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Kinetic Law (kinetic + law)
Selected AbstractsThermal decomposition of cyclic organic peroxides in pure solvents and binary solvent mixturesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2010Mariángeles Iglesias The thermal decomposition reaction of acetone cyclic triperoxide, acetone cyclic diperoxide, 4-heptanone cyclic diperoxide, and pinacolone cyclic diperoxide ca. 0.02 M was studied in pure solvents (acetone and 1-propanol) and in binary mixtures of acetone/1-propanol at 150°C. The kinetics of each system was explored by gas chromatography (GC) at different solvent compositions. The reactions showed a behavior accordingly with a pseudo-first-order kinetic law up to at least 90% peroxide decomposition. The main organic products derived from these thermolysis reactions were detected by GC analysis. Among them, the corresponding ketones, methane, ethane, and propane were the main identified products. The rates of decomposition of pinacolone diperoxide in the pure solvents were practically independent of the solvent characteristics, so it was of no interest to analyze its kinetic behavior in binary solvent mixtures. In acetone/1-propanol mixtures, the solvation effect on the cyclic peroxides derived from 4-heptanone and acetone molecules was slightly dominated by specific interactions between 1-propanol and a diradical-activated complex initially formed. This species was preferentially solvated by 1-propanol instead of acetone. Specific interactions between the O atoms from the peroxidic bond and the H from the OH in 1-propanol can be taken into account. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 347,353, 2010 [source] Formation and decay of the ABTS derived radical cation: A comparison of different preparation proceduresINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2002Carola Henriquez Bleaching of a preformed solution of the blue-green radical cation 2,2,-azinobis (3-ethylbenzothizoline-6-sulfonic acid) (ABTS+·) has been extensively used to evaluate the antioxidant capacity of complex mixtures and individual compounds. The reaction of the preformed radical with free-radical scavengers can be easily monitored by following the decay of the sample absorbance at 734 nm. The ABTS radical cation can be prepared employing different oxidants. Results obtained using MnO2 as oxidant show that the presence of manganese ions increases the rate of [ABTS]+· autobleaching in a concentration-dependent manner. The radicals can also be obtained by oxidizing ABTS with 2,2, -azobis(2-amidinopropane)hydrochloride (AAPH) or peroxodisulfate (PDS). The oxidation by AAPH takes place with a large activation energy and a low reaction order in ABTS. The data support a mechanism in which the homolysis of AAPH is the rate-limiting step, followed by the reaction of ABTS with the peroxyl radicals produced after the azocompound thermolysis. On the other hand, the low activation energy measured employing PDS, as well as the kinetic law, are compatible with the occurrence of a bimolecular reaction between the oxidant and ABTS. Regarding the use of ABTS-based methodologies for the evaluation of free radical scavengers, radical cations obtained employing AAPH as oxidant can be used only at low temperatures, conditions where further decomposition of the remaining AAPH is minimized. The best results are obtained with ABTS derived radicals generated in the reaction of PDS with an ABTS/PDS concentration ratio equal (or higher) to two. However, even with radicals prepared by this procedure, stoichiometric coefficients considerably larger than two are obtained for the consumption of the radical cation employing tryptophane or p -terbutylphenol as reductants. This casts doubts on the use of ABTS-based procedures for the estimation of antioxidant capacities. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 659,665, 2002 [source] Enhanced reactivity in the ammonolysis of phenyl thiolacetates in aqueous mediumINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2002D. Rajarathnam The ammonolysis of several substituted phenyl thiolacetates is kinetically studied in aqueous medium, 18°C, ionic strength 0.1 M (KCl). By following the leaving groups spectrophotometrically (,max = 260,410 nm), under excess free ammonia, pseudo-first-order rate coefficients (kobs) are obtained. The plots of (kobs , kH) against free ammonia concentration are linear at constant pH. The macroscopic nucleophilic substitution rate coefficients (kN) are obtained as the slopes of these plots and found to be pH-independent for all the thiolesters. The Brönsted-type plot (log kN against pKa of leaving groups) and the Hammett plot (log kN against , values of substituents) obtained for the title reactions of thiolesters are linear with slope values of ,lg = ,0.34 and , = 0.74 respectively. From the magnitude of these values, experimental data, the kinetic law, and the analysis of products, it is deduced that the ammonolysis of thiolesters proceeds through a simple bimolecular nucleophilic substitution pathway with a zwitterionic tetrahedral addition intermediate (T±), whereby its formation is rate-determining (k1 step). Comparison of this reaction of thiolesters with a similar reaction of analogue oxyesters shows a mechanistic difference. Further, for thiolesters there is a rate enhancement with larger kN values. The change in mechanism and enhanced reactivity observed by substitution of the oxygen atom by sulphur atom on the phenyl moiety are discussed in detail. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 18,26, 2002 [source] Kinetics of heavy metal vaporization from coal in a fluidized bed by an inverse modelASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2010Jing Liu Abstract This study addresses the kinetics of heavy metal vaporization during fluidized bed thermal treatment of coal. Both direct and inverse models were developed in transient conditions. The direct model predicts the time course of the metal concentration in the gas from the vaporization rate profile. The inverse model was developed and validated to predict the metal's vaporization rate from its concentration in the outlet gas. A method to derive the kinetic law of heavy metal vaporization during fluidized bed thermal treatment of coal from the global model and the experimental measurements is derived and illustrated. A first-order law was fitted for the mineral matrix and a second-order law was fitted for coal. This method can be applied to any matrix, whether it is mineral matrix or organic matrix. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Kinetic study of carbon nanotubes synthesis by fluidized bed chemical vapor depositionAICHE JOURNAL, Issue 2 2009R. Philippe Abstract Multi-walled carbon nanotubes (MWCNTs) have been produced with high selectivity by fluidized bed catalytic chemical vapor deposition from ethylene on Fe/Al2O3 catalysts. The influence of operating parameters such as deposition duration, temperature, ethylene and hydrogen partial pressures, and iron loading on MWCNT productivity, process selectivity, characteristics of final powders, and chemical composition of the outlet gases has been analyzed. Using gas phase chromatography, methane and ethane have been detected, whatever are the conditions used. Between 650 and 750°C, no catalyst deactivation occurs because nucleation remains active all along the synthesis, thanks to the explosion of the catalyst grains. Above 650°C, ethane itself produces MWCNTs, whereas methane does not react in the temperature range, 550,750°C. The formation of MWCNTs induces marked bed expansions and sharp decreases of grain density. Apparent kinetic laws have been deduced from the collected data. The apparent partial orders of reaction for ethylene, hydrogen, and iron were found to be 0.75, 0, and 0.28, respectively. © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source] | ||||||||||