Imperfect vapour phase

When the vapour phase deviates appreciably from the perfect gas laws it follows from the treatment of the last section that Baoult s and Henry s laws must be expressed in terms of fugacities. Thus 

This result may be compared with equation (3 72) which refers to an ideal gaseous solution and where // is the fugacity of the pure vapour component at the temperature and tote pressure under discussion. If it occurs that the liquid and vapour phases in equilibrium are both ideal solutions, and remain ideal up to we obtain 

The left-hand sides of these equations are given in terms of enthalpies and volumes by (2 113) and (2 111) respectively. Substituting from these relations we obtain 

Now fif, and therefore also its partial derivatives, is independent of composition. The same must therefore be true of and in an ideal solution the partial molar enthalpy and volume are independerU of composition. 

In the case of liquid mixtures which are ideal over the complete range of composition, from =0 to x, = 1, Hi and Vi are therefore the same as hi and t the enthalpy and volume respectively, per mole of the pure component. It follows that the total enthalpy T(=Z%jET ), and the total volume F(=Sn F ), of any such mixture, are the same as those of the components before mixing. Such mixtures can therefore be prepared, at constant temperature and pressure, without heat effect or volume change. 

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For systems in which the vapour phase imperfections are not significant, equation 8.32 reduces to the familiar Raoult s law equation (see Volume 2, Chapter 11) ... 

The equations for an imperfect vapour analogous to (6-23) and (6 24) may be obtained by a similar procedure. The following method of derivation is rather simpler, and could also have been adopted for the derivation of (6-23) and (6 24). Let / be the fugacity of the volatile substance in the vapour phase, where its chemical potential is Then from the fugacity definition (3 56)... 

It follows that the equilibria of solutions can always be discussed thermodynamically in terms of the partial pressures in the saturated vapour (or the fugacities if the vapour phase is imperfect). Consider, for example, the reaction of NO2 with water as in nitric acid manu-... 

Vapor-phase imperfection and the variation of the Gibbs energy of the pure liquid components are accounted for through the second molar virial coefficients and die molar volumes Plunder saturation pressmes. The vapour pressure P, Eq. (3.2), the vapour phase compositional, Eq. (3.3), die partial molar excess Gibbs energies, /i,, Eqs. (3.5) and (3.6), the excess Gibbs energy G, Eq. (3.7), and die activity coefficients at infinite dilution, 7 and 7 , Eqs. (3.8) and (3.9), are calculated ... 

The entropy of the compound in the ideal gas state at 298.15 K and the standard pressure (101.325 kPa) is calculated from the entropy at 298.15 K of the condensed phase, the enthalpy of vaporization at 298.15 K, and vapour pressure and gas imperfection data, using the equation ... 

The conhgurational energy of the reference component is the part of the internal energy which arises from interactions. Therefore it is the difference between the energy of the reference component in the condensed phase and its energy as a perfect gas. It has to be equated to RT —. SP where jgP is the latent heat of vaporization (for a vapour extrapolated to zero pressure in order to take account if necessary of the gas imperfections). 

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