Physical chemistry second-order rate constant

Miscellaneous Physical Chemistry. A kinetic study has been made of the electrochemical reduction of /8-carotene. The photoelectron quantum yield spectrum and photoelectron microscopy of /3-carotene have been described. Second-order rate constants for electron-transfer reactions of radical cations and anions of six carotenoids have been determined. Electronic energy transfer from O2 to carotenoids, e.g. canthaxanthin [/8,/3-carotene-4,4 -dione (192)], has been demonstrated. Several aspects of the physical chemistry of retinal and related compounds have been reported, including studies of electrochemical reduction, the properties of symmetric and asymmetric retinal bilayers, retinal as a source of 02, and the fluorescence lifetimes of retinal. Calculations have been made of photoisomerization quantum yields for 11-cis-retinal and analogues and of the conversion of even-7r-orbital into odd-TT-orbital systems related to retinylidene Schiff bases. ... 

Some reactions are difficult to study directly because the required instrumentation is not available or the changes in standard physical properties (light absorption, conductivity etc.) typically used in kinetic measurements are too small to be useful. Competition kinetics can provide important information in such cases. In some situations, the chemistry itself makes direct measurement inconvenient or even impossible. This is the case, for example, in studies of slow reactions of free radicals. Because of the ever-present radical-depleting second-order decomposition reactions, slow reactions of free radicals with added substrates are possible only at very low, steady-state radical concentrations. The standard methods of radical generation (pulse radiolysis and flash photolysis) are not useful in such cases, because they require micromolar levels of radicals for a measurable signal. The self-reactions usually have k > 10 M s , so that the competing reactions must have a pseudo-first-order rate constant of lO s or higher (or equivalent, if conditions are not pseudo-first order) to be observed. Competition experiments, on the other hand, can handle much lower rate constants, as described later for some reactions of C(CH3)20H radicals with transition metal complexes. 

In physical organic chemistry, kinetics has been used successfully to distinguish between Sn 1 and Sn2 mechanisms. When ion formation is the rate-determining step, the reaction rate does not depend on the concentration of other reagents such as solvent or monomer. Second-order kinetics does not necessarily mean that a direct bimolecular reaction (e.g., Sn2) takes place between the dormant species, D, and monomer, M. If the covalent precursor, D, is in dynamic equilibrium with the carbenium ion, C, and only the latter reacts with M to give the product P, then the overall kinetics depends on the ratio of the rate constants k i and k2 ... 

Activated rate processes are ubiquitous in condensed matter physics and chemistry. Their characteristic feature is the existence of an energy barrier separating the initial and final state. The system can undergo the necessary change only if it has sufficient energy to cross the barrier. It must be activated in order to react. The activation is usually effected by an external medium, such as a liquid solvent or a solid, which is in thermal equilibrium at temperature T. The second property is that one is probing a rate process. The change from reactants to products is accompanied by an exponential decay in time of reactants, such that a rate constant may be defined as the characteristic inverse time of the exponential decay. 

Use the Handbook of Chemistry and Physics or a website and locate information on kinetics, conversion factors, or rate constants. Which of the units for a second-order reaction found in this handbook or on another website is/are equivalent to the units used in this textbook ... 

In the School of Chemical Technology in Pardubice founded in 1950, at the department of physical chemistry, the smdy of reaction kinetics of redox reactions of second and third order, of two competing consecutive reactions, and the determination of rate constants of homogeneous reactions from potential-time curves... 

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