Effects on Chemical Equilibria

Micellar Effects on Chemical Equflibria.—A few studies have been made of acid-base equilibria in micelles. Hydronium ion activity in anionic micelles has been measured conductimetrically using hydrophilic indicators, it being found that a plot of mn+ versus [H ]-t-[Na ] is linear with a slope of 0.82. The quantity mH+ is defined as the number of micellized hydrogen ions per surfactant head group, namely mH = [H ]tot—[H ]w/ [D]tot c.m.c., where [DJtot is the total catalyst concentration. The use of fluorescent indicators (21a) and (21b) in anionic, neutral, and cationic surfactantspermitted the evaluation of the electrical potential at the micellar surface as a function of added electrolytes. Indicator pK values for mixed micelles and pK values of weak 

Baumgart, G. Klar, and R. Strey, Ber. Bunsenges. Phys. Chem., 1979, 83, 1222. 

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Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

Among many examples of the solvent effects on chemical equilibria and reactions, the solvent effect on tautomerization has been one of the most extensively studied. Experi-... 

The understanding of isotope effects on chemical equilibria, condensed phase equilibria, isotope separation, rates of reaction, and geochemical and meteorological phenomena, share a common foundation, which is the statistical thermodynamic treatment of isotopic differences on the properties of equilibrating species. For that reason the theory of isotope effects on equilibrium constants will be explored in considerable detail in this chapter. The results will carry over to later chapters which treat kinetic isotope effects, condensed phase phenomena, isotope separation, geochemical and biological fractionation, etc. 

C. J. Cramer and D. G. Truhlar,/. Am. Chem. Soc., 113, 8552 (1991). Molecular Orbital Theory Calculations of Aqueous Solvation Effects on Chemical Equilibria. 

Among the theories into which the B.O. approximation has been incorporated are the theory of isotope effects on the rotational-vibrational energy levels of a molecule, the theory of isotope effects on chemical equilibria, and various theories of isotope effects on chemical rates. The relative success of these theories in the interpretation of experimental results suggests that the B.O. approximation must be a relatively "good" approximation. 

Ben-Naim, A. 1975. Solute and solvent effects on chemical-equilibria. Journal of Chemical Physics. 63, 2064. 

We now inquire into the nature of solvent effects on chemical equilibria, taking noncovalent molecular complex formation as an example. Suppose species S (substrate) and L (ligand) interact in solution to form complex C, K, being the complex binding constant. 

The isotope effect on chemical equilibria can be illustrated with the example of acid-base equilibria. Consider the ionization reactions of the weak acids HA and DA in water and heavy water, respectively... 

The theory of isotope effects is well established and has been presented in detail in the books by Collins and Bowman (1970), Melander (1960), Melander and Saunders (1980) and Willi (1983). Only some general principles of isotope effects on chemical equilibria are presented here mainly to introduce the formulations and parlance of isotope effect theory. A frame of reference is given which is intended to allow the interpretations of the isotope effect studies to be followed with regard to the specific problems described. 

In contrast to thermodynamic isotope effects on chemical equilibria which result from changes in vibrational force constants and zero point energies between the initial and the final state, nmr isotope shifts of static molecules are single-state physical properties, intrinsic to that molecule and its particular set of vibrational force constants. Intrinsic isotope shifts normally do not change much with temperature whereas equilibrium isotope shifts show large temperature dependences. 

Pressure is a fundamental physical property that affects various thermodynamic and kinetic parameters. Pressure dependence studies of a process reveal information about the volume profile of a process in much the same way as temperature dependence studies illuminate the energetics of the process (83). Since chemical transformations in SCF media require relatively high operating pressures, pressure effects on chemical equilibria and rates of reactions must be considered in evaluating SCF reaction processes (83-85). The most pronounced effect of pressure on reactions in the SCF region has been attributed to the thermodynamic pressure effect on the reaction rate constant (86), and control of this pressure dependency has been cited as one means of selecting between parallel reaction pathways (87). This pressure effect can be conveniently evaluated within the thermodynamic framework provided by transition state theory, which has often been applied to reactions in solutions (31,84,88-90). This theory assumes a true chemical equilibrium between the reactants and an activated transition... 

Cramer, C. Truhlar, D. G. (1991). Molecular orbital theory calculations of aqueous solvation effects on chemical equilibria. Journal of the American Chemical Society, 113, 8552. 

The shift in electrode charge resulting from the addition of Fe or Fe may be thought of as an extension of Le Chateller s Principle which is often used as a guide to the prediction of temperature, pressure and other effects on chemical equilibria.The principle is applied as follows - Suppose a change (of temperature, pressure, chemical composition,.) is imposed on a system previously at equilibrium. Le Chateller s Principle predicts that the system will respond in a way so as to oppose or counteract the imposed perturbation. For example -... 

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