Dependence of Activity on Pressure

If the value of for a reaction is calculated from the value of AG, we must have values of the 7, to substimte into Equation (16.27) or Equation (16.30) to obtain equilibrium yields in terms of m, or A,. The determination of these quantities from experimental data will be discussed in Chapters 17 and 19. 

Although we have chosen to define the standard state at a fixed pressure, the vapor pressure of the pure solvent, the standard chemical potential is still a function of the pressure chosen, so that the first term on the right in Equation (16.32) is equal to Therefore, 

For solvents, 1, is equal to V because the standard state is the pure solvent, if we neglect the small effect of the difference between the vapor pressure of pure solvent and 1 bar. As the standard state for the solute is the hypothetical unit mole fraction state (Fig. 16.2) or the hypothetical 1-molal solution (Fig. 16.4), the chemical potential of the solute that follows Henry s law is given either by Equation (15.5) or Equation (15.11). In either case, because mole fraction and molality are not pressure dependent. 

the partial molar volume is constant along the Henry s-law line and equal to the standard partial molar volume. The only real solution along the Henry s-law line is the infinitely dilute solution, so 

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