Position of chemical equilibrium

In some cases the extent of reaction is limited by the position of chemical equilibrium, and this extent ( e) will be less than max. However, in many cases e is approximately equal to max- In these cases the equilibrium for the reaction highly favors formation of the products, and only an extremely small quantity of the limiting reagent remains in the system at equilibrium. We will classify these reactions as irreversible. When the extent of reaction at... 

Equation (11.9) shows how the equation of state affects the chemical potential, and enables us to calculate the influence of deviations from the perfect gas laws upon the position of chemical equilibrium. This is most readily achieved by substituting into equation (6.22), which gives the affinity in terms of the chemical potentials ... 

This formula shows clearly the influence of the equation of state on the position of chemical equilibrium we shall work out an example later in this chapter. We note that the parameter K, which is a pseudo-constant of equilibrium, is a function not only of T and p, but also of the mole fractions of the various components. 

This is a most useful definition of the position of chemical equilibrium. 

This important equation tells us how the position of chemical equilibrium can be defined in terms of the free energies of the reactants and products at 1 atm pressure. Such standard free energies can be determined experimentally and are tabulated for use in this way. We shall consider specific examples later. The equation is also valuable in a qualitative sense. If AG° is negative we know the equilibrium position will correspond to the presence of more product than reactants (lnK. > 0). If AG° is positive the reaction will not proceed to such an extent and reactants will predominate in the equilibrium mixture. With this result we have accomplished a major purpose of our study. 

Some of the most valuable results we have obtained from chemical thermodynamics are those that relate the position of chemical equilibrium to the thermodynamic properties of the reactants and products. With the aid of statistical thermodynamics we can go one step further and relate the position of equilibrium to the masses, dimensions, and vibrational frequencies of the molecules involved. 

In many biocatalyzed reactions, the position of chemical equilibrium is important, because it will place a limit on the eventual yield. In such cases, the choice of solvent will usually have a significant effect on the equilibrium position. Because this simply reflects the differential solvation of reactants and products, these effects can be predicted fairly confidently, at least to a reasonable approximation1271. 

Pressure can have a large influence on the position of chemical equilibrium for reactions occurring in the gaseous phase. An increase in pressure favors a shift in the direction that results in a reduction in the volume of the system. But for reactions occurring in solutions, normal pressure changes have a negligible effect on the equilibrium because liquids caimot be compressed the way gases can. 

Chemical reactions do not move in the forward direction only but in either direction and come to a "resting" point of concentrations known as the position of chemical equilibrium. Our goal in this section is to understand how we analyze such a common situation and at the same time to discover the interrelationships between kinetics and thermodynamics as they apply to chemical systems. 

Skill 26.3 Analyze the thermodynamics and kinetic dynamics that move a reversible reaction to a position of chemical equilibrium. 

The maximum extent of an irreversible reaction ( max,iiT) can be obtained by setting/in equation (1.1.9) equal to 1. However, for reversible reactions, the maximum extent of reaction is limited by the position of chemical equilibrium. For these situations, equation (1.1.9) becomes... 

A special circumstance in the use of alternative aldehydes like glyoxalic acid glutaraldehyde, glyoxal, succinaldehyde, etc., is the position of chemical equilibrium in the reaction with the polyhydroxy compounds. In the case of some aldehydes, it lies far to the right and gives a quantitative reaction. In other words, the backward reaction—splitting-off of aldehyde—is not observed because the thermodynamic stability of the intermediates than with formaldehyde-based aminoplast resin systems. 

So far, we have treated the stationary-state quantum mechanics of an isolated molecule. The molecular properties so calculated are appropriate for gas-phase molecules not at high pressure. However, most of chemistry and biochemistry occurs in solution, and the solvent can have a major effect on the position of chemical equilibrium and on reaction rates. (For a survey of solvent effects on rates, equilibria, IR, UV, and NMR spectra, see C. Reichardt, Solvents and Solvent Ejfects in Organic Chemistry, VCH, 1988.) We now examine solvent effects on molecular and thermodynamic properties. 

These organized media possess many unique properties including their ability to solubilize, concentrate, organize, and localize solutes, to modify the spectral parameters and the effective microenvironment about solubilized molecules, to alter chemical and photochemical pathways and rates, as well as the position of chemical equilibrium processes, among other effects. 

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