Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Equilibrium, chemical conversion

Let us note that no restriction has been imposed on the extent of irreversibility in chemical conversions at finite rate and also the complexities of reactions. In other words, far away from equilibrium, chemical conversions and complex reactions such as chemical oscillations, chemical chaos, diffusion-controlled reactions having multi-steps etc. are governed by the above equation provided the system is spatially uniform. [Pg.326]

The following presentation pertains to an equilibrium chemical conversion that is bimolecular in both directions... [Pg.305]

Aside from merely calculational difficulties, the existence of a low-temperature rate-constant limit poses a conceptual problem. In fact, one may question the actual meaning of the rate constant at r = 0, when the TST conditions listed above are not fulfilled. If the potential has a double-well shape, then quantum mechanics predicts coherent oscillations of probability between the wells, rather than the exponential decay towards equilibrium. These oscillations are associated with tunneling splitting measured spectroscopically, not with a chemical conversion. Therefore, a simple one-dimensional system has no rate constant at T = 0, unless it is a metastable potential without a bound final state. In practice, however, there are exchange chemical reactions, characterized by symmetric, or nearly symmetric double-well potentials, in which the rate constant is measured. To account for this, one has to admit the existence of some external mechanism whose role is to destroy the phase coherence. It is here that the need to introduce a heat bath arises. [Pg.20]

Catalysis opens reaction pathways that are not accessible to uncatalysed reactions. It should be self-evident that thermodynamics predict whether a reaction can occur. So, catalysis influences reaction rates (and as a consequence selectivities), but the thermodynamic equilibrium still is the boundary. Catalysis plays a key role in chemical conversions, although it is fair to state that it is not applied to the same degree in all sectors of the chemical industry. While in bulk chemicals production catalytic processes constitute over 80 % of the industrially applied processes, in fine chemicals and specialty chemicals production catalysis plays a relatively modest role. In the pharmaceutical industry its role is even smaller. It is the opinion of the authors that catalysis has a large potential in these areas and that its role will increase drastically in the coming years. However, catalysis is a multidisciplinary subject that has a lot of aspects unfamiliar to synthetic chemists. Therefore, it was decided to treat catalysis in a separate chapter. [Pg.59]

We define the rate of reaction as the speed at which a chemical conversion proceeds from start to its position of equilibrium, which explains why the rate is sometimes written as d /dt, where is the extent of reaction. [Pg.350]

Due to chemical conversion in the liquid-phase mass transfer film the mass flux of A at the vapour-liquid interface and the mass flux of A at the boundary between this film and the liquid bulk will differ. Figures 9(a) and (b) show the values of these fluxes as a function of the reaction rate constant ko for equilibrium constants K = 1 and X = 100. The... [Pg.12]

An important thermodynamic result is that the free-energy change can be related to the reaction equilibrium constant. Begin by considering again the reaction of Eq. 9.3, which describes chemical conversion of A and B to X and Y in the molar ratios shown. If the reaction proceeds by some infinitesimal amount dS, then the number of moles of each chemical species changes by an amount... [Pg.377]

Summing up, if the inventory of the main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid-phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable equilibrium state. Interactions through recycles can be exploited to create plantwide control structures that are not possible from a standalone unit viewpoint. [Pg.228]

However, we would like to point here not to the differences between the equilibrium tunneling mechanism and the above examples of mechanisms of the nonequilibrium type in low-temperature chemical conversions, but, on the contrary, to a simplifying assumption which relates them but which has to be rejected in a number of cases—and that is the subject matter of this chapter. In the above models the solid matrix itself was considered, in essence, from a special point of view, namely, as an ideal system, devoid of defects, which is in mechanical equilibrium. In other words, the fact that the systems in question are significantly out of equilibrium with respect to their mechanoenergetic state was ignored. This property of the experimentally studied samples was the result of both their preparation conditions and the ionizing radiation. [Pg.341]

In the systems that are far from equilibrium, the stratification into sub systems with fast and slow is also possible, with the subsystem with fast internal variables being characterized by the minimum of the relevant Lyapunov function (provided that such a function exists for the particular process scheme). The ways to describe systems that can be stratified in accordance with the timing hierarchy of the processes involved are under intensive study in modem chemical engineering and biophysics. The methods are based on models that take into account mechanistic (deterministic) and statistical degrees of freedom and their contribution to processes of energy transfer and chemical conversions in the systems with a very complicated process hierarchy (for example, catalytic and biological processes). [Pg.301]

The conclusive step in this quasi-chemical development is to recognize that a model for stoichiometric chemical equilibrium provides a correct description of Xq. We imagine following a specific solute molecule of interest through chemical conversions defined as changes in the inner-shell populations,... [Pg.145]

The equilibrium perturbed by the electrolytic process el) tries to be re-established. The compound C is replenished in the vicinity of the electrode by a chemical conversion of A with rate constant k[. In this way the amount of C that can undergo reduction at the surface of the electrode is increased and the wave becomes higher when compared with the conditions for a slowly established equilibrium (Fig. 18). The magnitude of this increase depends, for given experimental conditions, on the value of the formal rate constant k[, i.e. on the magnitude of ki and the concentration of the component B present in excess. [Pg.32]

Hvdrofonnybtion is a homogeneous catalysis reaction which employs a coordination complex of cobelt with the olefin. The catalyst precursor is a cobalt salt which is converted tn siru, in the presence of CO and H , to cobalt tetracarbonyl hydride in equilibrium with cobalt tricarbonyl hydride. The latter, which displays a vacant coordination position, forms a complex with the olefm. The olefin undergoes chemical conversions leading to the production of two isomeric aldehydes, and the normal aldehyde is favored in relation to the iso-compound in the ratio of 4/1. [Pg.83]

Amphidinolide Y (24) is a 17-membered macrolide obtained together with amphidinolide X (23), and it was elucidated to exist as a 9 1 equilibrium mixture of 6-keto and 6(9)-hemiacetal forms on the basis of 2D NMR data. The structure and absolute stereochemistry of the 6-keto form were assigned on the basis of spectroscopic data and chemical conversion of amphidinolide Y (24) into amphidinolide X (23) by Pb(OAc)4 oxidation. The 6-keto form of amphidinolide Y (24) is a 17-membered macrolide possessing a tetrahydrofuran ring, five branched methyl groups, a ketone, and two hydroxy groups. ° ... [Pg.285]

Wet deposition encompasses the removal of gases and particles from the atmosphere by precipitation events, through incorporation into rain, snow, cloud, and fog water, followed by precipitation (Hales, 1986). As in the case of dry deposition, wet deposition is a complex phenomenon which in this particular case involves transport to the surface of a droplet, absorption, and possible aqueous-phase chemical conversion. Wet removal of gases is frequently approximated by assuming that the species is in equilibrium between the gas and aqueous phases. The equilibrium partitioning is represented in terms of a washout ratio, Wg = [C]drop/[C]air, where [C]drop and [C]ajr are the concentrations of the chemical in the aqueous and gas phases (Mackay, 1991). [Pg.330]

Figure 16.45. Schematic representation of the chemical corrosion of polypyrrole in three steps I, initial state 2, distribution equilibrium for nucleophiles 3, partial chemical conversion + radical cationic centre O nucleophiles in solution 9 nucleophile in the solid and reacted slates. Adapted from Werkstoffe und Korrosion 42, 341 (1991), with permission of VCH,... Figure 16.45. Schematic representation of the chemical corrosion of polypyrrole in three steps I, initial state 2, distribution equilibrium for nucleophiles 3, partial chemical conversion + radical cationic centre O nucleophiles in solution 9 nucleophile in the solid and reacted slates. Adapted from Werkstoffe und Korrosion 42, 341 (1991), with permission of VCH,...
The influence of the initial C/H/0 composition of the carbon compounds used for non-equilibrium diamond synthesis on the composition of the deposited films is usually illustratedby the so-called Bachmann triangle, showninFig. 9-45. The Bachmanntriangle summarizes experimental data and indicates the C/H/0 compositions corresponding to deposition of non-diamond films, diamond films, or no deposition at all (which corresponds to domination of etching over deposition). Deposition of diamond films from the gas phase is typically performed at pressures of 10-100 Torr in a mixture of hydrogen with about 1-5% methane. The plasma-chemical conversion of carbon compounds can lead not only to the formation of diamond films but also to so-called diamond-like carbon (DLC). DLC films are... [Pg.668]

The historical development of chemical equilibrium has been described in several reviews (e.g., Berger, 1997 Laidler, 1985 Lindauer, 1962 Lund, 1965, 1968). The concept of chemical equilibrium was introduced in the 1860s in the context of empirical studies of incomplete and reversible chemical conversions. Explanations for these phenomena were formulated on the basis of two essentially different theoretical perspectives, a kinetic framework and a thermodynamic framework. [Pg.272]

Incomplete Conversion. In equilibrium reactions, components A and B do not react with each other to completion. One of the two substances A or B is generally used in excess to increase the extent of conversion to the other at equilibrium. Occasionally, the component present in excess is the reaction medium for the chemical conversion. The excess amount of A or B remaining at the end of the reaction can contain a higher or lower content of impurities, and must be disposed of in an environmentally friendly manner if it cannot be recycled directly after physical or chemical treatment. [Pg.7]

The substance A can undergo at the droplet (or particle) surface after adsorption chemical conversion Aads Bads (Fig. 4.19). This chemical flux is given by Fhe, = khet cn)o- Using the adsorption equilibrium condition K = (cn)oI (Civ)g it follows that ... [Pg.436]

Two distinct non-equilibrium situations arise in this context i.e., (i) spatially uniform systems in non-equilibrium and (ii) spatially non-uniform systems. These are discussed below [8]. If the system has the source of irreversibility only in chemical conversions at finite rates, then G function has the following expression, namely... [Pg.326]


See other pages where Equilibrium, chemical conversion is mentioned: [Pg.2696]    [Pg.256]    [Pg.314]    [Pg.219]    [Pg.57]    [Pg.12]    [Pg.328]    [Pg.348]    [Pg.84]    [Pg.18]    [Pg.101]    [Pg.198]    [Pg.186]    [Pg.89]    [Pg.445]    [Pg.106]    [Pg.170]    [Pg.51]    [Pg.291]    [Pg.2696]    [Pg.510]    [Pg.1]    [Pg.600]    [Pg.94]    [Pg.75]    [Pg.274]    [Pg.56]    [Pg.105]    [Pg.2]   
See also in sourсe #XX -- [ Pg.11 ]




SEARCH



Chemical conversion

Chemical reaction equilibrium conversion

Equilibrium conversion

© 2024 chempedia.info