Chemical reactions very fast

When the chemical reaction is fast (with large Damkohler numbers or with very low diffusivities) the reactant and product reach their equilibrium concentration throughout most of the film. The concentration gradients are very steep at the nonequilibrium region. The set of parameters Das = 1.0, DaP = 0.5, y = 0.0 represent slow reaction and nonequilibrium film. 

It is interesting to observe variations due to different reaction rates. Fast hydrolysis and condensation are reported on Fig. 6, slow hydrolysis and condensation are reported on Fig. 7. When the chemical reactions are fast compared to the diffusion of water, there is a very fast sur ce curing. The crosslinking is controlled by the water diffusion. As soon as the water diffuses in the uncured sealant, it reacts and is consumed by the hydrolysis reaction. It does not difiuse further until the sealant is fully cured. At the surface the full cure is quickly reached. A full cute at 5 mm is obtained in 100 h, diis is close to the experimental data observed for an acetic type sealant (Fig. 8). 

In case chemical reaction is fast compared to internal diffusion or when the porosity of the solid is very low, conversion starts at the outer surface of the particle. As reaction proceeds further, a reacting boundary layer progressively moves inwards. 

Some chemical reactions are reversible and, no matter how fast a reaction takes place, it cannot proceed beyond the point of chemical equilibrium in the reaction mixture at the specified temperature and pressure. Thus, for any given conditions, the principle of chemical equilibrium expressed as the equilibrium constant, K, determines how far the reaction can proceed if adequate time is allowed for equilibrium to be attained. Alternatively, the principle of chemical kinetics determines at what rate the reaction will proceed towards attaining the maximum. If the equilibrium constant K is very large, for all practical purposes the reaction is irreversible. In the case where a reaction is irreversible, it is unnecessary to calculate the equilibrium constant and check the position of equilibrium when high conversions are needed. 

Chemical methods involve removing a portion of the reacting system, quenching of the reaction, inhibition of the reaction that occurs within the sample, and direct determination of concentration using standard analytical techniques—a spectroscopic metliod. These methods provide absolute values of the concentration of the various species that are present in the reaction mixture. However, it is difficult to automate chemical mediods, as the sampling procedure does not provide a continuous record of tlie reaction progress. They are also not applicable to very fast reaction techniques. 

Cambridge) and G. Porter (London) studies of extremely fast chemical reactions, effected by disturbing the equilibrium by means of very short pulses of energy. 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

FIGURE 13.1 Reactions proceed at widely different rates. Some, such as explosions of dvnamite, are very fast. Charges have been set off to demolish this old building. The chemical reaction in each explosion is over in a fraction of a second the gases produced expand more slowly. 

The events that happen to an atom in a chemical reaction are on a time scale of approximately 1 femtosecond (1 fs = 10 ",5 s), the time that it takes for a bond to stretch or bend and, perhaps, break. If we could follow atoms on that time scale, we could make a movie of the changes in molecules as they take part in a chemical reaction. The new field of femto-cbemistry, the study of very fast chemical processes, is bringing us closer to realizing that dream. Lasers can emit very intense but short pulses of electromagnetic radiation, and so they can be used to study processes on very short time scales. 

Because of the possibility of fluorescence, any chemical reactions of the 5i state must take place very fast, or fluorescence will occur before they can happen. 

For reviews of such proton transfers, see Hibbert, F. Adv. Phys. Org. Chem., 1986,22,113 Crooks, J.E. in Bamford Tipper Chemical Kinetics, vol. 8 Elsevier NY, 1977, p. 197. Kinetic studies of these very fast reactions were first carried out by Eigen. See Eigen, M. Angew. Chem. Int. Ed. Engl., 1964, 3, 1. 

In processes where evaporation is compatible with the chemical reaction, evaporation could be used for heat removal. An additional downside of RPBs is the short residence time in these devices, usually in the range of 0.2-2 s, which makes them applicable to fast and very fast processes only. 

The diffusivity in gases is about 4 orders of magnitude higher than that in liquids, and in gas-liquid reactions the mass transfer resistance is almost exclusively on the liquid side. High solubility of the gas-phase component in the liquid or very fast chemical reaction at the interface can change that somewhat. The Sh-number does not change very much with reactor design, and the gas-liquid contact area determines the mass transfer rate, that is, bubble size and gas holdup will determine reactor efficiency. 

The situation is different when the chemical reaction is not very fast. In this case the equilibrium between substances Red and A in the solution layers near the electrode will be disturbed, and the rate at which reactant Red is replenished on account of reaction (13.37) decreases. When the chemical reaction is very slow, the limiting CD will approach the value... 

Chemical reactions will take place only when the reactant molecules are in intimate contact. In some cases, especially with very fast reactions or viscous liquids, segregation of the reactants can exist, which make the reaction rates and... 

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