Effects Kinetic

Many different approaches have been suggested as possible approaches to this problem, from the 1960s onwards [Verwer and Leusen 1998]. What is obvious from all of these ellorts is that this is an extremely difficult problem. Both thermodynamics and kinetics can be important in determining which crystalline form is obtained under a certain se1 of experimental conditions. Kinetic effects are particularly difficult to take into accouni and so are usually ignored. A proper treatment of the thermodynamic factors would lequire one to deal with the relative free energies of the different possible polymorphs... 

Finally, a consideration of equilibrium chemistry can only help us decide what reactions are favorable. Knowing that a reaction is favorable does not guarantee that the reaction will occur. How fast a reaction approaches its equilibrium position does not depend on the magnitude of the equilibrium constant. The rate of a chemical reaction is a kinetic, not a thermodynamic, phenomenon. Kinetic effects and their application in analytical chemistry are discussed in Chapter 13. 

From equation 12.1 it is clear that resolution may be improved either by increasing Afr or by decreasing wa or w-q (Figure 12.9). We can increase Afr by enhancing the interaction of the solutes with the column or by increasing the column s selectivity for one of the solutes. Peak width is a kinetic effect associated with the solute s movement within and between the mobile phase and stationary phase. The effect is governed by several factors that are collectively called column efficiency. Each of these factors is considered in more detail in the following sections. 

Electro-Kinetic Effects. The appHcation of d-c potential in filtration or sedimentation is known to have a beneficial effect on the separation. Although this has been known and studied since the beginning of the nineteenth century, practical appHcation and development have only accelerated since the late 1980s commercial appHcation is likely. 

Surfactants aid dewatering of filter cakes after the cakes have formed and have very Httle observed effect on the rate of cake formation. Equations describing the effect of a surfactant show that dewatering is enhanced by lowering the capillary pressure of water in the cake rather than by a kinetic effect. The amount of residual water in a filter cake is related to the capillary forces hoi ding the Hquids in the cake. Laplace s equation relates the capillary pressure (P ) to surface tension (cj), contact angle of air and Hquid on the soHd (9) which is a measure of wettabiHty, and capillary radius (r ), or a similar measure appHcable to filter cakes. 

The steric and electronic effects of substituents on the electrophilic attack at the nitrogen atom have been discussed in the general chapter on reactivity (Section 4.02.1.3). All the conclusions are valid for pyrazoles and indazoles. The effect on equilibrium constants will be discussed in detail in the sections dealing with values (Sections 4.04.2.1.3(iv) and (v)) and the kinetic effects on the rates of quaternization in the corresponding section (4.04.2.1.3(vii)). 

That the specific rate is affected by extremes of pressure—sometimes upward, sometimes downward—is well known. A review of this subject is by Kohnstam ( The Kinetic Effects of Pressure, in Pi ogi e.s.s in Reaction Kinetics, Pergamon, 1970). Three examples follow ... 

If the contact angle is known, insight into the extent of wetting to be expected at equilibrium can be obtained from calculations for idealised rough surfaces. The conclusions may require modification when kinetic effects, such as setting of the adhesive, are taken into account. 

Afterwards, kinetic effects determine whether many small islands are formed on a new layer, or only a few large islands develop there [12,19,81,94]. One must take into account many different length and time scales for the interface processes. The typical residential time of an adsorbed atom on the surface might be... 

In Eq. (76) we neglect the kinetic effects, that is, the dependence of the interface temperature on the growth velocity v . The approximation holds at sufficiently small undercoolings and velocities. 

It was shown that the effect of the particle size is not significant in HOPC (1). The experiments were conduced using silica gels of the same pore size but with a different average particle size between 15 and 100 /urn. A kinetic effect— enrichment of the mobile phase with high MW components is better at short times before equilibrium is reached—was cited as a possible reason for almost equal quality of separation by large particles. The back-pressure problem was not serious in that range of the particle size. 

Chemical themiodynamics provides tlie answer to tlie first question however, it provides information about tlie second. Reaction rates fall witliin tlie domain of chemical kinetics and are treated later in tliis section. Both equilibrium and kinetic effects must be considered in an overall engineering analysis of a chemical reaction. 

The dependence of reaction rates on pH and on the relative and absolute concentrations of reacting species, coupled with the possibility of autocatalysis and induction periods, has led to the discovery of some spectacular kinetic effects such as H. Landolt s chemical clock (1885) an acidified solution of Na2S03 is reacted with an excess of iodic acid solution in the presence of starch indicator — the induction period before the appearance of the deep-blue starch-iodine colour can be increased systematically from seconds to minutes by appropriate dilution of the solutions before mixing. With an excess of sulfite, free iodine may appear and then disappear as a single pulse due to the following sequence of reactions ... 

A similar kinetic effect was reported for the reaction of 4-chloro-pyridine 1-oxide with methoxide ion at 50°, and still larger effects were obtained with the 2- and 3-isomers at the same temperature. ... 

The kinetic effects of some substituents on the fused benzene ring (CHg, NOg, OCHg, Cl) havc been determined recently for the reaction between the methoxide ion and the series of compounds aj-R-2-chlorobenzothiazole. Some of these effects are compared in Table XVIII with those observed at the 6-position of 2-chloroquino-lines and quinoxalines which are of the conjugative class. The data for... 

Asano and co-workers have reported die kinetic effects of pressure, solvent, and substituent on geometric isomerization about die carbon-nitrogen double bond for pyrazol-3-one azomethines 406 (R = H), 406 (R = NO2) and 407, (Scheme 93). The results demonstrate the versatility of die inversion mechanism. The rotation mechanism has been invalidated. First-wder rate constants and activating volumes for diermal E-Z isomerization for 406 (R = H) and 406 (R = NO2) are given at 25°C in benzene and methanol (89JOC379). 

Estimate the flood point from Figure 8-137, which accounts for liquid flow effects and is a ratio of liq-uid/vapor kinetic effects [79]. Flooding velocity is obtained from... 

By measuring the kinetic effects of the fluid in motion, since at a given pressure drop, low-density steam will move at a much greater velocity than will high-density condensate, and the conversion of pressure energy into kinetic energy can be used to position a valve. 

In common with a number of heterocyclic iodinations, kinetic effects are found in the iodination of indole and 2-methylindole [68AC(R)1435], When the substituent effects for the reaction are examined it is clear that any resonance effects from the fused benzene ring are only poorly relayed to the reactive 3-position, and the rates appear to be controlled by inductive effects. A 5-methyl group was more activating than 5-methoxy [69AC(R)799]. 

In summary, the Third Law predicts that ordering processes are favored as the temperature is lowered, so that eventually perfect order should be obtained in any solid as its temperature approaches 0 K. But kinetic effects are such that the equilibration times needed to achieve this order are sometimes very long. 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

The use of internal viscosity forces permit us to take into account kinetic effects associated with deformation rates which were beyond the scope of most polymer... 

The kinetic effect of increased pressure is also in agreement with the proposed mechanism. A pressure of 2000 atm increased the first-order rates of nitration of toluene in acetic acid at 20 °C and in nitromethane at 0 °C by a factor of about 2, and increased the rates of the zeroth-order nitrations of p-dichlorobenzene in nitromethane at 0 °C and of chlorobenzene and benzene in acetic acid at 0 °C by a factor of about 559. The products of the equilibrium (21a) have a smaller volume than the reactants and hence an increase in pressure speeds up the rate by increasing the formation of H2NO. Likewise, the heterolysis of the nitric acidium ion in equilibrium (22) and the reaction of the nitronium ion with the aromatic are processes both of which have a volume decrease, consequently the first-order reactions are also speeded up and to a greater extent than the zeroth-order reactions. 

Addition of perchlorate ion had little kinetic effect, but addition of chloride ion decreased the rate and complicated the kinetics, probably through intervention of the equilibrium... 

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