Reagent concentration, effect

FIG. 14-12 Effects of reagent-concentration and reagent-conversion level upon the relative values Kca in the C02-Na0H-H 0 system. [Adapted from Eckeii et at, Ind. Eng. Chem., 59f2h 41 (1967).]... 

In certain cases this reduction (with lithium aluminum hydride) takes a different course, and olefins are formed. The effect is dependent on both the reagent concentration and the steric environment of the hydrazone. Dilute reagent and hindered hydrazone favor olefins borohydride gives the saturated hydrocarbon. The hydrogen picked up in olefin formation comes from solvent, and in full reduction one comes from hydride and the other from solvent. This was shown by deuteriation experiments with the hydrazone (150) ... 

Other considerations aside, the use of dilute reagents minimizes effects of nonideality. This allows the use of concentrations in place of activities. Of course, the time scale, the sensitivity of the analytical method at different concentrations, and the use of other reaction components introduce additional considerations. Tied closely to this decision is the choice of solvent. Reaction rates may (or may not) be affected by such variables as polarity, dielectric constant, hydrogen-bonding ability, donor capacity, and viscosity. A change in solvent may change not only the rate but also the mechanism and possibly even the products. One cannot even assume that the net reaction is the... 

Accident caused when using a solution at 15% of nitric acid in ethanol. These concentrations should never exceed 10%. These concentration effects are due to an inversion in the order in which reagents should be incorporated. 

The fact that WCirtz reactions take a long time to start with halogenous derivatives and sodium is due to a concentration effect which can lead to an acceleration of the reaction once the concentration of reagents is critical. It is aggravated by thermal phenomena. 

Inspection of Fig. 15.3 reveals that while for jo 0.1 nAcm , the effectiveness factor is expected to be close to 1, for a faster reaction with Jo 1 p,A cm , it will drop to about 0.2. This is the case of internal diffusion limitation, well known in heterogeneous catalysis, when the reagent concentration at the outer surface of the catalyst grains is equal to its volume concentration, but drops sharply inside the pores of the catalyst. In this context, it should be pointed out that when the pore size is decreased below about 50 nm, the predominant mechanism of mass transport is Knudsen diffusion [Malek and Coppens, 2003], with the diffusion coefficient being less than the Pick diffusion coefficient and dependent on the porosity and pore stmcture. Moreover, the discrete distribution of the catalytic particles in the CL may also affect the measured current owing to overlap of diffusion zones around closely positioned particles [Antoine et ah, 1998]. 

Since the kinetic constant is independent of the reagent concentration and of the extent of proton loss from the intermediate, the last step is surely not rate limiting and the formation of the intermediate is for this olefin completely rate determining. (20) Consistent with this hypothesis is the lack of a large primary kinetic isotope effect (KIE), kH/kD = 0.97 (0.01), expected for a slow L+ loss when the vinyl bromide is the main product. Taking into account that the rate determining step is the formation of a bromocarbenium-tribromide (bromide) ion pair and that the transition state should be closely related to the intermediate,... 

In previous sections, numerous examples of anion activation by cationic micelles and polysoaps were presented. The extent of rate augmentation— 102—lO -fold—cannot be rationalized in terms of concentration effects alone. We believe that these observations are explained most reasonably by the concept of the hydrophobic ion pair (Kunitake et al., 1976a). According to this concept, anionic reagents are activated probably due to desolvation when they form ion pairs with an ammonium moiety in a hydrophobic microenvironment. The activation of anionic species in the cationic micellar phase... 

The effects of various compounds and inhibitors on the enzyme activity were tested. The activity was measured in the presence of 1 -10 mM of various metal ions and compounds. The enzyme activity was inhibited by sulfhydryl reagents (concentration 1 mM) such as PbCl2 (relative activity, 18%), SnCl2 (17%),HgCl2... 

A continuous factor is a factor that can take on any value within a given domain. Similarly, a continuous response is a response that can take on any value within a given range. Examples of continuous factors are pressure, volume, weight, distance, time, current, flow rate, and reagent concentration. Examples of continuous responses are yield, profit, efficiency, effectiveness, impurity concentration, sensitivity, selectivity, and rate. 

Finally, Nudelman and coworkers examined the effects of temperature, reagent concentration, reaction time, a radical trap, light and solvent on the formation of radical byproduct, in the reaction of PhLi with fi-cinnamaldehyde. It was claimed that the PhLi dimer is the reacting species with the aldehyde and that the reaction is initiated by an FT from the PhLi dimer to the cinnamaldehyde. MO calculations at the MNDO level of theory were claimed to be consistent with the participation of a dimer species. ... 

For in situ geochemical fixation, site characterization data and reagent concentrations are more exacting than those required for a pump-and-treat approach. Therefore, the use of the system requires a detailed analysis of site conditions and careful system engineering. Site and contaminant characteristics have a large impact on treatment cost and effectiveness. 

Under such conditions, the effects of the rate of initiation, Vh the reagent concentration, and the oxygen pressure can be determined. A zero order was always obtained with respect to the oxygen at pressures varying from 0.2-1.5 atm. At low concentration, all of the reagents... 

The apparent order of Vp with respect to hydrocarbon depends somewhat on the solvent, as shown in Figure 1. In fert-butylbenzene, which is inert chemically and clearly resembles the xylenes, Vp is first order in hydrocarbon, but deviations occur in o-dichlorobenzene. This principle affects generally the choice of solvents (21). Thus, even in radical oxidation, where solvent effects are much weaker than in ionic reactions, proper choice of solvent is essential if kinetic laws are to be observed over a wide range of reagent concentrations. 

Closely related to the problem of the structure of the effective rate constant is the above-mentioned problem of the compensation mechanism. Without a knowledge of this mechanism, it would be impossible to understand why in such a complicated epoxyamine system one can frequently observe relatively simple kinetic principles, viz., a weak dependence of the effective rate constant on conversion, simple dependences of the initial rate on reagent concentrations, a linear dependence of the total heat release on conversion and almost equal values of the heat release and enthalpy of the epoxy ring opening. The latter two aspects have been discussed above, whereas the first two problems can be understood, say, from a consideration of a noncatalytic reaction. 

As expected, the effect of the increased viscosity during the reaction becomes more pronounced with decreasing A, /E0 ratio, where the subscript 0 refers to the reagent concentrations at t = 0 the limiting viscosity also increases. At Alo/E = 1.8,... 

Many thermal reactions are effectively irreversible under the conditions employed, but some are reversible and an equilibrium position is reached between substrates and products. The position of equilibrium depends on the standard free energy difference between the two (AC - = - RT In K and on reagent concentrations, and A varies with temperature. Such considerations rarely apply to photochemical reactions, the overwhelming maiority of which are effectively irreversible (1.3), and the products are not in thermodynamic... 

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