Arrhenius plot, observed reaction rate

The observed kinetic law, the type of rate profile (plot of log k vs. sulfuric acid concentration), the values of the Arrhenius parameters, the comparison of the observed reaction rates with the calculated encounter rates, and the agreement with the features of the nitration of quinoline127 are in favor of a reaction of nitronium ions with the azolium cations. Only at lower acidities (< 90% H2S04) can the reaction of the neutral azole molecules become important. 

Figure 4 Arrhenius Plot of the Observed Reaction Rate of 1-butene Dehydrogenation...

The isomerization of 8 -HOABA and 37 was observed as a first-order reaction in which the rate was proportional only to the concentration of the 8 -hydroxyl compound. As the pH and temperature increased the reaction proceeded more rapidly. At 25°C, the half-life of 8 -HOABA was 30 hr at pH 3, 4 hr at pH 7, and shorter than 1 min at pH 10, that is, 8 -HOABA was isomerized to PA more rapidly at pH 10 than at pH 3 by a fa.ctor of 2,000. The temperature dependence of the rate was greater under alkaline conditions than under acidic conditions. The Arrhenius plots of the rate constants gave the activation energies of Arrhenius and frequency factors, which were converted to the kinetic parameters, i.e. the activation enthalpy activation entropy (4S ) and activation free... 

The triplet excited state of deuterated and non-deuterated tetralone (370), and the triplet 1,4-biradicals formed by intramolecular hydrogen abstraction have been studied by flash photolysis at different temperatures. The authors propose that the rate of hydrogen transfer is largely governed by tunnelling effects since they observe a very large isotope effect as well as curved Arrhenius plots for the rate constant for the hydrogen abstraction reaction of the triplet excited state. 

The reasons for the change of apparent reaction rate during conversion are the development of surface area, porosity, char crystal structure change, exposure of different macerals, and ash effects [59]. An alternative evaluation of the change of reaction rate during particle conversion is to plot the observed reaction rate r of several intervals of conversion as shown in Figure 3.3d representing the experimental values. In a second step, the determined Arrhenius parameters and the pressure reaction order are used, accompanied by a dimensionless conversion function/(X)... 

A ten to hundredfold decrease in the velocity of the reaction, seen as a break down of the Arrhenius plot, is observed at a temperature which, for any given pressure, is always higher than that thermodynamically foreseen for the beginning of the a-/3 transition (this discrepancy is smallest at 265 mm Hg pressure). The marked decrease of the rate of reaction is characteristic of the appearance of the /3-hydride phase. The kinetics of reaction on the hydride follows the Arrhenius law but with different values of its parameters than in the case of the a-phase. 

The first indication of something unusual in the reactivity of this species was that the EPR signal of 2 was found to decay via a first-order process to produce a new radical. The product was shown to be the neophyl radical 3, whose EPR spectrum was identical with an independently prepared authentic sample. Over the temperature range -30 to -90°C, Arrhenius plots indicated an unusually low preexponential log A (s ) value of only 5.3, and a strikingly large k /ko ratio of ca. 50 observed at -30°C (based on comparison of reaction rates of 2 versus the in-A)-tert-b xiy analog) for the rearrangement. 

The TGA system was a Perkin-Elmer TGS-2 thermobalance with System 4 controller. Sample mass was 2 to 4 mgs with a N2 flow of 30 cc/min. Samples were initially held at 110°C for 10 minutes to remove moisture and residual air, then heated at a rate of 150°C/min to the desired temperature set by the controller. TGA data from the initial four minutes once the target pyrolysis temperature was reached was not used to calculate rate constants in order to avoid temperature lag complications. Reaction temperature remained steady and was within 2°C of the desired temperature. The actual observed pyrolysis temperature was used to calculate activation parameters. The dimensionless "weight/mass" Me was calculated using Equation 1. Instead of calculating Mr by extrapolation of the isothermal plot to infinity, Mr was determined by heating each sample/additive to 550°C under N2. This method was used because cellulose TGA rates have been shown to follow Arrhenius plots (4,8,10-12,15,16,19,23,26,31). Thus, Mr at infinity should be the same regardless of the isothermal pyrolysis temperature. A few duplicate runs were made to insure that the results were reproducible and not affected by sample size and/or mass. The Me values were calculated at 4-minute intervals to give 14 data points per run. These values were then used to... 

Mandelate racemase, another pertinent example, catalyzes the kinetically and thermodynamically unfavorable a-carbon proton abstraction. Bearne and Wolfenden measured deuterium incorporation rates into the a-posi-tion of mandelate and the rate of (i )-mandelate racemi-zation upon incubation at elevated temperatures. From an Arrhenius plot, they obtained a for racemization and deuterium exchange rate was estimated to be around 35 kcal/mol at 25°C under neutral conditions. The magnitude of the latter indicated mandelate racemase achieves the remarkable rate enhancement of 1.7 X 10, and a level of transition state affinity (K x = 2 X 10 M). These investigators also estimated the effective concentrations of the catalytic side chains in the native protein for Lys-166, the effective concentration was 622 M for His-297, they obtained a value 3 X 10 M and for Glu-317, the value was 3 X 10 M. The authors state that their observations are consistent with the idea that general acid-general base catalysis is efficient mode of catalysis when enzyme s structure is optimally complementary with their substrates in the transition-state. See Reference Reaction Catalytic Enhancement... 

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