--%>

Liquid Vapour Free Energies

The free energy of a component of a liquid solution is equal to its free energy in the equilibrium vapour.

Partial molal free energies let us deal with the free energy of the components of a solution. We use these free energies, or simpler concentration terms to which they correspond, when we deal with a variety of solution equilibrium matters. Here we begin by seeing how the partial molal free energy of a component of a liquid solution can be deduced.

We cannot count on the assuming of ideal behavior when we deal with liquid solutions. The components interact with one another and generally produce free energy effects characteristic of the particular system. Thus, liquid mixtures contrast with gas mixtures for which the ideal solution results are often satisfactory. The strategy in dealing with liquid systems is to relate the free energies of the components to those of the more easily treated equilibrium vapour.

Consider a binary system that can consist of a liquid, a vapour, or a liquid and vapour in equilibrium with one another. In view of the relation illustrated the free energy of the entire system, with superscript l for liquid and v for vapour, can be expressed as:

G = nlA GvA + nlB GlB + nA + nB GvA

For this binary system 

nlA = nvA = nA    and     nlB + nvB = nB

Or

nlA = nA - nvA    and    nlB = nB - nvB

For equilibrium between the liquid and vapour, the free energy will be a minimum with respect to the fraction, or amount of the components in the vapour phase. We can form d/GdnnA and dG/dnvB and set these derivatives equal to zero to obtain

GlA = GvA    and    GlB = GvB

The partial molal free energy of a component in a liquid solution is equal to its partial molal free energy in the equilibrium vapour. This result can be used to relate the partial molal free energies of components in liquid solutions to be partial molal free energies of the components in the equilibrium vapour.

Example: the vapor pressure of benzene and toluene over benzene toluene solutions are shown as plotted points. What do these vapor pressures tell us about the benzene-toluene solutions?

Solution: the vapor pressures of the components are very nearly proportional to the mole fractions of the components. With the subscript B for benzene and T for toluene, this behavior can be described by the equations:

PB = xBB and PT = xTT

Or, PB/P°A = xand PT/P°T = x
T

When these relations are used, we obtain:

GlB = G°B + RT In xB and GlT = G°T + RT In xT

This is the component free-energy behavior that, according to characterizes ideal behavior.

Also the volume of a benzene-toluene solution is very nearly equal to the sum of the volumes of the separate components, and no appreciable enthalpy change accompanies the mixing process. Liquid benzene-toluene solutions confirm closely to ideal-solution behavior. 

   Related Questions in Chemistry

  • Q : Calculating number of moles from

    Choose the right answer from following. If 0.50 mol of CaCl2 is mixed with 0.20 mol of Na3PO4, the maximum number of moles of Ca3 (PO2)2 which can be formed: (a) 0.70 (b) 0.50 (c) 0.20 (d) 0.10

  • Q : Basicity order order of decreasing

    order of decreasing basicity of urea and its substituents

  • Q : Determining maximum Osmotic pressure

    Which of the following would have the maximum osmotic pressure (assume that all salts are 90% dissociated): (a) Decimolar aluminium sulphate (b) Decimolar barium chloride (c) Decimolar sodium sulphate (d) A solution obtained by mix

  • Q : Utilization of glacial acetic acid What

    What is the utilization of glacial acetic acid? Briefly describe the uses.

  • Q : Question on Mole fraction Mole fraction

    Mole fraction of any solution is equavalent to: (a) No. of moles of solute/ volume of solution in litter (b) no. of gram equivalent of solute/volume of solution in litters (c) no. of  moles of solute/ Mass of solvent in kg (d) no. of moles of any

  • Q : Mole fraction Give me answer of

    Give me answer of following question. The sum of the mole fraction of the components of a solution is : (a) 0 (b) 1 (c) 2 (d) 4.

  • Q : What is solvent dielectric effect?

    Ionic dissociation depends on the dielectric constant of the solvent.The Arrhenius that ions are in aqueous solutions in equilibrium with parent molecular species allows many of the properties of ionic solutions to be understood. But difficulties began to

  • Q : PH of an Alkyl Halide Briefly state the

    Briefly state the pH of an Alkyl Halide?

  • Q : Problem based on molality of glucose

    Select the right answer of the question. If 18 gm of glucose (C6H12O6) is present in 1000 gm of an aqueous solution of glucose, it is said to be: (a)1 molal (b)1.1 molal (c)0.5 molal (d)0.1 molal

  • Q : Molecular mass from Raoults law Provide

    Provide solution of this question. Determination of correct molecular mass from Raoult's law is applicable to: (a) An electrolyte in solution (b) A non-electrolyte in a dilute solution (c) A non-electrolyte in a concentrated solution (d) An electrolyte in a liquid so