Reactions geometry

Wang 0, Akhremitchev B and Walker G 0 1997 Femtosecond infrared and visible spectroscopy of photoinduced intermolecular electron transfer dynamics and solvent-solute reaction geometries Coumarin 337 in dimethylaniline J. Rhys. Chem. A 101 2735-8... 

It is seen from these examples that, in appropriate systems, it is possible to introduce product into the reactant in such a manner that an effective reaction interface is established before the reactant has been heated to the decomposition temperature. Accordingly, the induction period is removed and the acceleratory process may be less pronounced or eliminated altogether. Artificial nucleation results in changes in reaction geometry with consequent variation in the a—time curve shape and the maximum value of da/dt but does not enhance the rate of interface advance. We have found no studies in which increases in reaction rates were quantitatively correlated with the increased interfacial area and/or density of nucleation which resulted from the reactant pretreatment. 

The analysis of fluid-solid reactions is easier when the particle geometry is independent of the extent of reaction. Table 11.6 lists some situations where this assumption is reasonable. However, even when the reaction geometry is fixed, moving boundary problems and sharp reaction fronts are the general rule for fluid-solid reactions. The next few examples explore this point. 

T. Bartsch, T. Uzer, and R. Hernandez, Stochastic transition states reaction geometry amidst noise, J. Chem. Phys. 123, 204102 (2005). 

Let us begin the discussion of the last example of solid state kinetics in this introductory chapter with the assumption of local equilibrium at the A/AB and AB/B interfaces of the A/AB/B reaction couple (Fig. 1-5). Let us further assume that the reaction geometry is linear and the interfaces between the reactants and the product AB are planar. Later it will be shown that under these assumptions, the (moving) interfaces are morphologically stable during reaction. 

For reactions of the type A + B = AB (or a+P = y), the situation is different. If one has a linear reaction geometry and the y product forms at different times and locations on the A/B interface, the patches of y eventually merge by fast lateral (interface) transport. Eventually, a full y layer is formed between a and / . At first, this layer has a non-uniform thickness (Fig. 6-4). In Chapter 11 we will show, however, that the uneven a/y and y/p interfaces are morphologically stable and become smooth during further growth. This leads to constant boundary conditions for the y formation after some time of reaction and eventually results in a parabolic rate law, as will be discussed later. 

P. Beak, Determinations of Transition-State Geometries by the Endocyclic Restriction Test Mechanism of Substitution at Nonstereogenic Atoms, Acc. Chem. Res. 1992, 25, 215-222 P. Beak, Mechanisms of Reactions at Nonstereogenic Heteroatoms Evaluation of Reaction Geometries by the Endocyclic Restriction Test, PureAppl. Chem. 1993, 65, 611-615. 

P. Beak, Mechanisms of reactions at nonstereogenic heteroatoms evaluation of reaction geometries by the endocyclic restriction test, Pure Appl. Chem. 1993, 65, 611-615. 

Figure 15.12 (A) Coverslip chemiluminescence reaction geometry scheme. Imaging chambers are affixed to No. 1 coverslips and filled with 6 uL of chemiluminescent material (blue circle). Coverslips are positioned on plain glass substrates and glass substrates modified with an aluminium triangle (12.3-mm length 75 nm thick 1 mm -gap size for two triangles geometries (insets, middle) (B) Enhancement is calculated as the ratio of diemiluminescence, with and wiftiout die coverslip. Adapted from Anal Chem 79 7042-7052 (2007).

Solid state decompositions Interpretation of observations and formulation of mechanisms (both reaction geometry and interface chemistry). A check list. 

The kinetics of several well-known electrochemical reactions have been studied in the presence of an ultrasonic field by Altukhov et al. [142], The anodic polarization curves of Ag, Cu, Fe, Cd, and Zn in various solutions of HC1 and H2S04 and their salts were measured in an ultrasonic field at various intensities. The effect of the ultrasonic field on the reaction kinetics was found to be dependent on the mechanism of metal anodic dissolution, especially on the effect of this field on the rate-determining step of the reaction. The results showed that the limiting factor of the anodic dissolving of Cu and Ag is the diffusion of reaction products, while in the case of Fe it is the desorption of anions of solution from the anode surface, and at Cd the limiting factor is the rate of destruction of the crystal lattice. Similar results were obtained by Elliot et al. [ 143] who studied reaction geometry in the oxidation and reduction of an alkaline silver electrode. 

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