exp_j

Chemistry 2301 Phase Diagram Fall 2011

of 7

Binary Liquid-‐Vapour Phase Diagram Recommended Preparatory Reading

Experiment 14 in Shoemaker (1) entitled Binary Liquid-‐Vapor Phase Diagram Experiment 6 in Sime (2) entitled Liquid-‐Vapor Equilibrium in an Azeotropic Mixture The following subheadings in section 6.1 of Mortimer (3)

o Raoult’s Law o Molecular Structure and Ideal Solutions o Phase Diagrams of Two-‐Component Ideal Solutions o Temperature-‐Composition Phase Diagrams

The following subheading in section 6.6 of Mortimer (3) o Liquid Vapour Phase Diagrams

Introduction This experiment is essentially Shoemaker’s Experiment 14: Binary Liquid-‐Vapor Phase Diagram (1). Two significant modifications to this experiment have been made. The cyclohexane/ethyl acetate system is studied, as described by Gordon, Kenkel, Prescia and Towns (4). A refluxing method is used, as described in the article by James W. Rogers and co-‐authors (5). The two component system that we use will exhibit a strong positive deviation from Raoult’s law, resulting in a boiling point minimum.

For a system of two components (A and B), Gibbs’ phase rule is:

𝐹𝐹 = 𝐶𝐶 − 𝑃𝑃 + 2 = 4 − 𝑃𝑃

Where 𝐶𝐶 is the number of components in the system, 𝑃𝑃 is the number of phases (number of physically differentiable parts of the system at equilibrium), and 𝐹𝐹 is the variance or the number of degrees of freedom (number of intensive variables that can be independently varied at equilibrium without altering the number or kinds of phases present). Give Gibbs' phase rule, and explain how it will be applied to your two-‐component, two-‐phase system. Give Raoult’s law. According to this law, the vapour pressure, 𝑝𝑝 , of component A at a given temperature, 𝑇𝑇 , is proportinal to its mole fraction, 𝑋𝑋 , in the liquid. Thus, on a plot of 𝑝𝑝 vs. 𝑋𝑋 , , the lines representing the vapour pressures of A and B and the total vapour pressure are straight lines. However, the total vapour pressure when plotted against vapour composition is not linear and lies below the line representing the total vapour pressure. If Raoult’s law holds true for a solution, the solution is said to be ideal.

O

O

Cyclohexane Ethyl Acetate


of 7

Vapour pressures of the pure liquids, 𝑝𝑝 and 𝑝𝑝 , increase with temperature. At a constant pressure (say 1 atm), the boiling points of the pure liquids are 𝑇𝑇 and 𝑇𝑇 . The boiling point of the solution 𝑇𝑇 , as a function of 𝑋𝑋 , or 𝑋𝑋 , , is not linear, the vapour curve lying above the liquid curve. In most binary liquid-‐vapour systems Raoult’s law is a good approximation for a component only when its mole fraction is close to unity (one). Large deviations are oftentimes encountered for the dilute component or when the mole fraction of neither is close to unity. If, at a given temperature, the vapour pressure is greater than that predicted by Raoult’s law the system is said to show a positive deviation from that law. For such a system the 𝑇𝑇 versus 𝑋𝑋 diagram will be convex downward at a given temperature. On the other hand, if at a given temperature the vapour pressure is lower than that predicted by Raoult’s law, the system is said to show a negative deviation from the law and the boiling point diagram will be convex upward. In many cases the deviations are large enough to produce a maxima or minima in the boiling point diagram. Note that at the maximum or minimum there is a point of tagency of the vapour and liquid curves. Also, at every 𝑋𝑋 the slope of the vapour and liquid curves have the same sign (i.e. positive or negative) and one is zero where and only where the other is zero.

Sketch vapour pressure and boiling point diagrams for ideal and non-‐ideal systems, drawing separate diagrams illustrating positive and negative deviations. Explain why these deviations occur in terms of intermolecular forces. See Shoemaker’s (1) introduction for help with this.

Liquid-‐vapour phase diagrams and, in particular, boiling point diagrams are of importance in

connection with distillation, which usually has the objective of separating a liquid solution into it’s individual components. Basically, the liquid solution is boiled and the vapour condensed into a separate container. Consider a simple distillation of an ideal binary solution having components C and D (the boiling point of D being higher than C). When the solution is boiled and a small portion of the vapour condensed, the drop of distillate (condensed vapour) obtained will be richer in C than in D. This causes the residue in the flask to be slightly richer in D. When the next drop of distillate is obtained it will be richer in D than was the first drop. If the distillation is continued until all of the residue is boiled away, the last drop of distillate obtained will be (virtually) pure D. To completely separate the solution in this way, the reciveing flask would have to be changed many times during the distillation and the separate portions of distillate subsequently distilled in the same way, and so on. This would be very time consuming as many subsequent distillations would be required.

Now consider the same solution but assume that it is not ideal (its boiling point diagram shows a

minimum or maximum). When distilled, the composition of the vapour and residue do not approach pure C or D but, instead, the composition corresponding to the maximum or minimum. Here the solution will distill without a change in composition because the vapour and liquid phases have the same composition. This solution is known as an azeotrope and cannot be separated by distillation. Azeotropes are sometimes useful (the aqueous azeotrope of hydrochloric acid is used as an analytical standard) but most times are nuisances (aqueous 95% ethanol is an azeotrope, the existance of which prevents the preparation of absolute (100 %) ethanol by direct distillation of dilute ethanol solutions).


of 7

Experimental

Safety Issues Since you are heating flammable solvents, please be aware of the location of the closest fire

extinguisher in case it needs to be used. The solvents used are quite volatile. Please keep all containers containing solvents capped

as much as possible to minimize fumes in the laboratory. MSDS sheets for the chemicals used can be found in the experiment’s folder placed near

experimental setup in C-‐3041. Please wear appropriate eye protection, a lab coat and disposable nitrile gloves for the

duration of this experiemnt. Your instructor will supply hints concerning the use and care of the apparatus, which will include the operation of the Mettler-‐Toledo digital refractometer. Treat the glass Cottrell pump with care because it is fragile. Take notice of the five positions of the stopcock (see diagram on following page). It is extremely time-‐consuming to select concentrations which will produce a good phase diagram and allow discovery of the azeotropic properties. Therefore a list of suggested volumes will be supplied.1

1. Unclamp the boiling tube from the ring stand. Place the Cotrell pump in the tube by sliding it down the side. Reclamp the boiling tube in position.

2. With the stopcock in position 1, introduce roughly 100 mL of cyclohexane, measured with a graduated cylinder, into one of the boiling tubes. Add a few boiling stones.

3. Position the thermometer and condenser in the boiling tube. The thermometer should be placed so that its bulb is in the middle of the three spouts on the Cotrell pump. Turn on the condenser water.

4. Mark the level of pure cyclohexane at room temperature on the outside of the boiling tube

using a glass marker.

5. Raise the heating mantle into position around the bottom of the tube and turn on the transformer at a setting of about 90.

1 Posted on the benchtop adjacent to the experimental setup (Thanks to Margaret Miller and Jerome Johnson).


of 7

6. While the cyclohexane is heating obtain 20 sample vials with caps and number two sets from 1 to 10 with one each for liquid (L) and vapour (V) samples (i.e. 1L, 2L, 3L, … and 1V, 2V, 3V, …)

7. Obtain a clean dry reserve flask with a stopper. Record the atmospheric pressure in mm Hg (an instructor will show you how to use the barometer in C-‐3041A). Two or three readings during the laboratory period will be sufficient.

8. When the liquid has been boiling for about 5 minutes. Turn the stopcock clockwise to position 2 to drain the contents of the condensed vapour column into the reserve flask.

9. Turn the stopcock back to position 3. Immediately, record the temperature to the nearest 0.01°C. Allow the column to fill to approximately ¾ full. Again, immediately record temperature.

10. If the two temperatures agree to within 0.20°C, immediately turn off the transformer and lower the heating mantle2, then begin sample

withdrawal. If temperatures do not agree, repeat steps 8 through 10.

Sample Withdrawal 11. Turn the stopcock clockwise to position 2. Condensed vapour will begin to flow out of the spout

into the reserve flask. When the column is half empty turn quickly back to position 3 again to close the stopcock and keep the other half of the condensed vapour in the column.

12. Collect a small sample of condensed vapour in the vial labeled 1V by returning the stopcock to

position 2, then turn back to position 3. It is only necessary to approximately half fill the vial.

2 Do not allow liquids to fall on the mantle. Move it well away from the boiling tube when you are not heating the solution. Also, notice the location of the fire extinguishers adjacent to the main lab doors!!


of 7

13. Drain roughly half of the liquid in the boiling tube into the reserve flask to flush the contents of the spout by turning the stopcock clockwise to position 4. Turn to position 5 to stop the flow when approximately half of the liquid is remaing in the tube.

14. Collect a liquid sample in the vial labeled 1L by returning the stopcock to position 4. Again, it is

only necessary to half fill the vial. Stop the flow be returning the stopcock to position 1.

15. Now, add the required amount3 of pure ethyl acetate, measured roughly with a graduated cylinder.

16. Top up the solution, by adding liquid from the reserve flask, to the 100 mL level as marked on

the boiling tube. Discard the remaining contents of the reserve flask in the organic dump or a waste flask (this is important).

17. Add a few more boiling stones (this is also important!). Raise the heating mantle into position

and turn on the transformer.

18. While the liquid is heating you may measure the refractive indices of the vapour and liquid samples collected in step 12. Your instructor will probably, however, ask you to collect several samples before measuring. Be sure sample vials are tightly sealed to prevent evaporation.

19. Repeat steps 8 through 17 until the suggested increments have been added.

20. When you are comfortable with the operation of the apparatus, return to step 1. This time use

pure ethyl acetate in the second boiling tube and repeat the procedure, adding increments of cyclohexane to the ethyl acetate.

21. If you have not done so already, measure and record the refractive indicies of each sample collected.

Have your data checked by an instructor before carefully disassmbling the apparatus. Do not

rinse any parts with water. Simply empty liquids into the organic dump and leave the apparatus open to the air to dry. If you have not already done so measure the atmospheric pressure. Before leaving the lab, be certain the water flowing through both condensers has been turned off.

Results Calculate the mole percent or mole fraction of cyclohexane in each sample. You may use a linear interpolation between the succesive data in the refractive index -‐ composition table located on the last page of this outline or the calibration equation provided. Calculate the mean temperature over which vapour condensed for each sample.

Construct a boiling point diagram, plotting 𝑇𝑇 versus 𝑋𝑋 . Expand your data to fill the page. This diagram should contain two separate sets of data points, one for the liquid phase and one for the vapour phase. To allow for differeitiation between phases, the data points corresponding to the liquid phase should have a different color or shape than the vapour phase data points. Use french

3 Amounts are posted on the benchtop.


of 7

curves (available in C-‐3041) to fit your data with very smooth curves, tangent at a single point at the minimum.4 If your data is smooth enough, with a good fit and a large, very fine-‐grid plot, your precision will not be limited by your graph, but rather by your thermometer and refractometer, or, if available reference data is not very precise, by your thermometer and refractive index/composition table or calibration equation. Using your completed phase diagram, report an estimate of the azeotropic composition, temperature, and pressure (if measured).

Discussion Your data should be very smooth, with both curves originating at the boiling point of pure cyclohexane on the left and terminating at the boiling point of pure ethyl acetate on the right. Explain any anomalies. Obtain literature values of the azeotropic composition, temperature, and pressure for the cyclohexane/ethyl acetate system from Gordon et al (4), and compare with your experimental values. Why might your experimental results not agree with the literature results?

Using molecular diagrams, show the intermolecular forces which seem to be important in the cyclohexane/ethyl acetate system. Explain how deviations from Raoult's Law occur in this system.

Works Cited 1. Shoemaker, David P., Garland, Carl W. and Nibler, Joseph W. Experiments in Physical Chemistry. 8th. New York : McGraw Hill Higher Education, 2009. ISBN 978-‐0-‐07-‐282842-‐9. 2. Sime, Rodney J. Physical Chemistry -‐ Methods, Techniques, and Experiments. Philadelphia : Saunders College Pubulishing, 1990. ISBN 03-‐0-‐009499-‐2. 3. Mortimer, Robert G. Physical Chemistry. 3rd. Boston : Elsevier Academic Press, 2008. ISBN-‐13: 978-‐1-‐12-‐370617-‐1. 4. Developement of Binary Liquid-‐Vapor Phase Diagram Laboratory Procedures to Replace the Traditional Tetrachloroethylene/Cyclohexanone System. Gordon, Kelly J., et al. s.l. : Chem. Educator, 2007, Vol. 12, pp. 177-‐178. 5. An Improved Apparatus for Determining Vapor-‐Liquid Equilibrium. Rogers, James W., Knight, Jack W. and Choppin, A. R. s.l. : Journal of Chemical Education, October 1947, Vol. 24, pp. 491-‐493.

4 This diagram is the centerpiece of your report. Make it a good one. It is worth a high percentage of the grade for your report.


of 7

Refractive Index / Composition Table Refractive Index and corresponding composition of cyclohexane/ethyl acetate mixtures according to the calibration equation:

𝑛𝑛 = 1.3699 + 0.0363 𝑋𝑋 + 0.0163 𝑋𝑋

𝑛𝑛 mol % C6H12




1.3700 0.00 1.3836 32.37 1.3972 59.56 1.4108 82.69 1.3706 2.62 1.3842 33.69 1.3978 60.70 1.4114 83.64 1.3712 3.62 1.3848 35.01 1.3984 61.83 1.4120 84.59 1.3719 5.61 1.3854 36.31 1.3990 62.95 1.4126 85.53 1.3725 7.10 1.3861 37.61 1.3996 64.07 1.4132 86.46 1.3731 8.57 1.3867 38.90 1.4003 65.17 1.4138 87.38 1.3737 10.04 1.3873 40.19 1.4009 66.27 1.4145 88.29 1.3743 11.50 1.3879 41.46 1.4015 67.36 1.4151 89.20 1.3749 12.95 1.3885 42.72 1.4021 68.44 1.4157 90.09 1.3756 14.39 1.3891 43.98 1.4027 69.51 1.4163 90.98 1.3762 15.82 1.3898 45.23 1.4033 70.58 1.4169 91.86 1.3768 17.25 1.3904 46.47 1.4040 71.63 1.4176 92.73 1.3774 18.66 1.3910 47.70 1.4046 72.68 1.4182 93.59 1.3780 20.07 1.3916 48.93 1.4052 73.72 1.4188 94.45 1.3786 21.47 1.3922 50.14 1.4058 74.75 1.4194 95.29 1.3793 22.86 1.3929 51.35 1.4064 75.77 1.4200 96.13 1.3799 24.25 1.3935 52.54 1.4071 76.78 1.4206 96.96 1.3805 25.62 1.3941 53.73 1.4077 77.79 1.4213 97.78 1.3811 26.99 1.3947 54.92 1.4083 78.78 1.4219 98.59 1.3817 28.34 1.3953 56.09 1.4089 79.77 1.4225 99.40 1.3824 29.69 1.3959 57.25 1.4095 80.75 1.4227 100.00 1.3830 31.03 1.3966 58.41 1.4101 81.73

** Notice that, in the table, composition is expressed in mole percent, not mole fraction.

exp_j

Documents

Transcript of exp_j