Reaction boundary

The first step is described by back-reaction boundary conditions with intrinsic rate constants Aj and k.d. This is followed by a diffusion second step in which the hydrated proton is removed from the parent molecule. TTiis latter step is described by the Debye-Smoluchowski equation (DSE). 

The FACTSage thermochemical database was used to identify the thermodynamically stable phases that could exist in a system comprised of a pure metal, oxygen, HC1 and Cl2 at 500°C (Suppiah, 2008). The predominant Fe, Ni, Cu and Cr phases in an 02/HCl/Cl2 environment were determined. The equilibrium reaction boundary was plotted as a function of the partial pressures of 02 and HC1, for a constant Cl2 partial pressure. The resulting predominance diagrams were plotted over an 02 and HC1 partial pressure range of 10-20 to 1 atm for Cl2 partial pressures between 10-6 and 1 atm. The predominant Ni and Cr species are solids, suggesting that a corrosion resistant protective layer could be formed on the metal. 

The phase (or rather reaction ) boundaries of the dimer and chain polymer phases have not yet been determined, and only the reaction coordinates for the two experiments reported are shown in Fig. 18. Also, for C70 the drawing of a reaction map is complicated by the topochemical requirements for polymerization described above. Dimers can be formed in both fee and hep crystals, but ordered chain structures can only form in hep crystals, and different initial structures thus probably also lead to different final structures. Although it has been reported that initially hep C70 reverts to fee after high-pressure treatment (see above), it is not known which of these two structural phases is more stable under pressure and whether a change in the stacking sequence can be induced directly by pressure and/or temperature. 

In the second case, as shown in Fig. 2, the entire surface of the reactant particle is covered with a thin layer of the solid product very soon after contacting the reactant gas and the reaction boundary advances inward as reaction proceeds. This model has been known as the shrinking core model [2, 3] and has been used by many investigators. Since the functional dependence of S on t behaves well in this model, provided that the assumption of smooth advancement of the reaction boundary without changing its shape is valid, consideration of a variety of forms of r can conveniently be included and the kinetics can be described up to the completion of reaction. Moreover, even in the first case, in which the... 

The reduction of U03 to U02 has drawn much attention in connection with the preparation of fuels for nuclear reactors. Above 400° C, in which temperature range most of the reductions have been studied, the oxides of uranium take the forms, U02+, U409, U308-x(U02 6) and U03, as in Fig. 5 [65], In common with other gas—solid reactions, the details of the kinetics vary depending on the origin of the sample [69], the surface area [68], etc. Some authors claim that the reduction by hydrogen proceeds stepwise U03 -> U3Og - U02 [66—69], but there is a report [70] that no evidence for the existence of the intermediate oxides was found by X-ray diffraction at the reaction boundary of U03 and U02. 

In making all operating and design decisions. It Is Important to keep in mind the definition of the true reaction zone. Fundamentally, this Is the Interfaclal area between the immiscible hydrocarbon and acid catalyst liquid phases in the reactor. Reactants and products flow across this boundary. The olefins In the feed stream react Instantaneously with the sulfuric acid catalyst and combine with the relatively small amount of isobutane present In solution In the acid catalyst to form alkylate. Alkylate passes out through the Interfaclal surface reaction boundary into the hydrocarbon phase while Isobutane passes in to resaturate the catalyst. To suppress undesirable polymerization and other reactions It Is necessary to ... 

Upon careful examination of an actinometric reaction, boundary conditions are frequently found at which the reaction can be evaluated using rather simple equations. For example, under certain conditions a linear relationship between the irradiation intensity and the change in photoproduct concentration over time is found as depicted in the general equation below ... 

Figure 7 Phase relationships in the system MgO-Si02-H20 for bulk composition bci (Figure 6 60% olivine, 40% orthopyroxene, with 40% serpentization of the olivine), showing the stable mineral assemblages as a function of pressure and temperature. Stability fields and topology of reaction boundaries after Ulmer and Trommsdorff (1999).

Figure 13 Experimental, estimated, and calculated phase relationships for chlorite stability. Thin solid lines experimentally determined reactions chi = fo -h prp -h spl -h H2O and chi = fo -h en -h sp -h H2O as summarized by Ulmer and Trommsdorff (1999). Short dashed line for chi + en = fo -h prp -h H2O as estimated by Ulmer and Trommsdorff (1999). Solid line for chi -h en = fo -h prp -h H2O from Pawley (2003). Long dashed lines calculated position of the same three reaction boundaries. Calculated with version 3.21 of THERMOCALC (source Holland and Powell, 1998).

Reaction-path calculation. A sequence of mass transfer calculations that follows defined phase (or reaction) boundaries during incremental steps of reaction progress. 

Based on their hydrothermal experiments, Aja et al. (1991a, 1991b) and Aja and Rosenberg (1992) have concluded that the I/S clays are not two-phase mixtures, but have several defined compositions. They have inferred these compositions from the solution chemistry and the slopes of reaction boundaries. For example, their data in Fig. 9.12, suggest the most stable I/S solids are Ko.48 io( H)2 Ko.69 U o(OF1)2 at 25°C, and Ko.3i/Oio(UH)2 and Ko,g5/0 o(OH)2 at 125°C. The... 

These redox reaction boundaries and their computed equilibrium Eh values for pH = 7.0 are summarized in Table 11.3. The Eh/pH equations generally have different slopes so that when plotted on an Eh-pH diagram some of the reaction boundaries cross above or below pH = 7. To illustrate the importance of this effect, Fig. 11.11 shows that for constant Eh and increasing pH, the solubilities of solids of Se, Tc, and U (radioelements important in nuclear waste disposal) may reverse in order, and also become more or less soluble than the corrosion products of possible waste-container metals Cu, Fe, and Ni. 

Part 1. The pH of Hydrolysis Reaction Boundaries, Computed for Equal Activities of the TWo Fe Species... 

Recognition of the chemical processes involved permits identification of the variables and mechanisms that regulate and control the mineral composition of natural waters. With the help of equilibrium constants for the pertinent reactions, boundary conditions towards which aquatic environments must proceed can be established. 

Entropy and molar volume can be quantified using a well-known equation describing the slope of any two-phase reaction boundary on a pressure-temperature diagram. From equation (8.2) it follows that... 

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