Oxides, thermochemical determination

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

Considering the right-hand side of this equation as a simple mathematical function, it can be plotted versus (Sir). The left-hand side is known for a given metal it contains known thermochemical and thermophysical properties, thus (Sir) is determined. The mass of the oxide formed is greater than the mass of the metal consumed, consequently the original size of the metal that would be pyrophoric (rm) can be calculated from S, r and the physical properties of the oxide and metal and their molecular weights. These results have been presented in the... 

A core aspect of the model is the determination of the chemical conversion of gasifier SG to hydrogen by the RP. For conditions where steam availability is not limiting, the chemical conversion relates to the difference between the initial and final combustion/fuel ratio of the fuel gas. The initial CP/SG ratio is determined by the gasifier and the biomass feedstock. The final CP/SG ratio is determined by the thermochemical properties of the metal oxide material. Ideally the difference between the initial (CP/SG),and final (CP/SG)j , ratios should be as large as possible. In reality the availability of steam for the re-oxidation of the metal oxide is limiting for conditions where the difference in the CP/SG ratios are large. 

The requirements for selecting a fuel and oxidizer as a liquid bipropellant system are usually a compromise between the demands of the vehicle system, the propulsion system, and the propellants themselves. The vehicle and propulsion system will determine performance levels, physical property requirements, thermal requirements, auxiliary combustion requirements, degree of storability and package-ability, hypergolicity, etc. The final propellant selection must not only satisfy such requirements but is also dictated by thermochemical demands which the fuel and oxidizer make on each other. Frequently, specifically required properties are achieved through the use of chemical additives and/or propellant blending. 

Using the dihydrogen salt, NaH2As03, von Zawidzki2 determined the equivalent conductivity in the presence of N/32 arsenious acid in order to diminish hydrolytic dissociation, and concluded that it resembled the salt of a monobasic acid. At extreme dilutions the equivalent conductivity increased, apparently owing to hydrolysis and oxidation. This view that arsenious acid is essentially a feeble monobasic acid is supported by Thomsen s thermochemical values for the heats of neutralisation of the acid (see p. 140). 

J. Thomsen s value 8 for the heat of formation is 2P+50=P205+369-9 Cals, for the solid and for the dissolved oxide, 405-5 Cals. H. Giran obtained with yellow phosphorus, 369-4 Cals. with red phosphorus, 362-0 Cals. and with red phosphorus, 360-0 Cals. Other determinations have been made by M. Berthelot, T. Andrews, J. J. B. Abria, and P. A. Favre and J. T. Silbermann. J. C. Thomlinson argued from the thermochemical data that phosphorus is quadrivalent. The heat of solution of the crystalline variety was found by P. Hautefeuille and A. Perrey to be 44 58 Cals. and 41-32 Cals, for the pulverulent variety and H. Giran found for the variety obtained by combustion, 34-37 Cals. for the crystallized variety, 40 79 Cals. for the amorphous variety, 33-81 Cals. for the vitreous or glassy variety, 29-09 Cals. P. Hautefeuille and A. Perrey give 3-26 Cals, for the heat of transformation of the crystalline into the amorphous variety. 

Here we consider the factors which determine whether a given compound prefers an ionic structure or a covalent one. We may imagine that for any binary compound - e.g. a halide or an oxide - either an ionic or a covalent structure can be envisaged, and these alternatives are in thermochemical competition. Bear in mind that there may be appreciable covalency in ionic substances, and that there may be some ionic contribution to the bonding in covalent substances. Since there is no simple means - short of a rigorous MO treatment - of calculating covalent bond energies, and since quantitative calculations based upon the ionic model are subject to some uncertainties, the question of whether an ionic or a covalent structure is the more favourable thermodynamically cannot be answered in absolute terms. We can, however, rationalise the situation to some extent. 

It would be useful for our purposes to examine in similar fashion all other oxidation states of metals forming metal halides. However, the thermochemical data in the gas phase for these oxidation states are less well known. Where AHi0n(g) data for MFn or MIn are not available, other halide differences may be used to indicate approximate comparable orders. Also where no thermochemical data in the gas phase are known, lattice energies, i. e., Mvarious metal halides are either approximately constant or vary in such a fashion that the final metal order will not be affected. 

The problem of determining AG° for organic redox couples can usually be attacked experimentally by either electrochemical methods or gas-phase measurements of ionization potentials and electron affinities. In both approaches, we can in principle determine thermochemical parameters for one-electron oxidation (83) and reduction (84) of an organic species. 

The primaiy emphasis in this review article is to showcase the use of LEISS to examine the outermost layers of Pt-Co alloys in order to correlate interfacial composition with electrocatalytic reactivity towards oxygen reduction. In some instances, it is desirable to compare the properties of the outermost layer with those of the (near-surface) bulk an example is when it becomes imperative to explain the unique stability the alloyed Co under anodic-oxidation potentials. In such cases. X-ray photoelectron spectroscopy and temperature-programmed desorption may be employed since both methods are also able to generate information on the electronic (binding-energy shift measurements by XPS) and thermochemical (adsorption enthalpy determinations by TPD) properties at the sub-surface. However, an in-depth discourse on these and related aspects was not intended to be part of this review article. 

In a landmark paper, Breslow and coworkers described the determination of pA), values of weak hydrocarbon acids by use of thermochemical cycles involving electrochemical reduction data for triarylmethyl, cycloheptatrienyl, and triphenyl- and trialkylcyclopropenyl cations and radicals [9aj. Later, they derived pATa data from standard oxidation potentials and bond-dissociation energies [9b, c]. The methodology was further developed by Nicholas and Arnold [10a] for the determination of cation radical acidities, and later modified and extensively used by Bordwell and coworkers [10b, c] so that homolytic bond-dissociation energies and cation radical... 

Scheme 6. Thermochemical cycle for determination of oxidatively induced M-X BDE changes in metal halides.

The data determined directly by Knudsen cell measurements, plus a strong correlation between the bond strengths of metal hydroxide bonds and metal halide (in particular, chloride and fluoride bonds) in the gaseous metal hydroxides and halides were developed and allow us to more reliably estimate the enthalpy of formation of many hydroxide and oxyhydroxide metal compounds whose values of thermochemical heat and formation were previously unknown. These thermochemical properties were then used to estimate volatility of various supporting oxide substrates and catal)dically-active solids that were relevant for the fabrication of catalytic combustors. 

Standard electrode potential data are available for an enormous number of halfreactions. Many have been determined directly from electrochemical measurements. Others have been computed from equilibrium studies of oxidation/reduction systems and from thermochemical data associated with such reactions. Table 18-1 contains standard electrode potential data for several half-reactions that we will be considering in the pages that follow. A more extensive listing is found in Appendix 5. ... 

Experimental facts and theoretical concepts existing in the hterature indicate that the formation of free radicals plays an important role in a number of catalytic oxidation reactions [1-5]. In the present paper we analyze the contribution of fi-ee radicals to several oxidative transformations of lower alkanes over oxide catalysts. Based on the thermochemical data and on the results of kinetic simulations it is shown that the observed reaction kinetics and product compositions in the mentioned above processes are determined by a set of interdependent heterogeneous and homogeneous reactions of fi ee radicals, i.e. they should not be considered as spectators taking part in side reactions, but as principal intermediates causing the main features of lower alkanes oxidation and design of catalysts. 

The main factors determining the efficiency of different oxides as catalysts for lower alkanes oxidation are the H-atom affinity of strong oxidizing surface sites and the oxygen binding energy. These thermochemical factors cause the rates and directions of free-radical reactions and, as a result, the catalytic activity and selectivity to certain products. 

At that time the radical nature of solutions of "Cr(CO)3Cp was not yet proven. Hydrogenation of solutions of the dimer had been reported by Muetterties and coworkers,42 but the mechanism of oxidative addition was not known. In addition, the complex [Cr(CO)3Cp]2 is known to catalyze hydrogenation of dienes via H-Cr (CO)3Cp that is regenerated under catalytic conditions. Thermochemical data for reaction 10.54 were obtained by indirect thermochemical cycles, however direct measurement of the enthalpy of reaction of H2(g) and "Cr(CO)3Cp allowed the determination of H-Cr(CO)3Cp bond strength as 62.3 1 kcal/mol.74... 

For gaseous flames, the LES/FMDF can be implemented via two combustion models (1) a finite-rate, reduced-chemistry model for nonequilibrium flames and (2) a near-equilibrium model employing detailed kinetics. In (1), a system of nonlinear ordinary differential equations (ODEs) is solved together with the FMDF equation for all the scalars (mass fractions and enthalpy). Finite-rate chemistry effects are explicitly and exactly" included in this procedure since the chemistry is closed in the formulation. In (2). the LES/FMDF is employed in conjunction with the equilibrium fuel-oxidation model. This model is enacted via fiamelet simulations, which consider a laminar counterflow (opposed jet) flame configuration. At low strain rates, the flame is usually close to equilibrium. Thus, the thermochemical variables are determined completely by the mixture fraction variable. A fiamelet library is coupled with the LES/FMDF solver in which transport of the mixture fraction is considered. It is useful to emphasize here that the PDF of the mixture fraction is not assumed a priori (as done in almost all other flamelet-based models), but is calculated explicitly via the FMDF. The LES/FMDF/flamelet solver is computationally less expensive than that described in (1) thus, it can be used for more complex flow configurations. 

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