Stoichiometry term explained

Haight et al. ° have published a detailed account of the kinetics and stoichiometry of the oxidation of buffered bisulphite ion by chromic acid. The reaction is fast and its study requires a rapid mixing technique. The stoichiometry varies from a Cr(VI)/S(IV) molar ratio of 1 2 to 2 3 as the initial concentrations are changed in the range 0.12 < [Cr(VI)]/[S(IV)] < 1.4 and this was explained in terms of competition between two overall reactions... 

It is not currently possible to examine the configuration of the adsorbed species unambiguously. However, since thermodynamic arguments do not require a specific model at the molecular level, it is still possible to analyze equilibrium data within a thermodynamic context. Most surface reactions are inferred from experimental observations of reaction stoichiometries and perhaps only in a limited range of T. Consequently, the choice of specific surface species is dependent on two considerations (1) the need to explain the observed measurements in terms of reaction stoichiometries, and (2) the selection of a model to allow the representation of metal/ surface site interaction intensities. 

Since the experimental kinetic data refer to a reaction rate and how this is affected by variables, such as concentration, temperature, nature of the solvent, presence of other solutes, structural variations of the reactants, and so forth, the assignment of a mechanism is always only indirectly derived from primary data. Therefore, it is not surprising that more than one mechanism has often been proposed to explain the same rate law and that reaction mechanisms, which were once consistent with all experimental information available on a system, have later on been considered erroneous and have been disregarded, or drastically modified, as long as new experimental evidence was accumulated. In general, the stoichiometry of the reaction, even when this is a simple one, cannot be directly related with its mechanism, and when the reaction occurs through a series of elementary steps, the possibility that the experimental rate law may be interpreted in terms of alternative mechanism increases. Therefore, to resolve ambiguities as much as possible, one must use aU the physicochemical information available on the system. Particularly useful here is information on the structural relations between the reactants, the intermediate, and the reaction products. 

The term stoichiometry is often used and is well understood in Chemistry, and the law of definite proportions and the law of multiple proportion are well-known examples deduced from the stoichiometric relation. The existence of non-stoichiometric compounds cannot be explained by a simple interpretation of the above mentioned laws, however, it is no exaggeration that all inorganic compounds exhibit non-stoichiometry. 

For our present purposes, we use the term reaction mechanism to mean a set of simple or elementary chemical reactions which, when combined, are sufficient to explain (i) the products and stoichiometry of the overall chemical reaction, (ii) any intermediates observed during the progress of the reaction and (iii) the kinetics of the process. Each of these elementary steps, at least in solution, is invariably unimolecular or bimolecular and, in isolation, will necessarilybe kinetically first or second order. In contrast, the kinetic order of each reaction component (i.e. the exponent of each concentration term in the rate equation) in the observed chemical reaction does not necessarily coincide with its stoichiometric coefficient in the overall balanced chemical equation. 

Throughout all the preceding chapters, we have discussed thin films that can be created by chemical vapor deposition in terms of their physical and chemical attributes. However, we did not explain how we secured the necessary physical or chemical information. For example, we discussed film deposition rates many times, but did not explain how we knew the film thickness after a specified amount of time. Similarly, when we spoke of the stoichiometry of deposited composite films, we did not indicate how we determined their chemical composition. 

There remains model 4, and MacQuarrie and Bernhard 175) have utilized the full-site reactivity by iodoacetamide and half-site reactivity by FAP to provide support for this model. Thus, di(2-furylacryloyl) enzyme was prepared, and the two remaining sites were blocked with iodoacetate. Acyl groups were then removed from this derivative by arsenolysis, and the resulting dialkyl enzyme was tested for stoichiometry with FAP. Only one acyl group could be incorporated into the dialkyl enzyme. This result cannot be explained in terms of an induced asymmetry model, and indeed, can only be explained by a preexisting asymmetry model if there is a subunit rearrangement. In addition, alkylation of the enzyme with varying quantities of iodoacetate, followed by acylation of these derivatives with FAP, showed a 2 1 ratio of alkylation to acylation, independent... 

Solute-solvent complexes of different stoichiometry have been observed between all the D-A compounds under consideration and various solvent molecules. Some of the clusters show structured excitation spectra and a narrow short-wave emission that has been assigned to the primary excited state of the vdW complex. The broad, red-shifted emission of other clusters can be explained in terms of the transformation of the vdW complexes of stoichiometry l n (n > 0) into excimers or the transition into an intramolecular CT state of the D-A chromophore which is induced by its polar partner(s) (for the complexes of stoichiometry 1 ). The main conclusion from the fluorescence behaviour of the jet-cooled vdW clusters is that dual luminescence is obviously connected with the preference of specific solute-solvent geometries. 

Applying Concepts When yonr campfire begins to die down and smolder, it helps to fan it. Explain in terms of stoichiometry why the fire begins to flare np again. 

The examples shown is Section D indicate that the shape of calculated uptake curves (slope, ionic strength effect) can be to some degree adjusted by the choice of the model of specific adsorption (electrostatic position of the specifically adsorbed species and the number of protons released per one adsorbed cation or coadsorbed with one adsorbed anion) on the one hand, and by the choice of the model of primary surface charging on the other. Indeed, in some systems, models with one surface species involving only the surface site(s) and the specifically adsorbed ion successfully explain the experimental results. For example, Rietra et al. [103] interpreted uptake, proton stoichiometry and electrokinetic data for sulfate sorption on goethite in terms of one surface species, Monodentate character of this species is supported by the spectroscopic data and by the best-fit charge distribution (/si0,18, vide infra). 

When components in solid and fluid phases react, the sequence of steps must be similar to those for fluid-solid catalytic reactions. In Sec. 8-2 catalytic reactions were explained in terms of a three-step process adsorption of the fluid molecule on the sohd surface reaction on the surface involving the adsorbed molecule, and desorption of product. We shall not be concerned here with the mechanism of these processes instead we shall start out on the basis that the rate equation is known from experimental measurements. Often observed data agree with a rate equation which is first order in concentration of the reactant injthe-fluid..phase and directly proportional to the surface of the reactant in the solid phase. For example, despite the stoichiometry of the reaction... 

As pointed out above, the desorption order markedly effects the shape of the desorption curve and the behaviour of the peak temperature with variation of initial coverage. Zero-order kinetics are shown by an increase in peak temperature with coverage and zero-order surface processes have now been observed for many systems [281—286]. Schwartz et al. [287] performed isothermal desorption measurements on the H2/Ti system and determined an order of 1.5, explaining this finding in terms of surface compound formation, with a stoichiometry of TiHl s. A very good example of the confusion which can reign in this field is exhibited by the... 

For a non-stolchiometrlc crystal, the concentration of each point defect, in each conjugate pair, is no longer equal. If there is an excess of Vm, , or Xx, then the compound will have a surplus of X (or deficiency of M, which is the same thing) over the ideal stoichiometric composition. This is called a positive deviation firom stoichiometry. Conversely, for a negative deviation, there will be an excess of Vx, Mi, or Mm. This explains the plus and minus in equation 2.6.9. In terms of the above given defects, 6 may be expressed as shown in the following Table ... 

Murakami et al. have synthesized numerous substituted [20]- and [lO.lOJparacyclo-phanes like 70 Because of low solubility and a high tendency of aggregation, a complexation with clear stoichiometry and geometry has not been achieved. Nevertheless, a remarkable acceleration of the hydrolysis of p-nitrophenol esters has been observed, which is explained in terms of the formation of inclusion complexes between host and substrate. 

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