Effect of pressure on equilibrium constants

We can also calculate the standard entropy change associated with the reaction as AG° - AH0 - TAS°. 

As we would expect, the energy of the molecules is large and negative with respect to the constituent atoms, thus the enthalpy change on dissociation is positive and unfavourable to the dissociation process. However, the additional freedom acquired by the dissociated atoms leads to a positive entropy change. This favours dissociation, which becomes more significant as the temperature is increased. 

As Kp may be defined in terms of the free energies of the participant substances in the standard states, i.e. at 1 atm pressure, it must be independent of pressure. 

If we express the partial pressures in terms of the degree of dissociation a, 

Thus as KP is independent of the total pressure, the degree of dissociation a must in fact decrease as P increases. 

---

The effect of pressure on equilibrium constants is given by equation 1.43 ... 

The effect of pressure on equilibrium constants has been explored in a number of instances, and the Van t Hoff equation (XV.5.8) has been verified from independent studies of the partial molar volumes. This has been reported for the isomerization of cis-dichloroethylene with reasonable accuracy and qualitatively for N2O4 dissociatioiF and the ionization of weak electrolytes. ... 

In addition to the effects of temperature and ionic strength, rates are also affected by changing pressure. By analogy to the effect of pressure on equilibrium constants, the expected effect of pressure on rate constants (Asano andleNobel, 1978 Drljaca etal, 1988 Eckert, 1972) is related to the change in volume that results from the conversion of the reactants to the activated complex. This activation volume (A F ) is the difference between the partial molal volumes of the reactants and the transition state. 

Because the rates of reaction are much less sensitive to the effect of pressure than of temperature, studies of the effects of external pressure for reactions in solution are more difficult to carry out, and need pressures of several thousand atmospheres (kilobars). However, they do yield important information on the transition state, which helps understanding mechanisms. The theory of pressure effects was initially formulated by van t Hoff in terms of the effect of pressure on equilibrium constants... 

I continue to feel that the study of the volume changes in protein reactions is sorely neglected. They may be determined by dilatometry and by the effects of pressure on protein equilibrium constants. The results complement the results of the determination of enthalpy changes as measured by calorimetry and the effects of temperature on equilibrium constants. Much useful insight at the molecular level can be obtained from a knowledge of volume changes... 

The Effect of Pressure With the addition of one restricting condition, we like the simple and direct statement of Rossini1 in describing the effect of pressure on the equilibrium constant ... 

The choice of reactor temperature depends on many factors. Consider first the effect of temperature on equilibrium conversion. A quantitative relationship can be developed as follows. Start by writing Equation 6.6 at constant pressure ... 

The effect of pressure on chemical equilibria and rates of reactions can be described by the well-known equations resulting from the pressure dependence of the Gibbs enthalpy of reaction and activation, respectively, shown in Scheme 1. The volume of reaction (AV) corresponds to the difference between the partial molar volumes of reactants and products. Within the scope of transition state theory the volume of activation can be, accordingly, considered to be a measure of the partial molar volume of the transition state (TS) with respect to the partial molar volumes of the reactants. Volumes of reaction can be determined in three ways (a) from the pressure dependence of the equilibrium constant (from the plot of In K vs p) (b) from the measurement of partial molar volumes of all reactants and products derived from the densities, d, of the solution of each individual component measured at various concentrations, c, and extrapolation of the apparent molar volume 4>... 

The effect of pressure on chemical processes which occur in RPC has not been widely investigated. At first ance, the effect may appear to be significant as a consequence of the isothermal relationship between the equilibrium constant and pressure ... 

The effect of pressure on polymerization, although not extensively studied, is important from the practical viewpoint since several monomers are polymerized at pressures above atmospheric. Pressure affects polymerization through changes in concentrations, rate constants, and equilibrium constants [Ogo, 1984 Weale, 1974 Zutty and Burkhart, 1962], The commercial polymerizations of most gaseous monomers (e.g., vinyl chloride, vinylidene chloride, tetrafluoroethylene, vinyl fluoride) are carried out at very moderate pressures of about 5-10 MPa (1 MPa = 145 psi), where the primary effect is one of increased... 

Water, however, has the opposite effect. Actual equilibrium constants at various temperatures are given in Table 7. A detailed study of the effect of pressure on urea conversion is given in Ref. 17. 

---