Compositional thermodynamic stability

The composition of the products from the isomerization of an unsaturated compound under the influence of a catalytic amount of a base is governed by the relative thermodynamic stabilities of the starting compound and the product. Of particular synthetic interest are isomerizations in which there is an accumulation of an isomer in the isomerization sequence. Isolation of the desired intermediate in a reasonable state of purity is often a matter of careful selection of the base and the solvent. The following reactions are representative examples ... 

Marin, D. Mendicuti, F. Polarographic Determination of Composition and Thermodynamic Stability Constant of a Complex Metal Ion, /. Chem. Educ. 1988, 65, 916-918. 

When one considers the potential high-energy release on rupture of a carborane unit, together with the thermodynamic stability of combustion products, it is hardly surprising that there is a body of literature that reports on the use of carbo-ranes within propellant compositions. Their use in energetic applications is to be expected when the enthalpy of formation (AH/) data for the products of combustion for boron are compared to those of carbon. Thermodynamic data for the enthalpy of formation of o-carborane and of typical boron and carbon combustion products is shown in Table 4. Measurements of the standard enthalpy of combustion32 for crystalline samples of ortho-carborane show that complete combustion is a highly exothermic reaction, AH = — 8994 KJmol. ... 

In the above examples the composition of the equilibrium mixture is, of course, governed by the relative thermodynamic stability of the... 

Thermodynamic stability. Let us consider a liquid binary mixture of two species, a and b, crystallizing to a solid solution. Let G be the free enthalpy (Gibbs) function of each phase, either solid or liquid. Calculate the change AG in molar free enthalpy when a liquid of molar composition XUqb crystallizes to a solid of molar composition... 

This reaction can occur either thermally or photochemically, with the equilibrium position (thermal reaction) depending on the thermodynamic stability of the reactant and product, and the photostationary-state composition depending on the relative values of the absorption coefficients at the wavelengths used. 

The early work of Schwarz and Johnson (1983) used a prediction of the underlying thermodynamics of the Au-La system to explain the relative stability of the liquid/amorphous phase in their elemental layered composites (Fig. 11.7). However, they utilised the method proposed by Miedema (1976) for thermodynamic stability of the liquid/amorphous phase. There are clear limitations to the Miedema approach firstly it is not guaranteed to produce the correct phase diagram and therefore phase competition is at best only approximated, and secondly, the thermodynamics of the terminal solid solutions are chosen quite arbitrarily. 

The Davison SOx Index of a catalyst decreases with increasing regenerator temperature. We have observed this for all the materials we have tested. These materials covered a variety of chemical compositions. This effect of temperature is believed to be due to the thermodynamic stability of the sulfate-type complex on the surface of the catalyst. Data are shown in Figure 4 for DA-250 + 10% Additive R. 

We do not need to regard YBa2Cus07 as a solid solution, but it is not a thermodynamically stable phase at any temperature/pressure condition (11). Thermodynamic stability exists in the YBa2Cus06+x system in a certain region of temperature and oxygen pressure, but only for values of x between zero and about 0.6. Even these compositions appear not to be thermodynamically stable at room temperature and below. 

This section is primarily intended to show that the relevant combination of the intrinsic physico-chemical and microscopic characteristics of HT materials, their behaviour under aggressive conditions of corrosion, and their modelled thermodynamic stability, yields a sound composite picture of these materials. On the basis of the knowledge acquired over decades on high-level nuclear waste glasses (Vernaz Dussosoy 1992 Bates et al. 1994 Thomassin 1995, 1996 Ewing 1996), these are the key parameters that may drastically influence the long-term durability of HT materials. 

It is convenient to use phase diagrams [46] to represent the thermodynamic properties that determine the stability and equilibrium composition of water-containing aerosols. The properties of interest are the temperature, the vapour pressure and composition of the various components in the condensed phases. This is particularly important with respect to the composition and stability of the various hydrates formed at low temperature in the nitric acid-water [47] and sulfuric acid-water binary systems [48], and the ternary systems HjSO/HNOj/HjO and HjSO/HCl/HjO [49],... 

Since the retention of lead is primarily determined by the thermodynamic stability of the lead deposits, the influence of the temperature may vary with the composition of the lead deposits, which in turn, depends on the fuel and catalyst composition. We shall briefly discuss how such reasoning pertains to lead retention. 

In some reactions, however, product composition does not remain constant, and the ratio of products is significantly different in the early (incomplete) stages of the reaction, from that which is found when all the starting material has reacted. In this case the reaction is said to be under thermodynamic control, where the final ratio of products is determined by their relative thermodynamic stabilities (i.e. their free energy differences, AG). Here the products are in equilibrium with either the starting material, or with a common intermediate (as illustrated in Fig. 1.2), so that although the initial product ratio is determined by the relative rates of the reactions, the final ratio reflects the relative stabilities of the products. Sometimes of course the kinetically controlled product is also the more stable, but clearly if this is not the case, attention to detail in specifying reaction conditions may enable the preparative isolation of either of the different products (see for example Section 5.5.7, p. 574, Section 5.10.1, p. 643 and Section 6.4.1, p. 874). 

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