Azeotropic composition for cyclohexane and ethanol from gas chromatographic data
 Comments to Beatriz H. Cardelino - cardelino@spelman.edu
Goal:   The composition of a binary solution with lowest Gibbs free energy of mixing corresponds to the azeotrope.  Gas chromatography data is used to calculate the Gibbs free energy of mixing for various cyclohexane and ethanol solutions.  Based on the uncertainty of all the measurements and using error propagation techniques, the property is calculated with its associated uncertainty. This exercise is adequate for the undergraduate physical chemistry curriculum.

Prerequisites:  Introductory thermodynamics, phase behavior.

Resources you will need:  EXCEL or similar software package

Background:

The molar Gibbs free energy of mixing (Gmix) for a binary solution is given by:

Gmix    = RT (x1 ln a1 + x2 ln a2) = RT (x1 ln g1 x1 + x2 ln g2 x2)
= RT (x1 ln g1 + x2 ln g2) + RT (x1 ln x1 + x2 ln x2)                                  [1]

where R is the ideal gas constant, T the absolute temperature, xi the molar fraction of the ith components, ai its activity, and gi its activity coefficient.  The ideal molar Gibbs free energy of mixing (Gmixideal) and the excess molar Gibbs free energy of mixing (∆Gmixexcess) are then defined as:

Gmixideal = RT (x1 ln x1 + x2 ln x2)                                                                     [2]
Gmixexcess = RT (x1 ln g1 + x2 ln g2)                                                                 [3]

The regular solution model assumes that the molar entropy of mixing
Smix corresponds to an ideal solution, assumption that is reasonable for small molecules of similar size:

Smix    =  - R (x1 ln x1 + x2 ln x2).                                                                        [4]

Since:

Gmix    Hmix - T Smix                                                                                    [5]

then, the molar heat of mixing
Hmix for a real solution is given by:

Hmix    = RT (x1 ln g1 + x2 ln g2) = Gmixexcess                                                 [6]

The composition with the lowest Gibbs free energy of mixing (see equation [1]) corresponds to the azeotrope.  The mole fractions (xi) can be calculated from the composition of the binary solution.  The activity coefficients (gi) can be derived from an analysis of the composition of the distillate of the binary solution, using Raoult's and Dalton's laws.

Considering the vapor pressure of the ith component above the binary solution (Pi), Raoult's law convention states that:

Pi = ai P0i = gi xi P0i.                                                                                          [7]

where P0i is the vapor pressure of the ith pure component.  If the vapor phase above the solution can be assumed to be ideal, Dalton's law states that:

Pi = yi Ptotal                                                                                                        [8]

where yi is the mole fraction for the ith component in the vapor phase and Ptotal is the total pressure.  If Ptotal, P0i, xi and yi are known, then the activity coefficients in the liquid phase (gi) can be calculated by equating [7] and [8]:
yi Ptotal = gi xi P0i                                                                                               [9]

Experimental Data:

A series of binary solutions of cyclohexane and ethanol were prepared.
1. The mole fractions of the solutions (xi) can be calculated from the composition of the binary solutions, knowing the density of the components and their molar mass.

The solutions were analyzed with a gas chromatograph (GC) and the peak heights were recorded (hi) for the two components.
1. The peak height functions (Hi) may be calculated, as follows:
[10]

The solutions were boiled, the distillates were collected and analyzed using the GC under the same conditions, and the peak heights were recorded (h'i) for the two components.
1. The peak height functions for the distillate (H'i) may be calculated following equation [10].

In order to calculate the molar fraction in the gas phase (yi), standard curves are constructed for the two components of the binary solutions, using mole fractions (xi) and peak height functions (Hi) from the liquid phase.
1. The standard curves consist of polynomial fits of the mole fractions (xi) as a function of peak height functions (Hi).  The mole fractions of the distillates (yi) are determined using the standard curves, when the peak height functions of the distillates (H'i) are used as the independent variables.

The barometric pressure and the room temperature were measured.  The reported value of barometric pressure includes a temperature correction.
1. Since the vapors from the solution displace the air in the apparatus, Ptotal in equation [9] is equal to the barometric pressure and to the sum of the vapor pressures of the two components.

1. The vapor pressure of the pure components (P0i) at the boiling temperature of the binary solutions are estimated using Antoine's equation and available parameters A and B for cyclohexane and ethanol:
[11]

The molar Gibbs free energy of mixing (
Gmix) for the various solutions is calculated from equation [1].  Other thermodynamic terms (GmixidealGmix excessSmixHmix) are also obtained.  All values are calculated with their associated uncertainty, based on the uncertainty in the data.

Experimental Conditions - further information that will be needed:

 room temperature 22.3oC atmospheric pressure 740.0±0.05 torr*1 cyclohexane normal bp 80.7oC*2 ethanol normal bp 78.5oC*2 cyclohexane vapor pressure function ln Pocy = 17.338 - (3789/T)*3 ethanol vapor pressure function ln Poet = 20.62 - (4915/T)*4 density of cyclohexane at 20oC (rcy) 0.7786*2 ± 0.00005*5 g ml-1 density of ethanol at 20oC (rcy) 0.7893*2 ± 0.00005*5 g ml-1 molar mass of cyclohexane (Mcy) 84.16*2 ± 0.005*5 g mol-1 molar mass of ethanol (Met) 46.07*2 ± 0.005*5 g mol-1 cyclohexane standard heat vaporization 29.957 kJ mol-1 *6 ethanol standard heat vaporization 38.744 kJ mol-1 *6

Composition of the solutions*7:

 Sample mL cyclohexane mL ethanol boiling point (oC) 1 0 75 ± 0.4*8 77.5 ± 0.05 2 10 ± 0.1 65 ± 0.4 68.0 ± 0.05 3 25 ± 0.2 50 ± 0.3 65.0 ± 0.05 4 35 ± 0.2 40 ± 0.2 64.5 ± 0.05 5 40 ± 0.2 35 ± 0.2 64.2 ± 0.05 6 50 ± 0.3 25 ± 0.2 64.2 ± 0.05 7 65 ± 0.4 10 ± 0.1 64.5 ± 0.05 8 75 ± 0.4 0 79.0 ± 0.05

GC peak heights for the solutions (cm):

 Sample cyclohexane ethanol 1 0.00 7.95 2 0.94 8.25 3 2.19 6.44 4 2.86 5.00 5 3.96 6.02 6 5.29 4.62 7 7.00 2.29 8 5.72 0.00

GC peak heights for the distillates (cm):

 Sample cyclohexane ethanol 1 0 7.95 2 4.40 6.75 3 4.31 4.79 4 4.45 4.75 5 5.11 4.60 6 4.25 3.22 7 3.81 2.65 8 5.72 0.00

Exercise:

(Note:  See Sample Calculations below)

1.      Calculate the mole fractions for the solutions (xi), and their uncertainties.  Remember that the error of sums or subtractions is the sum of the errors of the individual terms and that the relative error of products or divisions is the sum of the relative errors of the individual terms.

2.      Calculate the peak height functions for the solutions (Hi), as well as for the distillates (H'i).

3.      Obtain third-order polynomial fits of xi vs Hi for cyclohexane and for ethanol.

4.      Using the third-order polynomial functions, calculate the mole fractions in the distillates (ycy and yet) as a function of the peak height function of the distillates (H'cy and H'et).  Assume that the associated uncertainty for the y values is the standard error of the y estimates.

5.
Using the corresponding Antoine equation, calculate the vapor pressure of pure cyclohexane and ethanol (P0i) at the boiling temperatures.  Calculate the uncertainty for the vapor pressures of the pure compounds, at the boiling temperatures, using the formulas:

6.      Estimate the activity coefficients for the components (γi), and estimate their associated errors.

7.      Calculate the molar Gibbs free energies of mixing (∆Gmix) at the boiling temperature, with their associated errors.  Remember that

.

8.      Calculate ∆Hmix (or ∆Gmixexcess) and ∆Smix (or -∆Gmixideal/T) and estimate their associated errors.

9.      Plot
Gmix vs xi. Fit the data into a third-order polynomial function.  Calculate the azeotrope composition by finding the value of xi at the minimum of the Gmix curve.

10.  Estimate, by interpolation, the boiling temperature for the azeotrope.

Questions:

1.      What assumption is made concerning the vapor phase produced at the boiling point?  Is it valid?

2.      How do the boiling points of the pure components compare with the literature values?

3.      How do activity coefficients vary with mole fraction?

4.      Does a boiling point graph (boiling temperature vs mole fractions) give similar azeotrope as the Gibbs free energy calculation?

5.      What is the meaning of the signs associated with
Gmix and Hmix?

6.      If one had a mixture corresponding to the azeotropic composition and attempted to distill it, what would take place?

7.      If the accepted*9 value for the azeotrope is 69.5% cyclohexane by mass, and the boiling temperature is 64.9oC, what is the accuracy of the results?

Sample Calculations:

1.      Mole fraction for cyclohexane (xcy) in solution 2:

where ni is the number of moles, ri is the density, Vi is the volume, and Mi the molar mass of the ith component.

Associated uncertainty for xcy in solution 2 (xcy):

2.      Peak height functions for cyclohexane in solution 2 (Hcy and H'cy):

3.      Standard curve of xi vs Hi for cyclohexane:   x = a + b H + c H2 + d H3.

 Solution # xcy Hcy Hcy2 Hcy3 1 0.000 0.000 0.000 0.000 2 7.670E-02 1.023E-01 1.046E-02 1.070E-03 3 4 5 6 7 8

Using LINEST(known_y's,[known_x's],true,true) {Ctrl-Shift,Enter}

 d c b a ± Dd ± Dc ± Db ± Da 0.9996 0.0095
xcy = a + b Hcy + c Hcy2 +d Hcy3
Coefficient of determination = 0.9996
Standard error of the y estimates: 0.0095.

4.      Mole fraction for cyclohexane in the distillate of solution 2:
ycy = a + b H'cy + c H'cy2 + d H'cy3
Dycy = 0.0095

5.      Vapor pressure for pure cyclohexane in solution 2:

6.      Activity coefficient for cyclohexane in solution 2:

7.      Molar Gibbs free energy of mixing (Gmix) for solution 2;

Associated uncertainty in solution 2 [
(Gmix)]:

8.      Other thermodynamic properties:
Similarly to Gmix, selecting only certain terms.

9.      Location of the azeotrope:
Use LINEST to obtain the polynomial fit    Gmix = a + b xcy + c xcy2 + d xcy3.
Minimize the function with respect to xcy.

(d Gmix / xcy)  =  b + (2 c) xcy,min + (3 d) xcy,min2 = 0

Solve for xcy,min.

10.  Given that xcy,min is between xcy,j and xcy,k, with boiling temperatures Tj and Tk:
Solve for Tmin.

Notes and references:

*1 Includes a barometer temperature correction of -2.7 torr based on Lange's Handbook of Chemistry, Editor John A. Dean, Copyright 1973 by McGraw-Hill, Inc., page 2-30.
*2 http://organicdivision.org/organic_solvents.html; last accessed 02/01/2008.
*3 Based on data from B.E. Poling, J. M. Prausnitz, J. P. O'Connell, "The Properties of Gases and Liquids", 5th Edition, McGraw-Hill, 2001, fitted between 60o and 85oC.  Pressure in torr; temperature in K.
*4 Based on data from R.C. Reid, J. M. Prausnitz, B. E. Poling, "The Properties of Gases and Liquids", 4th Edition, McGraw-Hill, 1987, fitted between 60o and 85oC. Pressure in torr; temperature in K.
*5 The uncertainty is estimated from the unreported decimal place.
*6 Chemical Engineering Research Information Center; http://www.cheric.org/research/
kdb/hcprop/cmpsrch.php; last accessed
02/15/2008.
*7  Experiment performed 3/26/1989 by Beatriz H. Cardelino using a GOW-MAC 350 gas chromatograph, with thermal conductivity detector, Chromosorb (nonpolar) column, helium as a carrier gas, and column and inlet temperatures set to 83oC.  Setup similar to D. P. Shoemaker, C. W. Garland, J. W. Nibler, Experiments in Physical Chemistry, 5th edition, Copyright 1989 by McGraw-Hill, Inc., page 236.
*8 Accuracy of pipettes: 5mL = 1%; 10mL = 0.8%; in 20mL = 0.5%.
*9 CRC Handbook of Chemistry and Physics, 59th Edition, 1978-1979.

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