Decomposition, rate constant determination

The value of the forward decomposition rate constant, k = k Ceq, is evaluated from the experimentally determined values of ij>v by the regression analysis and the equilibrium dissociation pressures of CO2 reported by Hill and Winter (14)... 

For the experimental determination of the 0, it is necessary to quantify the light output of the direct chemiluminescent process. The experimental definition of the direct chemiluminescence quantum yield is given in Eq. 36, that is, the initial rate of photon production (/q ) per initial rate of dioxetane decomposition k )[D]o). Alternatively, the total or integrated light intensity per total dioxetane decomposed can be used. The /t )[Z)]o term is readily assessed by following the kinetics of the chemiluminescence decay, which is usually first order. Thus, from a semilogarithmic plot of the emission intensity vs. time, the dioxetane decomposition rate constant kjj is obtained and the initial dioxetane concentration [Z)]o is known,especially if the dioxetanes have been isolated and purified. In those cases in which the dioxetanes are too labile for isolation and purification, [/)]o is determined by quantitative spectroscopic measurements or iodometric titration. 

The experimental procedure to determine 4>wa is quite analogous to that discussed for The experimental definition is given by Eq. 38, in which all the terms have been already defined. Again the dioxetane decomposition rate constant kj) is determined by following the first-order kinetics of the DPA-enhanced chemiluminescence decay. The initial or total DPA fluorescence intensity is standardized with a suitable light standard, usually with luminol or the scintillation cocktail. The photomultiplier tube should be corrected for wavelength response. ... 

At low concentrations of Fe2+ ions, the effective synthesis rate constant and the decomposition rate constant are determined as pseudo-first-order rate constants since the change in the concentrations of other reactants is negligible [291]. The dependence of optical density on time is described by a first-order equation respectively in the concentration of iron(II) ions (Fig. 24)... 

The activation energies for several synthesis and decomposition rate constants, as well as equilibrium enthalpy at certain stages, have been determined from the temperature dependences, plotted with the Arrehenius coordinates (Fig. 29). 

The activation energy for the second stage k ) and enthalpy for the first equilibrium K ) have been determined from the temperature-dependent decomposition rate constants obtained in the presence and absence of boric acid. 

The equilibrium constant for zwitterion formation (logATgq = -15.95) may be obtained from the equilibrium constant for formation of the neutral adduct (logA q = -13.39)" and the values of hydroxyl and imidazolyl of the adduct (8.12 and 5.56 respectively), which are determined by calculation. The decomposition rate constant Z aic can be determined for the putative zwitterion intermediate by combining the equilibrium constant and the observed rate constant (0.583 M" sec" ) to give 4.96 x 10 sec". This rate constant exceeds the value expected for a vibration (10 sec" ) and indicates that the intermediate cannot have a discrete existence and that the mechanism must be a concerted process enforced by the short lifetime of the intermediate. In other words the putative intermediate cannot pass enough reaction flux to support the observed reactivity. 

Kim and Lee [14] have reviewed the kinetics of acid-catalyzed hydrolysis of hemicellulose and provide a useful tabulation of previously published kinetic rate constants. There is wide variation in reported rate constant values, reflecting again the differences in the substrate and pretreatment conditions, the model employed and the method by which the effective acid concentration (or weight percent) was determined [16]. Unfortunately, only a few researchers have performed detailed kinetic studies in which both hydrolysis and decomposition rates are determined. Kim and Lee [14] have carried out the only study including both the fast and slow xylan fractions and xylose decomposition. 

Growcock et al. determined the decomposition rate constant for vibration-ally excited ethane produced by chemical activation, C2H6 -> 2CH3.38 The... 

Another pioneering work concerning the photocatalytic decomposition of pro-pionaldehyde in the gas phase used several kinds of supports for Ti02, including activated carbon [9]. This work was very important from the standpoint of establishing the dependency of the photocatalytic activity on the nature of inert supports used. It was clearly demonstrated that when the adsorption constant was low, the decomposition rate was determined by the amount of adsorbed substrate while when the adsorption constant was very high, there was plenty of adsorbed substrate on the support, which was not transferred efficiently to loaded Ti02. 

Anharmonic corrections have also been determined for unimolecular rate constants using classical mechanics. In a classical trajectory (Bunker, 1962, 1964) or a classical Monte Carlo simulation (Nyman et al., 1990 Schranz et al., 1991) of the unimolecular decomposition of a microcanonical ensemble of states for an energized molecule, the initial decomposition rate constant is that of RRKM theory, regardless of the molecule s intramolecular dynamics (Bunker, 1962 Bunker, 1964). This is because a... 

Ttp 1 Initiators, initiator data, and initiator decomposition. Several books on polymer science and engineering cite information about commercial initiators (decomposition rate constants, activation energies, and half-lives). It should be noted that these initiator data may not be accurate for a particular monomer-polymer system. Commercial manufacturers usually report initiator decomposition data determined in organic solvents (toluene or benzene). These values are, at best, starting values for certain kinetic parameters. Published initiator decomposition data measured in the specific monomer-polymer environment are very rare, if at all available. 

Table A Parameters of Arrhenius equation determined from decomposition rate constants k of some peroxides in nonpolar low molecidar medium and calculated temperature T of peroxide decomposition related to the half-hfe 1 minute...

The same bimolecular decomposition rate constants were determined for SAHA and aceto-HX, i.e., 2fe= (4.5 0.5) x 10 s at zero ionic... 

A thermal deazotization studyof 4-methylpyrazoline-4-d in the temperature interval 170-290 °C has revealed that inctroduction of deuterium at the 4 position decreased the first-order decomposition rate constants by a factor of 1.06 + 0.03 at 241.75 °C and gave an isotope effect /chAd = in the product-determining step leading to cyclopropane... 

Figure 9 also reveals that the decomposition rate constants observed at 0% RH were greater than those at 79% RH in samples from CPG 75 to CPG 350. In the cases of the CPG 1000 and CPG 3000 mixtures, the decomposition at 79% RH was faster than that at 0% RH. Although hydrolysis in the solid state was generally accelerated by humidification, the opposite situation was observed in the mixtures with small-pore CPGs. The decomposition rate constant in the CPG 75 mixture at 0% RH and 50 C was determined as 4.90 x 10 h , indicating a half-life of 14.1 h for aspirin decomposition. 

In general, the rate constants determined by on-column chromatographic methods are a weighed sum of the rate constants within the stationary and the mobile phase, the retention capacity (and in non-first-order reactions also the partition coefficient) being the weighing factor. Certainly, for some catalyzed reactions occurring only within the mobile or the stationary phase, the decomposition is possible, if the dead time is known. 

To clarify the relationship between these rate constants In Equations (24) and (25) and the propane-propylene ratios, the decomposition rate constant of propane or propylene was determined from the experimental data with various ratios of both components... 

For simplification, let us consider the case when the seeond maximum in Fig. 4.11 lies mueh lower than the first one. This corresponds to the case when > Eo . Then decomposition in the direetion of reactants can be neglected, i.e., k/, is equalized to zero. The rate of formation of deeomposition produets D and the rate of formation of molecules AB are measured in experiment. We designate them as and Fab. The averaged over e decomposition rate constant [Pg.100]

Calculations using the RRKM theory rather well describe the experimentally determined plots of the decomposition rate constants vs. excitation energy. To illustrate this, some results of studying the elimination of hydrogen halide from chemically activated alkyl halides, which were obtained by radical recombination, are presented in Table 4.2. The experimental and theoretical values of the decomposition rate constants of these reactions are presented in the last column in the table. 

The gas phase decomposition A B -r 2C is conducted in a constant volume reactor. Runs 1 through 5 were conducted at 100°C run 6 was performed at 110°C (Table 3-15). Determine (1) the reaction order and the rate constant, and (2) the activation energy and frequency factor for this reaction. 

The determination of A V is illustrated by data for the thermal decomposition of di-ferf-butyl peroxide.10 The rate constants at 120 °C in toluene are as follows ... 

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