Reaction mechanism diffusion-controlled

These observations are consistent with the proposed mechanism of the reaction being diffusion controlled in the laminar flow regime. The mass transport is aided by the velocity gradient and thus the reaction rate increases as the Reynolds number is increased. 

The mechanisms described above similarly apply to the case of desorption with reaction (i.e., where the product of a liquid-phase reaction is volatile and desorbs in the gas phase). The word absorption in the above discussion will be replaced by the word desorption for this case. In most practical situations, more than one reaction occurs simultaneously. Under these situations, the terms "slow, fast, and instantaneous are applied to each reaction individually. Although the terms slow, fast, and "instantaneous reactions (or diffusion-controlled and mass-transfer-controlled regimes) are discussed with respect to gas-liquid reactions, they can also be applied to gas liquid-solid reactions, where the solid is either a catalyst or a reactant. 

Precipitate layer and leached layer dissolution mechanisms can also cause nonlinear dissolution rates. The precipitate layer hypothesis proposes that a more or less coherent, contiguous layer of secondary product precipitates on the reactant surfaces of the mineral that is dissolving. Consequently, reactants released from the surface must diffuse through the precipitate layer to reach the bulk solution. As the layer thickens, the diffusional path lengthens, and the integrated rate of diffusion decreases. If the integrated rate of diffusion of reaction products from the surface is lower than the rate of reaction at the surface of the primary mineral, then the dissolution reaction becomes diffusion controlled. 

As a sufficiently negative tip potential both ET reactions are diffusion controlled, and the rate of the overall process is limited by heterogeneous reaction (33b). Its rate constant can be determined from the current-distance curves as discussed in Sec. II. The kinetic analysis of a more complicated ECE mechanism can be reduced to the measurement of an effective heterogeneous rate constant at the ITIES. 

There is substantial evidence that R- diffuses in solution (.Sections 7.2,8 7.2,9). Consider the following mechanism in which -MgX also diffuses in solution. Here the products are formed in radical-radical reactions. Coupling/disproportio-nation c of R- is diffusion controlled or nearly so. In order for c not to dominate the products, the cross-coupling r of R- w ith -MgX wtiuld have to be competilive with c, that is. also (nearly) diffusion controlled. If all of these ratiical-rtKlical reactions were diffusion controlled, then ihc y ield ol RMgX... 

At very high bulk gas pressures, the ionic recombination reaction becomes diffusion controlled. The theory of this recombination mechanism was developed by Langevin [74] and Harper [75] who obtained the relationship... 

In steady-state voltammetry experiments, enzyme activity is viewed in the potential domain that can pinpoint the role of centers as electron relays, or reveal the presence of internal control mechanisms, such as a redox transformation that causes the enzyme to switch ofF at a certain potential. Such studies can also reveal and quantify how an enzyme is redox-biased to favor catalysis in a particular direction. Figure 6(b) shows the voltammetry of a film of fumarate reductase obtained in the presence of a low concentration of fumarate and a high concentration of succinate, from which it is easily seen how the catalytic activity of the enzyme is biased heavily in the direction of fumarate reduction [38]. This experiment has been carried out with a rotating disc electrode. The current for succinate oxidation is independent of rotation rate, while that for fumarate reduction is very sensitive because the reaction is diffusion controlled. 

Several investigations have reported a parabolic growth rate for the reaction layer, which is usually taken to mean that the reaction is diffusion controlled (32). The reaction between ZnO and FeiOs to form ZnFe204 is reported to occur by the counterdiffusion mechanism in which the cations migrate in opposite directions and the oxygen ions remain essentially stationary (33,34). The reaction... 

Kinetic studies have shown that the enolate and phosphorus nucleophiles all react at about the same rate. This suggests that the only step directly involving the nucleophile (step 2 of the propagation sequence) occurs at essentially the diffusion-controlled rate so that there is little selectivity among the individual nucleophiles. The synthetic potential of the reaction lies in the fact that other substituents which activate the halide to substitution are not required in this reaction, in contrast to aromatic nucleophilic substitution which proceeds by an addition-elimination mechanism (see Seetion 10.5). 

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

Reactions of the general type A + B -> AB may proceed by a nucleation and diffusion-controlled growth process. Welch [111] discusses one possible mechanism whereby A is accepted as solid solution into crystalline B and reacts to precipitate AB product preferentially in the vicinity of the interface with A, since the concentration is expected to be greatest here. There may be an initial induction period during solid solution formation prior to the onset of product phase precipitation. Nuclei of AB are subsequently produced at surfaces of particles of B and growth may occur with or without maintained nucleation. 

While there is agreement that the rates of clay dehydroxylations are predominantly deceleratory and sensitive to PH2G, there is uncertainty as to whether these reactions are better represented by the first-order or by the diffusion-control kinetic expressions. In the absence of direct observational evidence of interface advance phenomena, it must be concluded that the presently available kinetic analyses do not provide an unambiguous identification of the reaction mechanisms. The factors which control the rates of dehydroxylation of these structurally related minerals have not been identified. 

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