Stoichiometry heterogeneous reaction

The stoichiometry of decomposition of [Ni(NH3)4](NCS)2 was dependent on the method of salt preparation [1126]. Ammonia was lost in three successive steps (—NH3, —NH3, —2 NH3) from the solution-prepared salt, but the first intermediate could not be isolated from the similar reaction of material prepared by heterogenous reaction. The difference in behaviour was ascribed to differences in perfection of the crystallites resulting from the alternative preparative methods. 

Assume A, B, C, and D have similar diffusivities so that local stoichiometry is preserved. Under what circumstances will conversion be maximized by (a) complete segregation (b) by maximum mixedness Heterogeneous reactions are often modeled as if they were homogeneous. A frequently encountered rate expression is... 

The analytical concentrations change during the heterogeneous reaction. These changes are governed by the stoichiometry of the heterogeneous reaction and are proportional to one another. For example, chalcopyrite is dissolved in acidic ferric chloride solution according to... 

Selected entries from Methods in Enzymology [vol, page(s)] Association constant determination, 259, 444-445 buoyant mass determination, 259, 432-433, 438, 441, 443, 444 cell handling, 259, 436-437 centerpiece selection, 259, 433-434, 436 centrifuge operation, 259, 437-438 concentration distribution, 259, 431 equilibration time, estimation, 259, 438-439 molecular weight calculation, 259, 431-432, 444 nonlinear least-squares analysis of primary data, 259, 449-451 oligomerization state of proteins [determination, 259, 439-441, 443 heterogeneous association, 259, 447-448 reversibility of association, 259, 445-447] optical systems, 259, 434-435 protein denaturants, 259, 439-440 retroviral protease, analysis, 241, 123-124 sample preparation, 259, 435-436 second virial coefficient [determination, 259, 443, 448-449 nonideality contribution, 259, 448-449] sensitivity, 259, 427 stoichiometry of reaction, determination, 259, 444-445 terms and symbols, 259, 429-431 thermodynamic parameter determination, 259, 427, 443-444, 449-451. 

Many heterogeneous reactions give rise to an increase or decrease in the total number of moles present in the porous solid due to the reaction stoichiometry. In such cases there will be a pressure difference between the interior and exterior of the particle and forced flow occurs. When the mean free path of the reacting molecules is large compared with the pore diameter, forced flow is indistinguishable from Knudsen flow and is not affected by pressure differentials. When, however, the mean free path is small compared with the pore diameter and a pressure difference exists across the pore, forced flow (Poiseuille flow see Volume 1, Chapter 3) resulting from this pressure difference will be superimposed on molecular flow. The diffusion coefficient Dp for forced flow depends on the square of the pore radius and on the total pressure difference AP ... 

In the second step, with the hydrolysis constants and the specific interaction parameter for ZrOH" and for Zr3(OH) fixed to the values optimised as detailed above, the equilibrium constants and interaction parameter for all other species in the overall hydrolysis model were obtained by a global fit of the potentiometric, solubility, solvent extraction and ion exchange data mentioned above. The fit was extended to the determination of equilibrium constants for heterogeneous reactions ion exchange constants, solubility constants and liquid/liquid distribution coefficients. The fit was based on a preselection of the stoichiometries of dominant species which included invariably the species Zr(OH)4(aq), Zr ) ), Zr (OH)Jj and Zr4(OH)i6(aq) and various other mono-, di-, tri- and tetravalent species to improve the fit. The potential formation of chloride complexes of Zr was considered for chloride containing solutions, using the stability constants determined in Section V-4. If all fitted results were found insensitive to the equilibrium constants of a given species, the respective species was removed from the list of species. 

A chemical transition process is always characterized by its stoichiometry and its extent of reaction independent of the mode of process operation. The rate of change in extent, however, depends on the mode of process operation, on the reaction rate and, in the case of heterogeneous reactions, on the mass transfer. In order to account for the mode of operation appropriately, models are required for the different reactor types. This is all comprehensively described in the overall mass balance of the system. 

Ng at steady state is controlled by the stoichiometry of a reaction at a boundary. An example is gas A diffusing from the bulk gas phase to the catalyst surface, where it reacts instantaneously and irreversibly in a heterogeneous reaction as follows ... 

When n kinds of solvated building blocks B (solv), in a stoichiometry v , can self-assemble to give a crystalline product C(s), the heterogeneous reaction can be simplified as... 

The concept of a structure element and that of non-stoichiometry are most important in understanding phenomena involving solids, such as electrical conductivity, the mechanisms of heterogeneous reactions and certain optical and magnetic properties. 

No effort was made to characterize the intermediate organometallic species of this rather complex, heterogeneous reaction, nor were specific structures or stoichiometries advanced. In later work, Evans and co-workers [13] reported that, for R = t-butyl in octane, the reaction did not proceed to completion. Whether the reasons were thermodynamic or kinetic was not resolved. Seyam [14] subsequently demonstrated that the results of Evans were largely an artifact of inadequate UCI4 dispersion and reaction agitation. The reaction can be made to proceed to completion, and any assertion that it does not on thermodynamic grounds is unsupported. 

The synthesis of the already reported [PcFe(tz)] [4] is carried out by heating PcFe in a solution of s-tetrazine in chlorobenzene at 100 C for 24 h. The product of the heterogeneous reaction is a thermal-and light-stable blue powder. The stoichiometry and purity of the compound is well established by different methods such as IR-, far-IR-, Raman-, UV/Vis-spectroscopy, microanalysis and combined thermogravi-metric and differential thermal analysis. [ PcFe(tz) 3 splits off s-tetrazine above 125 C with an exothermic maximum at 310 C, PcFe remaining as residue. The synthesis of a complex PcFeCtz), containing two bisaxial coordinated molecules of s-tetrazine, is possible in a heterogeneous reaction of PcFe with s-tetrazine in ethanol. 

For a determined reaction stoichiometry and initial reactant composition, write the equilibrium constant in terms of the extent of reaction for gas-phase, liquid-phase, and heterogeneous reactions for ideal or nonideal systems. 

In this and the following chapters, we explore interactions between diffusion and chemical reaction. For heterogeneous reactions, we shall find that diffusion and reaction occur by steps in series, steps that can produce results much like mass transfer across an interface. For homogeneous reactions, we shall find that diffusion and reaction occur by steps partially in parallel, steps that are different than processes considered before. In both cases, we shall find that non-first-order stoichiometries lead to unusual results. 

Fig. 16.5-1. A second-order heterogeneous reaction. When the simple stoichiometry used earlier in this chapter is not followed, the overall rate no longer depends on a simple sum of the resistances of the various steps.

The second part of the chapter (Sections 16.3 through 16.5) is concerned with heterogeneous reactions. The key point is that the overall rate frequently varies with a sum of resistances in series. The results are similar to those involved in interfacial mass transport, but with chemical equilibrium constants replacing the Henry s law constants. Although this simplicity can be compromised by unusual stoichiometry or by concentrated solutions, the analogy with interfacial mass transfer is useful and worth remembering. 

Butane-Based Fixed-Bed Process Technology. Maleic anhydride is produced by reaction of butane with oxygen using the vanadium phosphoms oxide heterogeneous catalyst discussed earlier. The butane oxidation reaction to produce maleic anhydride is very exothermic. The main reaction by-products are carbon monoxide and carbon dioxide. Stoichiometries and heats of reaction for the three principal reactions are as follows ... 

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