Chemical reaction parameter

For the design and engineering of technical biphasic processes, it is necessary to gain insight into crucial physical and chemical reaction parameters [79] ... 

M — chemical reaction parameter — Da i/( l)2 (for first-order reactions)... 

Chemical reaction parameter (first-order reactions) 395 ... 

Af chemical reaction parameter D k,/(kt) (for fiist-order reactions)... 

The chemical response in the fuel processor is usually slow. It is associated with the time to change the chemical reaction parameters after a change in the flow of reactants. This dynamic response function is modeled as a first-order transfer function with a 5-s time constant. 

Whether the powder, which forms the starting point of the sample preparation, has been prepared in the laboratory (e.g., by the precipitation of hydroxides and their subsequent dehydration or by the oxidation of metal spheres or is purchased from a supplier, for the sake of reproducibility it is desirable to conduct series of measurements with particular batches of material. Substantial batch to batch variations have been found in the chemical reaction parameters of chemically identical (analytical grade) powders provided by the same supplier [3]. 

The key to experimental gas-phase kinetics arises from the measurement of time, concentration, and temperature. Chemical kinetics is closely linked to time-dependent observation of concentration or amount of substance. Temperature is the most important single statistical parameter influencing the rates of chemical reactions (see chapter A3.4 for definitions and fiindamentals). 

Chemical reactions can be studied at the single-molecule level by measuring the fluorescence lifetime of an excited state that can undergo reaction in competition with fluorescence. Reactions involving electron transfer (section C3.2) are among the most accessible via such teclmiques, and are particularly attractive candidates for study as a means of testing relationships between charge-transfer optical spectra and electron-transfer rates. If the physical parameters that detennine the reaction probability, such as overlap between the donor and acceptor orbitals. 

The differential material balances contain a large number of physical parameters describing the structure of the porous medium, the physical properties of the gaseous mixture diffusing through it, the kinetics of the chemical reaction and the composition and pressure of the reactant mixture outside the pellet. In such circumstances it Is always valuable to assemble the physical parameters into a smaller number of Independent dimensionless groups, and this Is best done by writing the balance equations themselves in dimensionless form. The relevant equations are (11.20), (11.21), (11.22), (11.23), (11.16) and the expression (11.27) for the effectiveness factor. 

A second requirement is that the rate law for the chemical reaction must be known for the period in which measurements are made. In addition, the rate law should allow the kinetic parameters of interest, such as rate constants and concentrations, to be easily estimated. For example, the rate law for a reaction that is first order in the concentration of the analyte. A, is expressed as... 

Sonochemistry is strongly affected by a variety of external variables, including acoustic frequency, acoustic intensity, bulk temperature, static pressure, ambient gas, and solvent (47). These are the important parameters which need consideration in the effective appHcation of ultrasound to chemical reactions. The origin of these influences is easily understood in terms of the hot-spot mechanism of sonochemistry. 

Knock is caused by unwanted chemical reactions in the combustion chamber. These reactions are a function of the specific chemical species which make up the fuel and the environmental conditions to which the fuel is subjected during the compression and power stroke in the engine. Therefore, both the chemical makeup of the fuel and the engine design parameters must be considered when trying to understand knock. 

Another parameter of relevance to some device appHcations is the absorption characteristics of the films. Because the k quantum is no longer vaUd for amorphous semiconductors, i -Si H exhibits a direct band gap (- 1.70 eV) in contrast to the indirect band gap nature in crystalline Si. Therefore, i -Si H possesses a high absorption coefficient such that to fully absorb the visible portion of the sun s spectmm only 1 p.m is required in comparison with >100 fim for crystalline Si Further improvements in the material are expected to result from a better understanding of the relationship between the processing conditions and the specific chemical reactions taking place in the plasma and at the surfaces which promote film growth. 

For many laboratoiy studies, a suitable reactor is a cell with independent agitation of each phase and an undisturbed interface of known area, like the item shown in Fig. 23-29d, Whether a rate process is controlled by a mass-transfer rate or a chemical reaction rate sometimes can be identified by simple parameters. When agitation is sufficient to produce a homogeneous dispersion and the rate varies with further increases of agitation, mass-transfer rates are likely to be significant. The effect of change in temperature is a major criterion-, a rise of 10°C (18°F) normally raises the rate of a chemical reaction by a factor of 2 to 3, but the mass-transfer rate by much less. There may be instances, however, where the combined effect on chemical equilibrium, diffusivity, viscosity, and surface tension also may give a comparable enhancement. 

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