Stoichiometry reactivity

The F/C ratio model accounts for the fact that for carbon-containing gases etching and polymerization occur simultaneously. The process that dominates depends upon etch gas stoichiometry, reactive-gas additions, amount of material to be etched, and electrode potential and upon how these factors affect the F/C ratio. For instance, as described in Figure 8, the F/ C ratio of the etchant gas determines whether etching or polymerization is favored. If the primary etchant species for silicon (F atoms) is consumed either by a loading effect or by reaction with hydrogen to form HF, the F/... 

Relationship between ionic Conductivity and Gelation Time (tge,) Molecular gelation takes place at a well-defined and generally calculable step in the course of the chemical reaction (Flory 1988). The reaction mechanism is assumed to be temperature-independent and free of noncross-linking side reactions but depends on stoichiometry, reactivity, and functionality of the reactants. Several attempts have been made to relate dielectric results and the gel point [e.g., see Fournier et al. (1996)]. An inflection point... 

The requirements of thin-film ferroelectrics are stoichiometry, phase formation, crystallization, and microstmctural development for the various device appHcations. As of this writing multimagnetron sputtering (MMS) (56), multiion beam-reactive sputter (MIBERS) deposition (57), uv-excimer laser ablation (58), and electron cyclotron resonance (ECR) plasma-assisted growth (59) are the latest ferroelectric thin-film growth processes to satisfy the requirements. 

Polyetherification is similar to a polycondensation process formation of high molecular weight polymer requires precise adjustment of composition to approximately 1 1 ratio of bisphenol to dihalosulfone. Trace amounts of water gready reduce the molecular weight attainable owing to side reactions that unbalance the stoichiometry (76). The reactivity of the halosulfone is in the order expected for two-step nucleophilic aromatic displacement reactions ... 

Barium metal is produced commercially by the reduction of barium oxide with a less reactive, nonvolatile element, usually aluminum (16—22). Depending on initial stoichiometry, two overall reactions occur in the BaO reduction ... 

When an equilibrium reaction occurs in a vapor-hquid system, the phase compositions depend not only on the relative volatility of the components in the mixture, but also on the consumption (and production) of species. Thus, the condition for azeotropy in a nonreactive system = x, for all i) no longer holds true in a reactive system and must be modified to include reaction stoichiometry ... 

The overall reaction stoichiometry having been established by conventional methods, the first task of chemical kinetics is essentially the qualitative one of establishing the kinetic scheme in other words, the overall reaction is to be decomposed into its elementary reactions. This is not a trivial problem, nor is there a general solution to it. Much of Chapter 3 deals with this issue. At this point it is sufficient to note that evidence of the presence of an intermediate is often critical to an efficient solution. Modem analytical techniques have greatly assisted in the detection of reactive intermediates. A nice example is provided by a study of the pyridine-catalyzed hydrolysis of acetic anhydride. Other kinetic evidence supported the existence of an intermediate, presumably the acetylpyridinium ion, in this reaction, but it had not been detected directly. Fersht and Jencks observed (on a time scale of tenths of a second) the rise and then fall in absorbance of a solution of acetic anhydride upon treatment with pyridine. This requires that the overall reaction be composed of at least two steps, and the accepted kinetic scheme is as follows. 

The carbides of the lanthanoids and actinoids can be prepared by heating M2O3 with C in an electric furnace or by arc-melting compressed pellets of the elements in an inert atmosphere. They contain the C2 unit and have a stoichiometry MC2 or M4(C2)3. MC2 have the CaC2 structure or a related one of lower symmetry in which the C2 units lie at right-angles to the c-axis of an orthogonal NaCl-type cell. They are more reactive than the alkaline-earth metal... 

The seven known sulfur fluorides are quite different from the other halides of sulfur in their stability, reactivity and to some extent even in their stoichiometries it is therefore convenient to... 

Stoichiometry in Reactive Systems. The use of molar units is preferred in chemical process calculations since the stoichiometry of a chemical reaction is always interpreted in terms of the number of molecules or number of moles. A stoichiometric equation is a balanced representation that indicates the relative proportions in which the reactants and products partake in a given reaction. For example, the following stoichiometric equation represents the combustion of propane in oxygen ... 

The number-average molecular weight of a polymer may be controlled by offsetting the stoichiometry of two dissimilar mutually reactive difunctional monomers. The polymer will have the same endgroup functionality as that of the monomer used in excess. For a generic polymer made from a difunctional monomer AA with A functional groups and an excess of difunctional monomer BB widt B functional groups, r is defined as... 

Linear step-growth polymerizations require exceptionally pure monomers in order to ensure 1 1 stoichiometry for mutually reactive functional groups. For example, the synthesis of high-molecular-weight polyamides requires a 1 1 molar ratio of a dicarboxylic acid and a diamine. In many commercial processes, the polymerization process is designed to ensure perfect functional group stoichiometry. For example, commercial polyesterification processes often utilize dimethyl terephthalate (DMT) in the presence of excess ethylene glycol (EG) to form the stoichiometric precursor bis(hydroxyethyl)terephthalate (BHET) in situ. 

Step-growth polymerization processes must be carefully designed in order to avoid reaction conditions that promote deleterious side reactions that may result in the loss of monomer functionality or the volatilization of monomers. For example, initial transesterification between DMT and EG is conducted in the presence of Lewis acid catalysts at temperatures (200°C) that do not result in the premature volatilization of EG (neat EG boiling point 197°C). In addition, polyurethane formation requires the absence of protic impurities such as water to avoid the premature formation of carbamic acids followed by decarboxylation and formation of the reactive amine.50 Thus, reaction conditions must be carefully chosen to avoid undesirable consumption of the functional groups, and 1 1 stoichiometry must be maintained throughout the polymerization process. 

These chemically reactive phases are prepared by slow cooling of melts with the appropriate composition under an inert atmosphere or vacuum. Equilibrium is slow to be attained at the low temperatures necessary to prevent dissociation at 6.9°C Na2K dissociates into a (solid solution of K in Na) and liquid (60/40 Na/K). The KjCs and K7CSJ phases are even less stable and result from cooling mixtures of the elements of the desired stoichiometry to — 100°C in a metal beaker under argon. ... 

In the general case of a piston flow reactor, one must solve a fairly small set of simultaneous, ordinary differential equations. The minimum set (of one) arises for a single, isothermal reaction. In principle, one extra equation must be added for each additional reaction. In practice, numerical solutions are somewhat easier to implement if a separate equation is written for each reactive component. This ensures that the stoichiometry is correct and keeps the physics and chemistry of the problem rather more transparent than when the reaction coordinate method is used to obtain the smallest possible set of differential... 

Self-condensing monomers of the form AMB avoid a stoichiometry restriction. The idea is to synthesize the monomer using conditions where the A and B endgroups are not reactive but to polymerize the monomer under conditions where the endgroups do react. An important example is the polymerization of PET. A large excess of ethylene glycol is reacted with terephthalic acid to form a nominal trimer, diglycol terephthalate ... 

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