Stoichiometry considerations

Chain metasilicates Si03 formed by comersharing of Si04 tetrahedra arc particularly prevalent in nature and many important minerals have this basic structural unit (cf, polyphosphates, p, 528), Despite the apparent simplieily of their structure motif and stoichiometry considerable structural diversity is encountered because of the differing conformations that can be adopted by the linked tetrahedra. As a result, the repeat distance along the c -axis can be (1). 

The second stoichiometry consideration is the ratio of catalyst to substrate. As noted in the preceding section, virtually all asymmetric epoxidations can be performed with a catalytic amount of Ti-tartrate complex if molecular sieves are added to the reaction milieu. A study of catalyst/substrate ratios in the epoxidation of cinnamyl alcohol revealed a significant loss in enantioselectivity (Table 6A.2) below the level of 5 mol % catalyst. At this catalyst level, the reaction rate also decreases, with the consequence that incomplete epoxidation of the substrate may occur. Presendy, the recommended catalyst stoichiometry is from 5% Ti and 6% tartrate ester to 10% Ti and 12% tartrate ester [4],... 

The other fluxes through the liquid film-liquid bulk boundary can be calculated based on stoichiometry considerations ... 

As far as two-electron oxidations such as hydroxylations, epoxidations, or sulfoxidations are concerned, stoichiometry considerations give rise to a mechanistic complication, because two different elementary steps are operative in the oxidation of the substrate. To clarify this argument, let us imagine a metal catalyst which can mediate the transformation 02 - 2 (O). For example, if a mononuclear metal complex [M] is involved, activation of O2 leads to a metal dioxide (or a metal peroxide) which may transfer one 0-atom to the substrate in a first step, thus producing the corresponding metal monoxide. A subsequent 0-atom transfer from [M] O to the substrate regenerates [M], thus closing the catalytic cycle Scheme 2,a). The key issue in what we term the stoichiometry problem is that two different elementary steps are involved in the oxidation of the substrate by O2, i.e., a first 0-atom transfer from a dioxo (or peroxo) species and then a second from a monoxo species. Activation parameters are unlikely to be comparable in both these... 

In order to allow a proper comparison of chemical conversion rates with transport (diffusional) rates of the reactants, it is more convenient to express eqn. (8) in terms of hydrogen conversion and the rate constant for hydrogen conversion, ku2, since rates of conversion depend on hydrogen pressure rather than on carbon monoxide pressure. The relation between ku2 and kco follows from stoichiometry considerations ... 

The most significant difference between the alkoxysilanes and siUcones is the susceptibiUty of the Si—OR bond to hydrolysis (see Silicon compounds, silicones). The simple alkoxysilanes are often operationally viewed as Hquid sources of siUcon dioxide (see Silica). The hydrolysis reaction, which yields polymers of siUcic acid that can be dehydrated to siUcon dioxide, is of considerable commercial importance. The stoichiometry for hydrolysis for tetraethoxysilane is... 

Synthesis Ga.s, Since petroleum prices rose abmpdy in 1974, the production of ethanol from synthesis gas, a mixture of carbon monoxide and hydrogen, has received considerable attention. The use of synthesis gas as a base raw material has the same drawback as fermentation technology low yields limited by stoichiometry. 

Although considerable study has been devoted to oxygen chemisorption (mainly on platinum) there is considerable ambiguity in the surface stoichiometry of the reaction. In some cases Pt20 is formed, in others PtO, the particular compound... 

Step 4 Define the System Boundaries. This depends on the nature of the unit process and individual unit operations. For example, some processes involve only mass flowthrough. An example is filtration. This unit operation involves only the physical separation of materials (e.g., particulates from air). Hence, we view the filtration equipment as a simple box on the process flow sheet, with one flow input (contaminated air) and two flow outputs (clean air and captured dust). This is an example of a system where no chemical reaction is involved. In contrast, if a chemical reaction is involved, then we must take into consideration the kinetics of the reaction, the stoichiometry of the reaction, and the by-products produced. An example is the combustion of coal in a boiler. On a process flow sheet, coal, water, and energy are the inputs to the box (the furnace), and the outputs are steam, ash, NOj, SOj, and CO2. 

Whenachemicalengineerbalancesanequationitmustbedone firstfromtheperspective of stoichiometry, and second takingvalue and profit into consideration. Thus for a given reaction ... 

The fixation of carbon dioxide to form hexose, the dark reactions of photosynthesis, requires considerable energy. The overall stoichiometry of this process (Eq. 22.3) involves 12 NADPH and 18 ATP. To generate 12 equivalents of NADPH necessitates the consumption of 48 Einsteins of light, minimally 170 kj each. However, if the preceding ratio of l ATP per NADPH were correct, insufficient ATP for COg fixation would be produced. Six additional Einsteins would provide the necessary two additional ATP. Prom 54 Einsteins, or 9180 kJ, one mole of hexose would be synthesized. The standard free energy change, AG°, for hexose formation from carbon dioxide and water (the exact reverse of cellular respiration) is +2870 kj/mol. 

Transition elements, for which variable valency is energetically feasible, frequently show non-stoichiometric behaviour (variable composition) in their oxides, sulfides and related binary compounds. For small deviations from stoichiometry a thermodynamic approach is instructive, but for larger deviations structural considerations supervene, and the possibility of thermodynamically unstable but kinetically isolable phases must be considered. These ideas will be expanded in the following paragraphs but more detailed treatment must be sought elsewhere. " ... 

Compounds whose molecular compositions are multiples of a simple stoichiometry are polymers, stricdy, only if they are formed by repetition of the simplest unit. However, the name polymerization isomerism is applied rather loosely to cases where the same stoichiometry is retained but where the molecular arrangements are different. The stoichiometry PtCl2(NH3)2 applies to the 3 known compounds, [Pt(NH3)4][PtCU], [Pt(NH3)4][PtCl3(NH3)]2, and [PtCl(NH3)3]2[PtCl4] (in addition to the cis and trans isomers of monomeric [PtCl2(NH3)2]). There are actually 7 known compounds with the stoichiometry Co(NH3)3(N02)3. Again it is clear that considerable differences are to be expected in the chemical properties and in physical properties such as conductivity. 

A considerable amount of work has been carried out into the corrosion of steels in the gases produced during the combustion of fossil fuel due to extensive use of low alloy steels as heat exchanger tubes in power generation. Combustion gases contain many species, such as CO, CO2, SO2, SO3, H2S and HCl, arising from elements within the fuel. The many different combinations of operating temperature and chemical stoichiometry of combustion reactions lead to many possible complex corrosion reactions. 

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