phenomena are shown here. 

Condensed species and trapped solvents [-C-M-0-M-H- + trapped solvents] 

Grained film microstructure with a crystal structure 

Some features of polymerizations are manifested in the kinetic factors and may be stationary (in special cases either living or immortal) or non-stationary. 

Very recently, a paper on immortal polymerization [7] has been published. This is a special (and so far rare) kind of polymerization where active centres 

Non-stationary polymerization are complicated from the kinetic point of view. The changing concentrations of active centres, of monomer and possibly even of further components produce conditions unsuitable for an analysis of the process. Even technical and technological difficulties occur. Nevertheless, these have to be solved as most known coordination and cationic, and a considerable number of anionic, polymerizations are non-stationary. Information on the polymerization mechanisms of the more conventional monomers are summarized in Table 3. 

In a sense, the difficulty of generating five-membered hetarynes could be totally discussed in terms of kinetic factors. If these arynes are of very high energy content due to whatever electronic or geometric effect, then their rate of formation will be slow and other reactions will have the opportunity to intervene before aryne formation can occur. In this section two types of kinetic effects on potential five-membered hetaryne precursors will be discussed, those which stabilize toward aryne formation and those which labilize toward non-aryne reactions. 

The most prominent example of the former situation is the unusual stability of the o-halometal derivatives where the metal is a to the heteroatom of a five-membered heterocycle. Compared to the lability of the benzene analog 32b, which loses LiBr at 100°C to give benzyne, heterocyclic compounds 

TABLE 8. Nonaryne Reactions of Five-Membered Hetaryne Precursors 

Several examples of such processes have been noted in Section III and include transhalogenation to the very stable a-metallated compounds 678 or 679, ring opening, and stepwise elimination to nonaryne intermediates which can rearrange or attack the aryne traps which are present. The lower resonance energy of five-membered heterocycles compared to benzenoid compounds makes them much more susceptible to addition reactions, and hence both cine-substitution and Diels-Alder products have been shown to sometimes arise by addition-elimination and not elimination-addition mechanisms. A summary of all the demonstrated nonaryne reactions of five-membered hetaryne precursors discussed in Section III is collected in Table 8. 

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For liquids that are reasonably fluid around their melting points, the kinetic factors in Eq. DC-15 come out about 10 /cm sec, so that Eq. IX-15 becomes... 

Reversibly fonned micelles have long been of interest as models for enzymes, since tliey provide an amphipatliic environment attractive to many substrates. Substrate binding (non-covalent), saturation kinetics and competitive inliibition are kinetic factors common to botli enzyme reaction mechanism analysis and micellar binding kinetics. 

More recently, studies of the hysteresis of these phase transitions have illuminated the importance of kinetic factors in solid-solid phase transitions [224]. The change between crystal stmctures does not occur at the same point when pressure is increasing, as when it is decreasing the difference between this up-stroke and down-stroke pressure... 

Before we proceed to discuss energy changes in detail it is first necessary to be clear that two factors determine the stability of a chemical system—stability here meaning not undergoing any chemical change. These two factors are the energy factor and the kinetic factor,... 

Prediction of solubility for simple ionic compounds is difficult since we need to know not only values of hydration and lattice enthalpies but also entropy changes on solution before any informed prediction can be given. Even then kinetic factors must be considered. 

We shall take up the kinetics of crystallization in detail in Secs. 4.5 and 4.6. For the present, our only interest is in examining what role kinetic factors play in complicating the crystal-liquid transition. In brief, the story goes like this. Polymers have a great propensity to supercool. If and when they do crystallize, it is an experimental fact that smaller crystal dimensions are obtained the lower the temperature at which the crystallization is carried out. The following considerations supply some additional details ... 

Processes in which solids play a rate-determining role have as their principal kinetic factors the existence of chemical potential gradients, and diffusive mass and heat transfer in materials with rigid structures. The atomic structures of the phases involved in any process and their thermodynamic stabilities have important effects on drese properties, since they result from tire distribution of electrons and ions during tire process. In metallic phases it is the diffusive and thermal capacities of the ion cores which are prevalent, the electrons determining the thermal conduction, whereas it is the ionic charge and the valencies of tire species involved in iron-metallic systems which are important in the diffusive and the electronic behaviour of these solids, especially in the case of variable valency ions, while the ions determine the rate of heat conduction. 

The aldol addition can be carried out under either ofitwo broad sets of conditions, with the product being determined by kinetic factors undenone set of conditions and by thermodynamic factors under the other. To achieve kinetic control, the enolate that is to... 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

If contact with a rough surface is poor, whether as a result of thermodynamic or kinetic factors, voids at the interface are likely to mean that practical adhesion is low. Voids can act as stress concentrators which, especially with a brittle adhesive, lead to low energy dissipation, i/f, and low fracture energy, F. However, it must be recognised that there are circumstances where the stress concentrations resulting from interfacial voids can lead to enhanced plastic deformation of a ductile adhesive and increase fracture energy by an increase in [44]. 

Kinetic factors may be important (we have only compared two equilibrium structure energies without investigating the barrier between them). 

Trends in chemical reactivity are also apparent, e.g. ease of hydrolysis tends to increase from the non-hydrolysing predominantly ionic halides, through the intermediate halides to the readily hydrolysable molecular halides. Reactivity depends both on the relative energies of M-X and M-0 bonds and also, frequently, on kinetic factors which may hinder or even prevent the occurrence of thermodynamically favourable reactions. Further trends become apparent within the various groups of halides and are discussed at appropriate points throughout the text. 

The modes of thermal decomposition of the halates and their complex oxidation-reduction chemistry reflect the interplay of both thermodynamic and kinetic factors. On the one hand, thermodynamically feasible reactions may be sluggish, whilst, on the other, traces of catalyst may radically alter the course of the reaction. In general, for a given cation, thermal stability decreases in the sequence iodate > chlorate > bromate, but the mode and ease of decomposition can be substantially modified. For example, alkali metal chlorates decompose by disproportionation when fused ... 

Interestingly, true hydrides, such as NaH and KH, do not reduce carbonyl groups. Using energies of hydride and methoxide (at left), calculate AH xn for the reduction of formaldehyde by H. Is this reaction more or less favorable than those based on ZH4 Can the low reactivity of NaH and KH be attributed to thermodynamic factors, or must kinetic factors be responsible ... 

Okamoto et al. found that A-oxidation activates 4-halogeno-quinolines in the reaction with piperidine in aqueous alcohol by kinetic factors of 9 to 25, at 100°. This rate-enhancing effect is accompanied by a fairly large decrease in the enthalpy of activation (up to 10 kcal/mole in the chloro compounds), the effect of which is partly offset by a decrease in the entropy of activation. 

A1 is more noble than Ti, and so at room temperature only codeposits and alloys can be obtained. Furthermore, kinetic factors also play a role in the electrodeposition of the element. 

The oxidation of the deposited germanium is also a complicated process we found that mainly chemical oxidation by Gel4 takes place, together with some electrooxidation. It is likely that kinetic factors play a dominant role. 

Environments are considered in detail in Chapter 2, but some examples of the behaviour of normally reactive and non-reactive metals in simple chemical solutions will be considered here to illustrate the fact that corrosion is dependent on the nature of the environment the thermodynamics of the systems and the kinetic factors involved are considered in Sections 1.4 and 1.9. 

Figures 1.27a to d show how the Evans diagram can be used to illustrate how the rate may be controlled by either the polarisation of one or both of the partial reactions (cathodic, anodic or mixed control) constituting corrosion reaction, or by the resistivity of the solution or films on the metal surface (resistance control). Figures 1. lie and/illustrate how kinetic factors may be more significant than the thermodynamic tendency ( , u) and how provides no information on the corrosion rate.

Equations 1.83 and 1.84, or the equations derived from them (1.85 to 1.89), may be used to calculate and E an., providing the various parameters involved are known. The equations also serve to illustrate how and corr, depend upon a thermodynamic factor ( r,ceii. °r r.c and E, ) and the kinetic factors a and / o for each of the half reactions that constitute the corrosion reaction. 

The rate of a chemical reaction is influenced by pressure, temperature, concentration of reactants, kinetic factors such as agitation, and the presence of a catalyst. Since the viability of a plant depends not only on reaction efficiencies but also on the capital cost factor and the cost of maintenance, it may be more economic to alter a process variable in order that a less expensive material of construction can be used. The flexibility which the process designer has in this respect depends on how sensitive the reaction efficiency is to a change in the variable of concern to the materials engineer. 

Principles The reduction reaction is controlled essentially by the usual kinetic factors such as concentration of reactants, temperature, agitation, catalysts, etc. Where the reaction is vigorous, as, for example, when a powerful reducing agent like hydrazine is used, wasteful precipitation of A/, may occur throughout the whole plating solution followed by deposition on all exposed metallic and non-metallic surfaces which can provide favourable nucleation sites. In order to restrict deposition and aid adhesion, the selected areas are pre-sensitised after cleaning the sensitisers used are often based on noble metal salts. 

The usual kinetic factors govern reaction and therefore plating rates. 

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