Thermodynamic factors

There are difficulties in making such cells practical. High-band-gap semiconductors do not respond to visible light, while low-band-gap ones are prone to photocorrosion [182, 185]. In addition, both photochemical and entropy or thermodynamic factors limit the ideal efficiency with which sunlight can be converted to electrical energy [186]. 

Many different approaches have been suggested as possible approaches to this problem, from the 1960s onwards [Verwer and Leusen 1998]. What is obvious from all of these ellorts is that this is an extremely difficult problem. Both thermodynamics and kinetics can be important in determining which crystalline form is obtained under a certain se1 of experimental conditions. Kinetic effects are particularly difficult to take into accouni and so are usually ignored. A proper treatment of the thermodynamic factors would lequire one to deal with the relative free energies of the different possible polymorphs... 

Before concluding this section, there is one additional thermodynamic factor to be mentioned which also has the effect of lowering. Since we shall not describe the thermodynamics of polymer solutions until Chap. 8, a quantitative treatment is inappropriate at this point. However, some relationships familiar from the behavior of low molecular weight compounds may be borrowed for qualitative discussion. The specific effect we consider is that of chain ends. The position we take is that they are foreign species from the viewpoint of crystallization. 

For the equiUbrium-controUed enzyme-catalyzed peptide synthesis the equiUbrium position Hes far over in the direction of the hydrolysis, and under physiological conditions, the product yield is negligible. The equiUbrium position is deterrnined exclusively by thermodynamic factors and like any other catalysts the enzymes only accelerate the attainment of the equiUbrium. 

The performance of adsorption processes results in general from the combined effects of thermodynamic and rate factors. It is convenient to consider first thermodynamic factors. These determine the process performance in a limit where the system behaves ideally i.e. without mass transfer and Idnetic limitations and with the fluid phase in perfect... 

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... 

This is our principal result for the rate of desorption from an adsorbate that remains in quasi-equihbrium throughout desorption. Noteworthy is the clear separation into a dynamic factor, the sticking coefficient S 6, T), and a thermodynamic factor involving single-particle partition functions and the chemical potential of the adsorbate. The sticking coefficient is a measure of the efficiency of energy transfer in adsorption. Since energy supply from the... 

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 ... 

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 simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

Various /-alkoxy radicals may be formed by processes analogous to those described for /-butoxy radicals. The data available suggest that their propensities for addition vs abstraction are similar.72 However, rate constants for [3-scission of /-alkoxy radicals show marked dependence on the nature of substituents a to oxygen (Figure 3.6).210 420,421 Polar, steric and thermodynamic factors are all thought to play a part in favoring this trend.393... 

One way to measure the thermodynamics of the reaction is by the ceiling temperatures, Tc (the temperature at which the free energy of reaction 56 is zero). Bowmer and O Donnell74 found that G(S02) increases with decreasing Tc showing the importance of the thermodynamic factor. Although kinetic factors can explain also the increase in G(S02) with temperature, they do not explain the observed correlation with Tc. 

Another factor in step-growth polymerizations is cyclization versus linear polymerization.1516 Since ADMET is a step-growth polymerization, most reactions are carried out in the bulk using high concentrations of the reactant in order to suppress most cyclic formation. A small percentage of cyclic species is always present but is dependent upon thermodynamic factors, typical of any polycondensation reaction. 

The various kinetic and thermodynamic factors involved in vinyl free radical polymerization have been considered for the case of a batch (or semi-batch) polymerization being carried out to very high conversion. In particular, computations have been done for the final stage of the reaction when monomer concentration is reduced from approximately 5 volume % to 0.5 volume %. 

The double arrows indicate reversibifity, an intrinsic property of all chemical reactions. Thus, for reaction (1), if A and B can form P and Q, then P and Q can also form A and B. Designation of a particular reactant as a substrate or product is therefore somewhat arbitrary since the products for a reaction written in one direction are the substrates for the reverse reaction. The term products is, however, often used to designate the reactants whose formation is thermodynamically favored. Reactions for which thermodynamic factors strongly favor formation of the products to which the arrow points often are represented with a single arrow as if they were irreversible ... 

This last inflammability parameter presents problems. After stating its definition it will be seen that measuring autoignition temperature proves to be a difficult exercise because its measurement is sensitive to the experimental conditions, even more sensitive than for flashpoints. Worse, this parameter seems to be controlled by kinetic factors far more complex to master than the thermodynamic factors that probably control flashpoints (in fact it is a liquid/vapour equilibrium). So whilst the influence of the nature of the cup metal on a flashpoint has never been demonstrated, this demonstration was easily made with autoignition temperatures. 

For the thermodynamic factors Stull takes into account the decomposition temperature . This is defined as the temperature reached by the decomposition compounds of the particular substance when the latter decomposes into these constituent elements. It is therefore calculated using the standard enthalpy of formation of the compound. 

Two-dimensional H-bond descriptors are included in Table 6.1. Considering information content, they may be classified as indirect descriptors (no direct link with the H-bonding process), H-bond indicators (atoms having potential H-bond capability) and thermodynamic factors (calculated on the basis of experimental thermodynamic data of H-bonding). 

The variables that control the extent of a chromatographic separation are conveniently divided into kinetic and thermodynamic factors. The thermodynamic variables control relative retention and are embodied in the selectivity factor in the resolution equation. For any optimization strategy the selectivity factor should be maximized (see section 1.6). Since this depends on an understandino of the appropriate retention mechanism further discussion. .Jll be deferred to the appropriate sections of Chapters 2 and 4. 

Full exploitation of the synthetic potential of enolates requires control over the regioselectivity of their formation. Although it may not be possible to direct deprotonation so as to form one enolate to the exclusion of the other, experimental conditions can often be chosen to favor one of the regioisomers. The composition of an enolate mixture can be governed by kinetic or thermodynamic factors. The enolate ratio is governed... 

The crystallization process of flexible long-chain molecules is rarely if ever complete. The transition from the entangled liquid-like state where individual chains adopt the random coil conformation, to the crystalline or ordered state, is mainly driven by kinetic rather than thermodynamic factors. During the course of this transition the molecules are unable to fully disentangle, and in the final state liquid-like regions coexist with well-ordered crystalline ones. The fact that solid- (crystalline) and liquid-like (amorphous) regions coexist at temperatures below equilibrium is a violation of Gibb s phase rule. Consequently, a metastable polycrystalline, partially ordered system is the one that actually develops. Semicrystalline polymers are crystalline systems well removed from equilibrium. 

We have shown that only Io(oc D) depends on ve, while C(a AG ) does not depend on ve. This means that the topological nature of nucleation is reflected only on the kinetic factor (D) and not on the thermodynamic factor (AG ) as... 

Nevertheless, their real value in chemical synthesis depends as much on kinetic as thermodynamic factors. A reaction (where M is... 

---