Defining Stoichiometry

COOKES The amount of each reactant present at the start of a chemical reaction determines how much product can form. 

Real-World Reading Link Have you ever watched a candle burning You might have watched the candle burn out as the last of the wax was used up. 

maybe you used a candle snuffer to put out the flame. Either way, when the candle stopped burning, the combustion reaction ended. 

Relationships Derived from a Balanced Chemical Equation 

Interactive Table Explore balanced chemical equations at qlencoe.com. 

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First reported by Fredenhagen in 1926 F3, F4), the graphite-alkali-metal compounds possess a relative simplicity with respect to other intercalation compounds. To the physicist, their uncomplicated structure and well defined stoichiometry permit reasonable band-structure calculations to be made S2,12) to the chemist, their identity as solid, "infinite radical-anions frequently allows their useful chemical substitution for such homogeneous, molecular-basis reductants as alkali metal-amines and aromatic radical anions N2, B5). 

A number of synthetic procedures are available (Ai2). (2) For precisely defined stoichiometries, the isobaric, two-bulb method of Herold is preferred H5, H6, H2). (2) To generate compounds suitable for organic synthesis work, graphite and alkali metal may be directly combined, and heated under inert gas (Pl, lA). (5) Electrolysis of fused melts has been reported to be effective iN2). 4) Although alkali metal -amine solutions will react with graphite, solvent molecules co-inter-calate with the alkali metal. Utilization of alkali metal-aromatic radical anion solutions suffers the same problem. 

Complex ions, also called coordination complexes, have well-defined stoichiometries and structural arrangements. Usually, the formula of a coordination complex is enclosed in brackets to show that the metal and all its ligands form a single structural entity. When an ionic coordination complex is isolated from aqueous solution, the product is composed of the complex ion and enough counter-ions to give a neutral salt. In the chemical formula, the counter-ions are shown outside the brackets. Examples include the sulfate salt of [Ni (NH3)g, ... 

If the electrolysis parameters (precursor concentrations, pH, temperature, cur-rent/potential, substrate) be defined in a precise manner, a self-regulated growth of the compound can be established, and highly (111 )-oriented zinc blende (ZB) deposits up to several p,m thickness are obtained at potentials lying at the anodic limit of the diffusion range (Fig. 3.3) [60]. Currently, the typical method of cathodic electrodeposition has been developed to yield quite compact and coherent, polycrystalline, ZB n-CdSe films of well-defined stoichiometry. The intensity of the preferred ZB(f 11) orientation obtained with as-deposited CdSe/Ni samples has been quite high [61]. 

One difficulty with many synthetic preparations of semiconductor NCs that complicates any interpretation of NMR results is the inevitable distribution of sizes (and exact shapes or surface morphologies). Therefore attempts to make semiconductors as a sort of molecular cluster having a well-defined stoichiometry are of interest to learn potentially about size-dependent NMR parameters and other properties. One approach is to confine the semiconductor inside a template, for instance the cuboctahedral cages of the sodalite framework or other zeolite structures, which have been characterized by multinuclear NMR methods [345-347], including the mesoporous channel material MCM-41 [341, 348]. 

We chose to show the application of techniques employed to study supramolecular dynamics to host systems that have defined binding sites and, therefore, form host-guest complexes with defined stoichiometries. CDs were chosen because they represent a host with only one binding site, and therefore can be viewed as a model system for hosts with the lowest degree of complexity. DNA was chosen because it provides multiple binding sites for small molecules, i.e. intercalative and groove binding, while the DNA is structurally fairly well defined. 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

Alkali metal doping of Cjq is also possible by solution-phase techniques [1,118-121]. K CgQ and Rb CgQ containing small fractions of the superconducting M3C50 phases were prepared by allowing toluene solutions of Cjq to react with the alkali metal [118, 119]. During the reaction, the alkali metal fullerides form a black precipitate. In another example, sonication of a solution of Cjq and excess potassium in TMEDA yields K3C5q(THF)j4 with a defined stoichiometry [104],... 

The deposit does not contain sulfur in a well-defined stoichiometry but is a mixture of different nickel sulfides, in particular Ni2S3, and amorphous nickel (72). It is not really clear whether the catalytic activity is due to amorphous nickel [compare Kreysa (55)] or to Ni2S3. A comparison of the... 

The chemiosmotic hypothesis had the great virtue of predicting the following consequences which could be tested (1) electron-transport driven proton pumps with defined stoichiometries and (2) a separate ATP synthase, which could be driven by a pH gradient or membrane potential. Mitchell s hypothesis was initially greeted with skepticism but it encouraged many people, including Mitchell and his associate Jennifer Moyle, to test these predictions, which were soon found to be correct.178... 

Solutions of copper(I) salts such as CuCl or Cu(CF3S03) readily absorb alkenes but it is frequently difficult to isolate crystalline solids with well defined stoichiometries, unless more traditional ligands are present, such as dien or pyridine.256"259 Complexes may alternatively be prepared from the copper(II) salt in ethanol in the presence of an alkene by a suitable reducing agent (see Figures 1-3). If ethylene is added to an aqueous solution of copper metal and... 

In the Hn phase a defined stoichiometry of phospholipid and a-tocopherol molecules exists [79]. 

The electronic spectra of the solid complexes are practically identical to those of the same complexes in aqueous solution101), as reported in Table 8. This means that, contrary to all previously described complexes of metal ions with polymeric ligands, the poly(amido-amines) complex compounds have a well defined stoichiometry. 

Case 6 Partial formation of solid solutions (Fig. 8). A more extensive reaction between a precursor and the support is often observed. This can lead to the formation of solid solutions of the supported element in the support. Other compounds of ill-defined stoichiometry can also form between the two partners. This can occur during the precipitation stage, by partial dissolution, and by precipitation, as shown in very recent work [31]. A still more extensive reaction can occur during calcination aimed at producing supported oxide precursors. 

A molecular complex may be dissociated in the ground state, and yet be associated in an excited electronic state. An excited molecular complex of defined stoichiometry, which dissociates in the ground state, is known as an exciplex [110,111], the term being derived from exci (ted comp) plex by analogy to excimer (= exci (ted di) mer ). 

The preparation of starch blue in aqueous solution with iodine-iodide mixtures is the most common procedures. According to the method of Pals56 starch (10 mg) of water (5 mL) is boiled for 5 min and blended with 7.6 X 10 5 mol/L (5 mL) of an aqueous solution of iodine and 4.27 x 10 5 mol/L (5 mL) of an aqueous solution of KI. The complex is precipitated by the addition of an aqueous solution of KF (10 mol/L). The precipitate is centrifuged (5000 rpm for 10 min) and washed with aqueous KF (1 mol/L). The Vanino reagent (barium permanganate) also precipitates iodized starch.57 The resulting complex is stable and has a defined stoichiometry. A similar procedure was described by Meyer and Bemfeld.58 Chinoy... 

The story begins in this chapter with the clusters of simplest geometric and electronic structure. These are clusters of p-block elements with defined stoichiometry and structure in which the cluster surface-atom valences are terminated with ligands. The large number known provide the factual base from which clever people have derived models that connect atomic composition with structure. In turn, these p-block models provide a foundation on which to build an understanding of more complex clusters such as condensed clusters, bare clusters and transition-metal clusters. A more comprehensive account of the structural chemistry will be found in older books and reviews, a selection of which will be found in the reading list at the end of each chapter. 

Almost all of the work diich has been done to date has involved a substrate of loosely defined stoichiometry, that is to say, an experimentally treated polymer surface of unknown composition. In several cases indicated, chemical reaction produced addition of an oiq gen species, which was not expected. For instance, no oxygen-containing reagent was used in the sodium-treated PTFE or in some of N2 or Ar corona-treated low-density polyethylene, yet copious amounts of oxygenated species were formed. What is needed for a method that can be said to quantitatively label the surface of a polymer is outlined below. Method Requirements for quantitative derivatization of polymers ... 

These reactions may be carried out under mild conditions (60°, benzene, 4-8 hr) and are clean, in that no colloidal iron or other iron carbonyls are produced. The work-up is thus much easier, and the reactions can be monitored by infrared spectroscopy. In addition, the well-defined stoichiometry of the displacement is in marked contrast to the uncertain stoichiometry of the reactions of the iron carbonyls, which are commonly used in large excess. 

Complexation is the association between two or more molecules to form a nonbonded entity with a well-defined stoichiometry. The two types of complexation that are most useful for increasing the solubility of drugs in aqueous media are stacking and inclusion. Stacking complexes are formed by the overlap of the planar regions of aromatic molecules, while inclusion complexes are formed by the insertion of the nonpolar region of one molecule into the cavity of another molecule (or group of molecules). 

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