Experiment 7

Distillation of a Mixture of n-Hexane and n-Heptane

Experiment 7

Distillation of a Mixture of n-Hexane and n-Heptane

Note: This laboratory will be conducted in the new chemistry building in labs 121 and 127. Go directly there at the start of your scheduled lab section on Tuesday. Introduction

The goal of this experiment is to carry out a simple distillation of a liquid binary mixture of two organic substances, n-hexane and n-heptane, analyze the contents of samples collected from both the liquid and vapor phase during the distillation, and create a liquid - vapor composition diagram similar to Figure 6.17 in Oxtoby. Hexane and heptane are straight chain hydrocarbons containing six and seven carbon atoms and are major constituents of gasoline. To a reasonable degree such hydrocarbons behave as ideal binary mixtures: (a) their gas - liquid equilibria can be described by Raoult's law, (b) they obey Dalton's law of partial pressures, and (c) their vapor pressures as a function of temperature can be approximated by the Clausius - Clapeyron equation. The original mixture prepared for distillation is 80% by volume hexane and 20% by volume heptane. The basic procedure you will be performing in this laboratory is as follows. During the course of the distillation at preselected temperature intervals, you will collect a pair of samples. Each pair will consist of one sample of approximately 0.5 ml from the boiling flask, and one sample of approximately 0.5 ml from the end of the condenser. At the same time you will record the temperature of the. After the distillation has finished, you will determine the composition of the liquid and condensed vapor samples by the technique of gas chromatography (GC). For each sample, there will be two peaks, the first from the n-hexane and the second from the n-heptane. By running a standard mixture of n-hexane and n-heptane, you can determine separate response factors that relate peak height to moles of each compound. Using a ratio method, you can convert these values to mole fractions in the sample. You will tabulate the data and present a graphical temperature - composition diagram for n-hexane / n-heptane pair.

Materials Condenser with two hoses 10 mL graduated cylinder Thermometer 3-4 boiling chips Heating well 10 sample vials with plastic tops Boiling flask with side arm Pipette and bulb Neck piece 10 microliter syringe Procedure You will work in teams of three to four students, as assigned by the instructor. Two lab partners will work with the distillation apparatus and one to two will work with the GC. 1. Wear goggles, as always. Please note that the samples are highly flammable. No open flames will be permitted in the laboratory.

Distillation

2. If the distillation apparatus is not already assembled, assemble it to resemble the model apparatus in the lab. If the hexane/heptane mixture has not already been added to the boiling flask the teaching assistant will add it. Make sure that your heater is properly connected to a variac (variable AC) power controller. Do not plug the heater directly into the wall. 3. Have your teaching assistant check the apparatus before proceeding. This is mandatory. 4. From Table 1 below, which lists the properties of n-hexane and n-heptane, preselect 5 temperatures which are evenly spaced between the boiling points of the two substances(i.e., your points should not include the boiling points). Label your 10 sample vials with these temperatures and whether it comes from the boiling flask or the end of the condenser. NOTE: The final data table will contain seven data points. The five data points you selected plus a mole fraction of 0 for heptane at the hexane boiling point temperature and a ole fraction of 1 for heptane at the heptane boling point temperature. 5. Place 100 mL of a 80/20 mixture of n-hexane / n-heptane into your bioling flask. Place the volumetric cylinder under the condenser outlet. 6. Cautiously begin heating. Adjust the heating rate so that the mixture distills slowly, approximately one drop every 2 - 3 seconds. Be careful not to burn yourself. 7. As the temperature approaches the first collection point, obtain the first pair of sample vials and prepare them for sample collection. 8. When the temperature reaches the first collection point, one lab partner will quickly withdraw a sample of approximately 0.5 mL from the boiling flask and one will replace

the volumetric cylinder with the appropriate sample vial and collect 0.5 mL. of sample from the condenser. When the samples have been collected, immediately replace the caps to prevent selective evaporation of the hexane. While one lab partner delivers the samples to the GC station for analysis, the other lab partner will continues to record the temperature and volume. 9. As the distillation proceeds, you may need to turn up the power on the variac to the heater to maintain the higher heptane content mixture at a boil. 10. If the volume in the boiling flask becomes so small that sample collection is impossible, add a 50/50 mixture of hexane/heptane to the flask and continue to supply heat. When the mixture begins to boil, the temperature will stabilize briefly at the boiling temperature of a mixture will start to boil at the boiling temperature of a 50/50 mixture and continue to . (What would be the mole fraction of heptane in a 50/50 mixture by volume?) 11. Terminate the distillation by turning off the power when only 2-3 mL of liquid remains in the flask - never allow the flask to boil dry. 12. Each sample is taken immediately to the GC room for analysis. Below are instructions for the gas chromatographic analysis.

GC Analysis

13. As the distillation is being started, the lab partner assigned to the GC turns on the chart recorder, makes sure the chart speed is set to one cm/min. and lowers the pen so it begins to make a line on the chart. With the zero control on the GC move the pen to the next to the last division on the chart. Set the attenuation to a scale recommended by your TA (approximately 8). Remember the higher the attenuation setting, the smaller the signal. 14. Start with the calibration sample, which is a 50/50 mixture of n-hexane and n-heptane by volume. Use the data in Table I to convert this to moles of each component present in the sample volume chosen. 15. Make sure the microliter syringe is working properly by withdrawing a full 10 microliter sample and expelling it. Ten microliters is a tiny quantity, but enough to form a drop on the end of the syringe. If you cannot observe such a drop when you expel the contents of the syringe, call the TA. First, remove air bubbles by quickly pumping the syringe up and down several times. Then draw into your syringe a 2 microliter sample of the standard. 16. Inject 2 microliters of sample into the GC. Mark the point of injection on the chart. Your first peak (hexane) should appear at approximately one minute. The second peak (heptane) will appear in approximately one more minute. If you see no peaks after 5 minutes, perform a second injection. If nothing appears, consult your teaching assistant.

17. It is desirable that you set the attenuator so that the most intense peak of the calibration sample is no more than 50% of full scale. 18. When you have the proper chromatogram for the calibration, run ten 10 samples labeling each clearly on the chart paper. 19. If there is any time left, rerun the samples. 20. Leave the syringe on top of the GC, be sure the chart pen is up, and the paper on the chart recorder is not moving before leaving. 20. Empty the contents of the sample vials into the waste container provided and leave them in the hood with the distillation apparatus with the caps OFF and the open end facing UP. Hexane and heptane are very volatile and will completely evaporate leaving the vials clean and dry for the next class.

Data Analysis 1. From the calibration run on the GC, find a conversion factor for each component that converts peak height in millimeters to moles. For the standardization sample you will have to convert from 50% by volume to moles of each component per microliter. Your conversion factor is then the total moles in the sample divided by the peak height. This conversion factor, whose units are moles/mm can be used to multiply the corresponding peak height of the unknown sample and convert it to moles. A Calibration Table is provided below for these calculations. Once you have the moles of the two components, you can convert to mole fractions by dividing the moles of one of the components (use heptane) by the total moles obtained from the sum of the converted peak heights. Tables A to D below are provided for you to enter data and present calculated results. Finally summarize the results in Table E to be plotted in a temperature – composition graph as described in paragraph 2. 2. Place the seven composition data pairs on a temperature - composition graph (mole fraction of heptane from 0 to 1on the x axis, temperature on the y axis). Portray each pair as a tie line, i.e. connect each composition of the corresponding vapor and liquid by means of a horizontal line at each temperature for which samples were collected. (See the red arrow in Figure 6.17 in Oxtoby.) You may also connect the vapor phase data points and the liquid phase data points to show two curves indicating the temperature boundaries of each phase as represented by the heavy black curves in Figure 6.17

Table I. Properties of Hexane and Heptane Hexane Density = 0.663 g/mL ∆Hvap = 7,627.2 cal/mol Mol. Mass = 86.18 g/mol B.P. = 68.2 C Heptane Density = 0.684 g/mL ∆Hvap = 8,928.8 cal/mol Mol. Mass = 100.21 g/mol B.P. = 98.4 C

Calibration Factors n-Hexane n-Heptane Units Measured Peak Heights

mm

Amt. Injected µL Density 6.63e-04 6.84e-04 g/ µL Mol Mass. 86.1 100.1 g/mol

Atten. Factor

Calculated Calibration Factor

mol/mm

Experimental Gas-Chromatograph Data

A. Vapor Phase Hexane Temp (K) Peak Ht.

(mm) Amt. Inj. (microL)

Atten. Factor

Corrected Peak Ht.

Moles Hexane

M.F. Hexane, Vapor

B. Vapor Phase Heptane Temp (K) Peak Ht.

(mm) Amt. Inj. (microL)

Atten. Factor

Corrected Peak Ht.

Moles Heptane

M.F. Heptane, Vapor

C. Liquid Phase Hexane Temp (K) Peak Ht.

(mm) Amt. Inj. (microL)

Atten. Factor

Corrected Peak Ht.

Moles Hexane

M.F. Hexane, Liquid

D. Liquid Phase Heptane

Temp (K) Peak Ht.

(mm) Amt. Inj. (microL)

Atten. Factor

Corrected Peak Ht.

Moles Hexane

M.F. Heptane, Liquid

E. Summary

Temp (K) Hexane M.F.,

vapor Heptane M.F. vapor

Hexane M.F., liquid

Heptane M.F. liquid

341.35 1 0 1 0

371.55 0 1 0 1