Effects on Equilibrium Position

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. 

One of the equilibria most commonly of interest is esterification. It may be desired to hydrolyze an ester, or reverse this in condensation of an alcohol and acid. Alternatively the hydrolytic equilibrium may be an undesirable side-reaction during transesterification. In this case, at a given water activity, the equilibrium position is quite strongly solvent dependent. The fraction of ester will increase dramatically on going from a polar solvent to non-polar solvent (Fig. 8-7). Hence alkanes are preferred solvents for esterification, while acetonitrile, a ketone or tertiary alcohol would be best for ester hydrolysis. If the equilibrium constant is expressed in terms of concentrations (including that of water), it is relatively solvent independent. However, optimal enzyme behavior in the different solvents usually requires maintaining the same water activity. At fixed water activity, the ratio of ester to acid and alcohol concentrations will be maximized in the least polar solvents. 

---

Change in concentration of Effect on equilibrium position of reaction... 

Hailing, P. J., Solvent selection for biocatalysis in mainly organic systems predictions of effects on equilibrium position, Biotechnol. Bioeng., 35, 691-701, 1990. 

Disturbance Effect on Equilibrium Position Effect on Value of K... 

Substituent Effects on the Position of the Equilibrium of Thiadiazine 1,1 -Dioxides 123... 

The application of this principle to several different systems is shown in Table 12.7. In system 2, the number of moles of gas decreases from to 1 as the reaction goes to the right. Hence increasing the pressure causes the forward reaction to occur a decrease in pressure has the reverse effect. Notice that it is the change in the number of moles of gas that determines which way the equilibrium shifts (system 4). When there is no change in die number of moles of gas (system 5), a change in pressure has no effect on the position of the equilibrium. 

A catalyst has no effect on the position of equilibrium. A catalyst increases the rate at which equilibrium is attained. As discussed in the Reaction Rates chapter, a catalyst provides an alternative route of lower activation energy. Because the rates of both the forward and backward reactions are increased, there is no change in the position of equilibrium. In industry, the presence of a catalyst allows a process to be carried out at a lower temperature (thereby reducing heat energy costs) whilst maintaining a viable rate of reaction. 

Why does a catalyst increase the rate of a chemical reaction, yet have no effect on the position of equilibrium (4)... 

Concentration (1), pressure (1) and temperature (1) may, if changed, alter the position of a chemical equilibrium. These factors often, but not always, have an effect on the position of equilibrium. 

CaO(s) is a pure solid, its concentration does not appear in the equilibrium constant expression and thus its addition will have no direct effect on the position of equilibrium. 

As a consequence, a change in overall volume or total gas pressure will have no effect on the position of equilibrium. In the equilibrium constant expression, the two partial pressures in the numerator will be affected to exactly the same degree as the two partial pressures in the denominator, and Qp will continue to equal Kp. 

Anisothermal Transport Across a Phase Boundary. Once we know the effect of temperature on equilibrium position, we need know only its effects on diffusivities and the condensation coefficient to complete our task. The Stephan-Maxwell equation states that diffusivity in the vapor increases with the square root of the absolute temperature. In the condensed phase the temperature effect is expressed by an Arrhenius-type equation. 

Aminoimidazoles might be expected to prove more complicated than the analogous hydroxy and mercapto derivatives because of the possibility that an R group on NHR could have a marked effect on the position of tautomeric equilibrium, particularly if R is an acyl or sulfonyl function. However, amines are much weaker acids than hydroxy or mercapto compounds and so it should not be necessary to consider zwitterionic structures to any extent an NH group is not at all likely. 

When the volume of a system is decreased, its total pressure increases. Another way to increase the total pressure is to add an inert gas such as argon to the reaction mixture without changing the total volume. In this case the effect on the equilibrium is entirely different. Because the partial pressures of the reactant and product gases are unchanged by an inert gas, adding argon at constant volume has no effect on the position of the equilibrium. 

If the four gases behaved ideally, pressure would have no effect on the position of equilibrium, since there is no change in the number of molecules accompan3ring the homogeneous reaction. However, because of deviations from ideal behavior, a shift in the composition is possible when the pressure is changed. The equilibrium constant may be written as... 

The systematic IR studies of the functional group and heteroatom effect on the position of the tautomeric equilibrium indicated that the stability of thiols (with respect to the corresponding thione forms) is considerably higher than the stability of the hydroxy forms (with respect to the oxo forms) in the same heterocyclic systems, so the mercapto tautomers of mercaptopyridines should be more favored in the equilibrium than their oxygen analogs (92JPC6250). 

Fig. 3.22 Grapefruit plots showing the effect on equilibrium magnetization of (a) a 90° pulse about the y axis and (b) a 90° pulse about the -x axis. Note the position of the Bj field in each case.

---