Prelab Report

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Binary Liquid-Vapor Phase Diagrams

Teaching Assistant: TA Name

Chem 374-005

Group #6

10/17/96

Lab Partner: Their Name

Introduction & Theory

Raoult's law states that, in an ideal mixture, the partial pressure of a component is proportional to its molar percentage-where an ideal mixture is a mixture that obeys Raoult's law throughout its ranges of composition 3. While some mixtures do obey Raoult's law (especially those similar in nature), many do not. Mixtures that have temperatures where the vapor pressure of a component is higher than that predicted by Raoult's law are said to show a positive deviation, while mixtures with components with lower vapor pressures are said to show negative deviations. Often, these deviations are large enough to cause maxima or minima in vapor-pressure and boiling-point curves.

At a maximum or minimum, the plot of the temperature or pressure of L, the liquid phase, and V, the vapor phase become tangent. The mixtures defined by these tangent points are called azeotropes. As a result, azeotropes are mixtures that cannot be separated by simple distillation, as the liquid and vapor phases are of the same composition.

Goal

The goal of this experiment is to plot a liquid-vapor phase diagram for a binary mixture. This diagram will be made from experimental data taken from the liquid/vapor equilibrium of 1,1',2,2'-tetrachloroethane and cyclohexanone.

Apparatus

Simple distillation apparatus consisting of a distilling flask, heat source, mercury thermometer (accurate to 0.1C), one hole stopper, two rubber hoses, distilling adapter, straight-tube condenser, two clamps and clamp holders, two ring stands, one iron ring, 20 small vials (with caps), 100-mL graduated cylinder, two wide-mouthed 250-mL flasks, 2-mL pipette (with bulb), and two 500-mL glass stoppered Erlenmeyer flasks.

Also, a refractometer is used, and must be thermostated to 25C (a sodium vapor lamp is optional). Eye droppers, clean cotton wool, acetone wash bottles, pure 1,1',2,2'-tetrachloroethane (300 mL), pure cyclohexanone (350 mL), rinsing acetone, and a waste disposal bottle are additionally required.















Simple Distillation Apparatus

Figure not available

Figure 1

Experimental Observables

During the distillation, the temperature at the exit of the distillation flask is monitored. Samples are taken to be analyzed for composition.

Experimental Method

A simple distillation apparatus is used, from which small samples of the residue and distillate are analyzed periodically. The samples are analyzed to determine their percent composition, which is plotted against temperature. The temperature against which distillate samples are plotted is the average of the temperatures at the beginning and end of the taking of the sample. For the residue, the temperature is as recorded when the distillation is stopped for the taking of the sample.

Three potential methods for sample analysis include titration, refractive indexing, and density determinations. As physical analytical methods are preferred to chemical methods, except in the case of simple titrations, measurement of the refractive index or density is suggested. Pure cyclohexanone has a nD20 value of 1.4507, and pure 1,1',2,2'-tetrachloroethane has a nD20 value of 1.4942. Log nD20 is nearly a linear function of weight percent of cyclohexanone in this system, and weight percentages may be interpolated from the following table:

Table 1
Weight% Weight% Weight%
Log nD20 C6H10O Log nD20 C6H10O Log nD20 C6H10O
0.17441 0 0.16864 40 0.16360 80
0.17298 10 0.16719 50 0.16256 90
0.17155 20 0.16582 60 0.16158 100
0.17010 30 0.16473 70



These weight percentages may then be converted into mole fractions.

The use of density to determine composition has the disadvantage of being nonlinear. Therefore, a calibration curve must be created from known samples or referenced. Pure cyclohexanone has a density of 0.9478 g/mL and pure 1,1',2,2'-tetrachloroethane has a density of 1.600 g/mL (both at 20C).

Experimental Procedure

A simple distillation apparatus is used, with the thermometer bulb level with the side arm to the condenser, so that the temperature of coexisting liquid and vapor will be measured. A receiving container should be set under the lower end of the condenser to collect distillate that is not taken as samples.

Twenty 5-mL shell vials should be labeled 1L, 1V, 2L, 2V, ... , 10V. This designates ten collections of two samples (L=Liquid residue, V=Condensed Vapor distillate).

With the water circulating through the cooling jacket throughout the experiment, the distillation should proceed at a moderate rate. When the desired temperature is reached, the vapor temperature should be read, a ~2 mL sample taken and sealed, and the temperature read again. These two temperature readings are averaged to determine the vapor temperature. Remove the heat from the flask to halt the distillation. Just as the temperature beings to fall, record the temperature again. This reading serves as the liquid temperature. As soon as the temperature falls by ~15C, remove 2 mL of the residue with a 2 mL pipette, and seal it into the appropriate vial. The distillation should then be resumed.

Following the above general procedures, 10 collections of samples should be taken.

  1. Place 125 mL 1,1',2,2'-tetrachloroethane in the flask. Distill until a constant temperature near 146C is reached. Take samples of pure tetrachloroethane 1V and 1L.
  2. Cool the distilling flask, and add 38 mL cyclohexanone as well as the excess distillate from step #1 to the flask. Resume the distillation, collecting samples 2V and 2L when the temperature reaches 149C.
  3. At 151C collect samples 3V and 3L (it may take some time for the temperature to get to this point)
  4. At 154C collect samples 4V and 4L.
  5. At 157C cool the flask somewhat, then add 35 mL tetrachloroethane and 25 mL cyclohexanone. Resume the distillation. When the temperature reaches 157C again, collect samples 5V and 5L.
  6. Cool the flask somewhat and add 36 mL tetrachloroethane as well as 54 mL cyclohexanone. Resume distillation until the temperature becomes relatively constant and take samples 6V and 6L. If the temperature fails to become constant, make up a 100 mL sample of the composition found in the vapor. Distill this sample to constant temperature and take the samples again.
  7. Clean the flask and fill with 105 mL pure cyclohexanone. Distill until constant temperature is reached (i.e., boiling point). This should occur around 155C. Collect samples 7V and 7L.
  8. Cool the distilling flask, return the unused distillate from paragraph #7 to the flask, and add 20 mL tetrachloroethane. Resume the distillation, and take samples 8V and 8L at 156.5C.
  9. Cool the flask somewhat, and add 50 mL cyclohexanone and 17 mL tetrachloroethane. Distill and collect samples 9V and 9L at 157C.
  10. Distill until constant temperature is reached. Take samples 10V and 10L.

Measure the indexes of refractions of all the samples as soon as possible, as they will decompose. Convert the indexes to molar percent and plot them on the x-axis against temperature. Hand draw a separate best fit line for the vapor and liquid phases.

Experimental Precautions

Preliminary Calculations

Molar percentages (M%) may be determined from weight percentages (W%) with the following formula, where W is the weight of each component, and MW is the molecular weight of each component:

Linear regression of the data in table 1 leads to the following relationship between refractive indexes and weight percentages:

This regression fits with a coefficient of correlation of -0.998

Safety 1

Mercury and mercury vapor are highly poisonous. Inhalation of vapor may lead to fever, nausea, vomiting, diarrhea, headache, chest pain, and possibly death. Skin contact may lead to a rash or allergic reaction, and if extensive may also cause the same effects as inhalation of vapor. Affected persons should be removed to fresh air and contaminated clothing removed. Necessary first aid techniques should be performed. Seek medical attention immediately.

Flammable. Harmful if swallowed, inhaled or absorbed through skin. Affects the central nervous system. If swallowed, inhaled in large amounts, or contacts large areas of skin, seek medical attention and perform appropriate first aid.

Extremely flammable and volatile. Vapors are heavier than air. Slightly toxic. If swallowed, may cause vomiting. Repeated contact with skin may cause dryness. Chronic inhalation may cause irritation of mucous membranes, headache, dizziness, or nausea.

Carcinogen. Overexposure can cause kidney, liver, and delayed lung damage. High vapor concentrations may cause drowsiness. May be absorbed through skin. If swallowed, inhaled in large amounts, or contacts large areas of skin, seek medical attention and perform appropriate first aid.

References

  1. Safety data taken from MSDS database on the Internet at "http://www.hazard.com/msds".
  1. D. P. Shoemaker, C. W. Garland, J. W. Nibler, Experiments in Physical Chemistry, 6th ed., chap. 8, experiment 14, The McGraw-Hill Companies (1996).
  2. P. Atkins, Physical Chemistry, 5th ed., p. 217, W. H. Freeman and Company (1994).