Approximation des états quasi stationnaires

L'approximation des états quasi stationnaires (AEQS) est une hypothèse parfois prise en physique, en chimie et tout particulièrement en cinétique chimique, elle est alors également appelée le principe de Bodenstein. Principe (en cinétique chimique) Soit X* un intermédiaire réactionnel (donc très réactif). Selon l'AEQS, sa vitesse de création est à peu près égale à sa vitesse de disparition car il est consommé par la réaction juste après sa création. En considérant que sa concentration reste tellement faible qu'elle est constante, la dérivée de sa concentration est nulle. Cela permet de lier directement la vitesse de la réaction qui donne cet intermédiaire aux réactions qu'il déclenche (soit une simplification du calcul). Cette simplification ne peut s'appliquer que si la vitesse de formation du composé B (k2) est égale à la vitesse de disparition de A (ou de formation de X) (k1). L'AEQS définit donc la dérivée d'un intermédiaire réactif comme nulle. \frac{\mathr

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Publications associées (12)

Publications associées

Chargement

Personnes associées

Chargement

Unités associées

Chargement

Concepts associés (16)

Concepts associés

Chargement

Cours associés (19)

Cours associés

Chargement

Séances de cours associées (42)

Séances de cours associées

Chargement

A systematic investigation on the bactericidal transient species generated by photo-sensitization of natural organic matter (NOM) during solar and photo Fenton disinfection of surface waters

Ling Feng, Stefanos Giannakis, Mona Kohantorabi, César Pulgarin

In this work, the role of dissolved oxygen in the solar and the photo-Fenton-mediated E. coli inactivation process was put under scrutiny. The effect of transient species that were produced in the presence of various natural organic matter isolates (NOM), namely Suwannee River (SR) NOM, Nordic Reservoir (NR) NOM, SR Humic acid (SRHA), and SR Fulvic acid (SRFA) was studied in detail. The role of (1) O-2 in this reaction was systematically evaluated by modifying the O-2 concentration (N-2/O-2 purging) and the matrix composition (10, 50, and 100% deuterium oxide (D2O) v/v). In the presence of NOM, (1) O-2 was generated and the enhancement of E. con inactivation rate due to charge transfer from triplet state to molecular oxygen. The comparison between SR and NR NOM showed that for these compounds, triplet state of NOM ((NOM)-N-3*), and (1) O-2 were the more favorable active species in E. coli inactivation, respectively. Also, the second order rate constants (k(E. coli)(2nd)) of E. coli with (NOM)-N-3*, and (1) O-2 were calculated by using the steady state approximation. The obtained results showed that the rate values of (1) O-2 related to NR NOM was similar to 5.6 times higher than SR NOM, while the rate values of (NOM)-N-3* for SR NOM was similar to 8.7 times higher than NR NOM. We also determined the effect of these organic matter isolates in the photo-Fenton process and its constituents (solar/Fe2+, solar/H2O2, and solar/Fe2+/H2O2). In presence of NOM, the photo-Fenton process inactivation rates increased which confirmed that the combined processes has additional pathways generated with disinfecting effect during solar exposure of bacteria.

ELSEVIER SCIENCE BV2019

Chargement

In-situ characterization of the surface intermediates for the ethanol dehydration reaction over g-alumina under dynamic conditions

Albert Renken

The dynamic behavior of the ethanol adsorption on g-alumina were investigated at 180 and 200 DegC by the transient-response method coupled with FT-IR data of the catalyst surface. The existence of three adsorbates was demonstrated: a reacting species which is the precursor for the formation of the gas-phase ethene; an inhibiting species responsible for the low steady-state reaction rate; and a spectator species accumulating on the catalyst surface. Their IR spectra indicate an ethoxide-like structure for the three adsorbates. Their C-H stretching bands can be depicted by four Lorentzians whose parameters indicate different surface environments. The surface concn. of the reacting species was detd. on the basis of the transient ethene response. A value of 0.77+-0.07 mol/kgcat was found at 180 DegC. The surface concn. of the spectator species was detd. by ex-situ thermogravimetric expts. A value of 0.13+-0.01 mol/kgcat was found at 180 DegC. The most likely structure of this species corresponded to an ethanol mol. coordinated in a bidentate manner on Al3+ cations, with stabilization of the alc.-hydroxyl group via lateral hydrogen bonding with an adjacent surface hydroxyl. [on SciFinder (R)]

1998

Chargement

Fracture toughness of Al replicated foam

Etienne Combaz, Andreas Mortensen

Elastoplastic toughness testing of pure aluminium replicated foam is conducted following a J-procedure adapted from the ASTM E1820-08a standard using disc-shaped compact tension specimens. Tests cover a wide range of foam relative density V-m (10-24%). Resulting data show pronounced R-curve behaviour, computed J values increasing steadily beyond the crack blunting line before reaching a plateau, corresponding to a "steady-state" J value. Fractography reveals that the crack propagates via the rupture of struts normal to the crack plane, this being accompanied by limited plastic deformation of the struts near the crack plane, the extent of which, made visible by slip markings along the strut surface, increases in extent with V-m. Intact and fractured struts coexist over a significant portion of the crack plane, indicating that the strut rupture is stochastic. Measured crack initiation and steady-state propagation J values vary roughly as V-m(3); this is a stronger dependence than has been observed in commercial (closed-cell) metal foams. A simple model is proposed to describe the initiation toughness of these open-cell foams, which are characterized by a greater matrix ductility than other metal foams characterized in the literature. The model is based on an estimate of the crack tip opening displacement at the instant when a strut aligned in the loading direction fails by ductile rupture just ahead of the crack tip. The model accurately predicts the scaling law observed here between the toughness and the relative density of the foams, and provides a relatively good predictor of the absolute value of fracture toughness up to a relative density near 20%, while at higher densities measured toughness values exceed predictions. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Elsevier2010

Chargement

ChE-403: Heterogenous reaction engineering

The theoretical background and practical aspects of heterogeneous reactions including the basic knowledge of heterogeneous catalysis are introduced. The fundamentals are given to allow for the use of chemical reactors to study reaction kinetics and test various mechanistic assumptions.

CH-310: Dynamics and kinetics

The course covers the principles of chemical kinetics, including differential rate laws, derivation of exact and approximate integral rate laws for common elementary and composite reactions, fundamentals of collision and transition state theories, and applications such as enzymatic catalysis.

ENV-507: Fate and behaviour of environmental contaminants

The student will learn the important processes that control the transport and transformation of organic chemicals in the environment, as well as the formulation and solution of quantitative models to describe these processes.

Réaction chimique

Une réaction chimique est une transformation de la matière au cours de laquelle les espèces chimiques qui constituent la matière sont modifiées. Les espèces qui sont consommées sont appelées réactif

Rate equation

In chemistry, the rate law or rate equation for a chemical reaction is a mathematical equation that links the rate of forward reaction with the concentrations or pressures of the reactants and const

Cinétique chimique

La cinétique chimique est l'étude de la vitesse des réactions chimiques. Sur le plan disciplinaire, elle fait partie de la chimie physique. Certaines réactions sont totales et très rapides, voire ins