Reaction stoichiometry simultaneous reactions

In calculating the metallic surface area, one has to take proper care of the reaction stoichiometry. In the ideal case, a molecule occupies one site, as shown for terminal adsorbed CO in Fig. 3.46.a. Alternatively, a molecule may chemisorb on more than one metal atom, as shown in Fig. 3.46.b and c for bridged-site adsorbed CO and in Fig. 3.46.d for valley-site adsorbed CO, respectively. In some specific cases of really big molecules, one can imagine that a molecule adsorbs on only one site, while simultaneously blocking adjacent sites for geometric reasons. In case an adsorbate molecule adsorbs dissociatively, it will occupy more than one site as shown in Fig. 3.46.e. 

As(in), a reaction that takes place too slowly to be of direct value unless catalyzed. Moreover, the mechanisms are often obscure. For example, the reduction of Mn(VII) to Mn( is complex. A further complication in redox reactions has to do with the stoichiometry frequently, several redox reactions occur simultaneously. For a slow reaction, a catalyst often can be found to accelerate it. For a reaction where the stoichiometry is unsatisfactory, a more complete understanding of the mechanism often can lead to selection of conditions under which it (a single reaction) can be made to proceed quantitatively. 

The H atom is retained on the lefthand side of this stoichiometric equation to emphasis its role (because its concentration occurs explicitly in the rate law). Thus, the overall reaction can be regarded as the conversion of H2 to 2H, catalyzed by H and with the simultaneous conversion of 2H2 + O2 to 2H2O. The latter provides the free energy driving-force for the process. In terms of the reaction stoichiometry, the stoichiometric coefficient for H atoms is now -1-2 (i.e.,-1-3-1) so that H is a product but also appears with a positive exponent (reaction order) in the rate expression. Thus, it clearly plays a role that parallels B in the simplified representation with A corresponding to O2 as the latter appears as the reactant in the rate law. 

Two types of precursor can be used in CVD of superconducting mixed oxides metal halides and metal-organics The use of metal-organic molecules results in some carbon in the films, which can hinder performance, so metal halides gained popularity in early work. Halide precursors call for a much higher deposition temperature. In the case of metal halide reactions, the chemistry follows that presented earlier (see 17.2.5.2.2) but with multiple reactions occurring simultaneously. The gas phase composition and temperature are controlled to obtain the desired stoichiometry. An example of such a reaction would be that used to make Bi2Sr2CaCu20s+ c(or Bi-2212) at 760-820°C ... 

Equations (1-85) also apply to this problem however, there are three additional unknown quantities Nv yl8, and y28. The mole fractions at the interface (z = 8) cannot be specified arbitrarily, but are fixed by the reaction stoichiometry. Thus, the interface mole fractions must be determined simultaneously with the molar fluxes. The flux of... 

Frequendy, it is not easy to determine the temperatures Ti,T2,..., owing to the nearly horizontal nature of the curve in those regions and the attendant indistinct beginning and ending temperatures of the reaction steps. This means also that it is not easy to determine accurately the mass loss in such cases. Because of this, some instruments also compute the trace of the first derivative of the mass curve simultaneously. This DTG (derivative thermogravimetric analysis) makes it much easier to determine where a zero slope indicates that the sample is undergoing no change in mass. Therefore, the reaction stoichiometry can be more accurately determined. 

In this section we consider a few additional factors in reaction stoichiometry— both in the laboratory and in the manufacturing plant. First, the calculated outcome of a reaction may not be what is actually observed. Specifically, the amount of product may be, unavoidably, less than expected. Second, the route to producing a desired chemical may require several reactions carried out in sequence. And third, in some cases two or more reactions may occur simultaneously. 

The essential information implied by the chemical equation is the stoichiometry at the macroscopic level, ie, if a moles of M react, then b moles of B do also p moles of P are formed, etc. No inference should be made about behavior at the microscopic or atomic level, ie, there is no implication thatp molecules of P appear simultaneously. There may or may not be intermediates that appear and disappear in the course of the reaction. 

The reformer outlet composition is deterrnined by an approach to the simultaneous equiUbria of reactions 3 and 4, where m = 2n + 2 represents the paraffinic nature of natural gas. The stoichiometry of the reformed gas can be conveniently characterized by the ratio R, where... 

Although the reaction has the overall stoichiometry of a dehydration it is more complex than this and involves a mutual redox reaction between N and N. This is at once explicable in terms of the volt-equivalent diagram in Fig. 11.9 which also interprets why NO and N2 are formed simultaneously as byproducts. It is probable that the mechanism involves dissociation of NH4NO3 into NH3 and HNO3, followed by autoprotolysis of HNO3 to give N02, which is the key intermediate ... 

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... 

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