Chemical reaction rates competing reactions

Complexity in multiphase processes arises predominantly from the coupling of chemical reaction rates to mass transfer rates. Only in special circumstances does the overall reaction rate bear a simple relationship to the limiting chemical reaction rate. Thus, for studies of the chemical reaction mechanism, for which true chemical rates are required allied to known reactant concentrations at the reaction site, the study technique must properly differentiate the mass transfer and chemical reaction components of the overall rate. The coupling can be influenced by several physical factors, and may differently affect the desired process and undesired competing processes. Process selectivities, which are determined by relative chemical reaction rates (see Chapter 2), can thenbe modulated by the physical characteristics of the reaction system. These physical characteristics can be equilibrium related, in particular to reactant and product solubilities or distribution coefficients, or maybe related to the mass transfer properties imposed on the reaction by the flow properties of the system. 

The bottleneck of very short lifetimes of resonace states (10 14s) becomes less severe once one assumes that the primary role of resonance states is to provide doorways to bound valence anionic states, with lifetimes determined by kinetics of the following chemical reactions [36], The reactions might proceed on these regions of potential energy surfaces, at which valence anions are bound with respect to the neutral species. The rates of these chemical transformations, e.g., the SSB formation, do not have to compete with short lifetimes of resonance states. It is worth noting that even for a kinetic barrier of ca. 20 kcal/mol, the half lifetime amounts (at 298 K) to about 30 seconds. Hence, if the kinetic barrier for SSB formation were lower than 20-23 kcal/mol, all nucleotides that could form stable anions would have enough time to cleave the C-O bond on the timescale of the electrophoretic assay of DNA damage. 

In the oxidizing converter, it has been accepted that heat and mass transfer processes are the main controlling steps except at cold start. An analysis of the converter behaviour at stoichiometric conditions shows that this conclusion is not a general one, and that transfer processes and chemical reactions are competing in three-way converters. This suggests that more reliable kinetic rate expressions are required to obtain reliable simulation results. 

Simultaneous Absorption of Two Reacting Gases In multi-component physical absorption the presence of one gas often does not affect the rates of absorption of the other gases. When chemical reactions in which two or more gases are competing for the same hquid-phase reagent are involved, selectivity of absorption can be affected by... 

Micellar catalysis to enhance or diminish the rate of chemical reactions is well known [97]. Of somewhat greater interest is the influence of micelles on competing reactions, e.g., proton-catalyzed reactions. An example related to the effect of alkanesulfonates is the epoxidation of simple aliphatic olefins. The reaction of olefins and hydrogen peroxide catalyzed by strongly acidic Mo(VI)... 

The outcome of a chemical reaction can be determined by relative rates of competing reactions and by relative stabilities of the final products. 

For reactions in parallel, it is the fast step that governs. Thus, if A B and A C are two competing reactions, and if kAB kAC, the rate of formation of B is much higher than that of C, and very little C is produced. Chemical rates can vary by very large factors, particularly when different catalysts are involved. For example, a metal catalyst favors dehydrogenation of an alcohol to an aldehyde, but an oxide catalyst often favors dehydration. 

The much larger energy difference between Si and S0 than between any successive excited states means that, generally speaking, internal conversion between Si and S0 occurs more slowly than that between excited states. Therefore, irrespective of which upper excited state is initially produced by photon absorption, rapid internal conversion and vibrational relaxation processes mean that the excited-state molecule quickly relaxes to the Si(v0) state from which fluorescence and intersystem crossing compete effectively with internal conversion from Si. This is the basis of Kasha s rule, which states that because of the very rapid rate of deactivation to the lowest vibrational level of Si (or Td, luminescence emission and chemical reaction by excited molecules will always originate from the lowest vibrational level of Si or T ... 

The polarographic technique can be used to measure the rates of rapid reactions. Because an internal process is examined the problem of mixing is avoided, as it is in the relaxation and other non-flow methods. The rate of diffusion of a species (which can be oxidized or reduced) to an electrode surface competes with the rate of a chemical reaction of that species, for example... 

Quantitatively, many observed deviations from simple equilibrium processes can be interpreted as consequences of the various isotopic components having different rates of reaction. Isotope measurements taken during unidirectional chemical reactions always show a preferential emichment of the lighter isotope in the reaction products. The isotope fractionation introduced during the course of an unidirectional reaction may be considered in terms of the ratio of rate constants for the isotopic substances. Thus, for two competing isotopic reactions... 

To observe the kinetic effect of the chemical reaction of an electronically excited atom, the rate must be significantly different from that of the ground state atom, as spin orbit relaxation may compete with reaction. The separation of these processes is inherent in studies of the reactions of excited halogen atoms. Clearly, where a strong attractive potential facilitates chemical reaction, this chemical interaction will also aid spin orbit relaxation itself (Section VII). Thus relatively strongly endothermic, slow, chemical reaction... 

As pointed out earlier, CVD is a steady-state, but rarely equilibrium, process. It can thus be rate-limited by either mass transport (steps 2, 4, and 7) or chemical kinetics (steps 1 and 5 also steps 3 and 6, which can be described with kinetic-like expressions). What we seek from this model is an expression for the deposition rate, or growth rate of the thin film, on the substrate. The ideal deposition expression would be derived via analysis of all possible sequential and competing reactions in the reaction mechanism. This is typically not possible, however, due to the lack of activation or adsorption energies and preexponential factors. The most practical approach is to obtain deposition rate data as a function of deposition conditions such as temperature, concentration, and flow rate and fit these to suspected rate-limiting reactions. 

Triplet decay processes most interesting to photochemists involve molecular rearrangements and reactions with other compounds. It must be emphasized that the rates of these chemical reactions vary over many orders of magnitude so that some occur to the exclusion of physical decay while others barely compete with phosphorescence,... 

Because of the relatively slow rates of unimolecular reactions of excited acetone in solution at room temperature, acetone makes a convenient solvent-sensitizer for photosensitizatioh studies, provided that the substrate does not undergo competing chemical reactions with triplet acetone. A recent study of the effects of high-energy radiation on dilute acetone solutions of polynuclear aromatic molecules revealed that the triplet states of these compounds were being formed at close to the diffusion-controlled rate by collision with some pre-... 

The transient nature of the cavitation event precludes conventional measurement of the conditions generated during bubble collapse. Chemical reactions themselves, however, can be used to probe reaction conditions. The effective temperature realized by the collapse of clouds of cavitating bubbles can be determined by the use of competing unimolecular reactions whose rate dependencies on temperature have already been measured. The sonochemical ligand substitutions of volatile metal carbonyls were used as... 

If a first-order chemical reaction is occurring, the orifice outflow competes with reactant removal and can be treated as a parallel rate process. The total rate of disappearance of a reactant will be the sum of the reactive and flow contributions, with the overall rate coefficient given by k, + kp, where kx is the reaction rate coefficient ... 

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