Reactions thermodynamic consideration

Reaction (5. EE) is particularly useful for the discussion of thermodynamic considerations because of the way differences in thermodynamic state variables are independent of path. Accordingly, if we know the value of AG for reaction (5. EE), we have characterized the following ... 

Reaction with Metals. Thermodynamic considerations for the reaction... 

However, it was found that the effect on the equilibrium formation of aromatics is not substantial due to thermodynamic considerations. A more favorable effect was found for the reaction between ethylene (formed via cracking during aromatization of propane) and hydrogen. The reverse shift reaction consumes hydrogen and decreases the chances for the reduction of ethylene to ethane byproduct. 

It follows from the electrochemical mechanism of corrosion that the rates of the anodic and cathodic reactions are interdependent, and that either or both may control the rate of the corrosion reaction. It is also evident from thermodynamic considerations (Tables 1.9 and 1.10) that for a species in solution to act as an electron acceptor its redox potential must be more positive than that of the M /M equilibrium or of any other equilibrium involving an oxidised form of the metal. 

Pyrolyses of formates, oxalates and mellitates yield CO and C02 (H2, H20 etc.) as the predominant volatile products and metal or oxide as residue. It is sometimes possible to predict the initial compositions from thermodynamic considerations [94], though secondary reactions, perhaps catalyzed by the solids present, may result in a final product mixture that is very different. The complex mixtures of products (hydrocarbons, aldehydes, ketones, acids and acid anhydrides) given [1109] by reactants containing larger organic groupings makes the collection of meaningful kinetic data more difficult, and this is one reason why there are relatively few rate studies available for the decompositions of these substances. 

It should be born in mind, however, that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different signs from those of the decrystalUzation proper. Specifically, the contribution to the activation parameters of the interactions that occur in the solvent system should be taken into account. Consider the energetics of association of the solvated ions with the AGU. We may employ the extra-thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols [99]. Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in Eq. 7 is endothermic ... 

Thermodynamic considerations postulate BjH to be a better boron source than BCl, in CVD of TaB2Using reaction (f) at < 1200 K deposits with extremely small crystal sizes are obtained on graphite substrates . They contain amorphous B at deposition temperatures < 873 K and are substoichiometric in B above this T. Carbon from the substrate substitutes for B, thereby stabilizing the diboride structure at high deposition T... 

Apparent activation energies are approximately double for the dehydrogenation reaction on most types of Pt catalysts [18,74]. Therefore temperature is the most straightforward variable to control selectivity. The temperature influences selectivity because of not only thermodynamic considerations, but also the influence of temperature on coverage drastically influences selectivity. 

Thermodynamic considerations are important in leaching in that they provide basic guidance in choosing the combination of reagents and their concentrations so as to obtain favorable free energy changes associated with any proposed reaction. Kinetic considera-... 

From thermodynamic considerations C0/H20 must be a better reductant than H2, thus milder reaction conditions might be possible. 

The equilibrium constant Kf for reaction i is computed from thermodynamic considerations using Gibbs free energies. The Arrhenius form for the rate constants is written in terms of the pre-exponential factor A,, the temperature exponent /( , the activation energy Ei, and the universal gas constant R in the same units as the activation energy. 

In the presence of trace amounts of iron, superoxide can then reduce Fe3+ to molecular oxygen and Fe2+. The sum of this reaction (13.2) plus the Fenton reaction (13.1) produces molecular oxygen, hydroxyl radical and hydroxyl anion from superoxide and hydrogen peroxide, in the presence of catalytic amounts of iron. This is the Haber-Weiss reaction (13.3), originally described by Haber and Weiss (1934), but manifestly impossible from thermodynamical considerations in the absence of catalytic amounts of redox metals such as iron and copper ... 

In both reactions, electron transfer induces the dissolution of the solid phase i.e., reductive and oxidative dissolution, respectively. Although no kinetic implications follow directly from the thermodynamic considerations, there are cases where the redox rate is related to the redox equilibrium (see e.g., Eq. 9.12). 

The thermodynamic feasibility of redox reactions at the semiconductor-electrolyte interface can be assessed from thermodynamic considerations. Since typical redox potentials for many redox couples encountered in electrolytes of natural or technical systems often lie between the band potentials of typical semiconductors, many electron transfer reactions are (thermodynamically) feasible (Pichat and Fox, 1988). With the right choice of semiconductor material and pH the redox potential of the cb can be varied from 0.5 to 1.5 V and that of the vb from 1 to more than 3.5 V (see Fig. 10.4). 

Despite the importance of the precipitation of calcium phosphates, there is still considerable uncertainty as to the nature of the phases formed in the early stages of the precipitation reactions under differing conditions of supersaturation, pH, and temperature. Although thermodynamic considerations yield the driving force for the precipitation, the course of the reaction is frequently mediated by kinetic factors. Whether dicalcium phosphate dihydrate (CaHPO HoO, DCPD), octacalcium phosphate (Ca HfPO, 2.5 H20, OCP), hydroxyapatite (Cag (PO fOH), HAP), amorphous calcium phosphate (ACP), or a defect apatite form from aqueous solution depends both upon the driving force for the precipitation and upon the initiating surface phase. Thermodynamically, the relative supersaturation, o, is given by... 

Reactions 1 and 6 in Table 8.4 were identified as the reactions that control the S2 and S02 concentrations, respectively, while reactions 2 and 4 control the S concentration with some contribution from reaction 1. Reaction 6 was the major one for the SO flux rate. The three reactions involving H2S were said to play an important role. SH was found to be controlled by reactions 1-5. Because the first eight reactions were found to be fast, it was concluded that they rapidly establish and maintain equilibrium so that the species S, S2, H2S, SH, SO, and S02 are efficiently interrelated. Thus relative concentrations, such as those of SO and S02, can be calculated from thermodynamic considerations at the local gas temperature from the system of reactions... 

Many SHS processes include an oxide as one of the reactant materials. As a convenience, all the elements to be discussed in the various SHS processes will be referred to as metals. The selection of a metal-metal oxide reacting combination is readily made without detailed thermodynamic considerations. Nevertheless, it is fruitful to examine the overall thermodynamics that govern the choice of a particular SHS combination. Consider the classic thermite reaction... 

The energy storage and power characteristics of electrochemical energy conversion systems follow directly from the thermodynamic and kinetic formulations for chemical reactions as adapted to electrochemical reactions. First, the basic thermodynamic considerations are treated. The basic thermodynamic equations for a reversible electrochemical transformation are given as... 

Ir(IV), Pt(IV), with the states from Rh(III) being termed inert. Thus, kinetic factors tend to be more important, and reactions that should be possible from thermodynamic considerations are less successful as a result. On the other hand, the presence of small amounts of a kinetically labile complex in the solution can completely alter the situation. This is made even more confusing in that the basic chemistry of some of the elements has not been fully investigated under the conditions in the leach solutions. Consequently, a solvent extraction process to separate the precious metals must cope with a wide range of complexes in different oxidation states, which vary, often in a poorly known fashion, both in kinetic and thermodynamic stability. Therefore, different approaches have been tried and different flow sheets produced. 

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