Effect of pressure on the equilibrium constant

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 ... 

Pressure Transition-state theory accounts for pressure effects on solution reaction rates through the effect of pressure on the equilibrium constant K. At constant temperature and electrolyte concentration (constant ionic strength),... 

Qualitative experimental observations on the effect of pressure on the equilibrium constant, K... 

Exactly the same principles apply to the study of ultrasonic absorption in acid-base systems, except that the quantity displacing the equilibrium is now the oscillating pressure of the sound wave rather than the electric field. The effect of pressure on the equilibrium constant is given by... 

For reactions in solntion an additional thermodynamic property that can be helpful is available. The effect of pressure on the equilibrium constant of a reaction yields the volume change of reaction, AT, given by Equation 1.28. 

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 temperature on the equilibrium constant follows from Eq. (4-41) written for pure speciesj in its standard state (wherein the pressure P° is fixed) ... 

Oxygen isotope fractionations. Isotopic fractionation is generally thought to be independent of pressure. However, in specific cases, pressure can lead to significant changes in the magnitude of fractionation factors. From classical thermodynamics, the effect of pressure on an equilibrium constant is given by ... 

In 9.8 we discussed the effect of pressure on an equilibrium constant, in the special case in which the A,.V° can be considered constant. This equation is... 

All partitioning properties change with temperature. The partition coefficients, vapor pressure, KAW and KqA, are more sensitive to temperature variation because of the large enthalpy change associated with transfer to the vapor phase. The simplest general expression theoretically based temperature dependence correlation is derived from the integrated Clausius-Clapeyron equation, or van t Hoff form expressing the effect of temperature on an equilibrium constant Kp,... 

By applying equation 49 to standard-state partial molar volumes of species in a reaction, we obtain the basic expressions for describing the effect of change in pressure on the equilibrium constant of the reaction. 

It should be noted that although JRT, and hence Kf, is independent of the pressure, the equilibrium functions K f and Kp for a homogeneous gas reaction should vary with the pressure, as is actually found to be the case (see Table XXII). This is, of course, due to the dependence of the activity coefficients on the pressure at constant temperature, as indicated in 32d. Since a change of pressure is usually accompanied by a change in the composition of the system it is not possible to use equation (30.19) to determine the effect of pressure on the activity coefficient factor. The calculation can be made by thermodynamic methods, along lines which will be described in Chapter XIV in connection with physical equilibria, but the results have no direct practical application without introducing a number of approximations. 

It is opportune to recall here the remarks made in 32g in connection with heterogeneous reactions. If the activities of pure solids and liquids taking part in the process are taken as unity, then the true equilibrium constant is obtained only if the total pressure of the system is 1 atm. Unless allowance is made for the effect of pressure on the activities, the constants obtained at other pressures vary with pressure in a manner dependent on the volume change of the solid and liquid phases involved in the reaction [cf. equation (31.7)]. 

THE EFFECT OF CHANGES IN TEMPERATURE AND PRESSURE ON THE EQUILIBRIUM CONSTANT... 

For chemical reactions, we have repeatedly assumed that a small but essentially constant concentration of the transition state is in equilibrium with the reactants. It is the concentration of the transition state that determines the magnitude of the rate constant. In Section 2.8, we dealt with the effects of temperature on the rate constant, but it should also be apparent that pressure can affect the value of fe if the transition state occupies a different volume than that of the reacting species. If the transition state occupies a smaller volume than the reactants, increasing the pressure will shift the equiftbrium toward the formation of a higher concentration of the transition state, which will increase the rate of the reaction. If the transition state occupies a larger volume than the reactants, increasing the pressure will decrease the concentration of the transition state and decrease the rate of the reaction. As will be discussed in Chapter 5, the effect of internal pressure caused by the solvent affects the rate of a reaction in much the same way as does the external pressure. 

Comments This example demonstrates the effect of pressure on the composition of the equilibrium mixture. The equilibrium constant itself is independent of pressure but the equilibrium composition is affected by pressure. This dependence comes through the term (on the left-hand side in eq. ri4.281i... 

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