Homework answers / question archive / Chem 347, Lab Section Reviewed 8-27-18 By JJ Koh EXPERIMENT: Macro-scale Fractional Distillation We will explore only fractional distillation which provides better separation of liquid mixture than simple distillation

Chem 347, Lab Section Reviewed 8-27-18 By JJ Koh EXPERIMENT: Macro-scale Fractional Distillation We will explore only fractional distillation which provides better separation of liquid mixture than simple distillation

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Chem 347, Lab Section Reviewed 8-27-18 By JJ Koh EXPERIMENT: Macro-scale Fractional Distillation We will explore only fractional distillation which provides better separation of liquid mixture than simple distillation. Safety Information Add boiling chips before heating liquids. NEVER add boiling chips to a hot liquid! Never allow a distillation flask to be boiled to dryness. Some compounds, such as ethers, secondary alcohols, and alkenes, can form peroxides through air oxidation, which can cause an explosion if the flask is heated to dryness. TEXTBOOK READING ASSIGNMENT ? Experiment 7 – Simple and Fractional Distillation (page 56 - 63) ? Technique 22 – Gas Chromatography (page 829 – 848) EXPERIMENTAL PROCEDURE: Fractional Distillation (Experiment video will be posted separately for online students) Experiment video: https://youtu.be/SFacWoGrJlo 1. Place 20 mL of toluene/hexane (1:1) mixture in a clean 50 mL round bottom flask containing one or two boiling chips. 2. Assemble the flask in a fractional distillation apparatus using 10 mL graduated cylinder as your collection receptacle. 3. Make sure the thermometer is correctly positioned with the top of the thermometer bulb just below the bottom of the distilling head side arm. 4. Connect a rubber tubing to the condenser inlet and another rubber tubing to the outlet. Connect the other end of the inlet tube to a cold water supply and place the other end of the outlet tube end in the sink. 5. Make sure there are no kinks in the hoses and then turn water on SLOWLY or you will blow off the rubber hose. NOTE: READ Steps 6 ~ 14 thoroughly before starting the actual distillation. 6. Heat the boiling flask gently with the heat source until liquid begins to boil. 7. As the liquid in the boiling flask is heated to boiling, you will observe a liquid/vapor interface begin to climb up the fractionating column as the boiling of the liquid progresses with time. When this interface reaches the thermometer bulb, the temperature will rapidly rise to around 66 ~ 71 °C (the approximate boiling point of hexane) and liquid should soon start to be collected in the receiver. 8. Record the temperature when the first drops of distillate are collected in the receiver (10 mL graduate cylinder) and then for each subsequent 1 mL of distillate that is collected. Chem 347, Lab Section Reviewed 8-27-18 By JJ Koh 9. Adjust the heat to distill the liquid at a rate of ~ 1 – 2 mL per minute (1 – 2 drops every 3 seconds). NOTE: As the distillation progresses, you may have to increase the heat controller setting to compensate or the increase in energy needed to distill the higher boiling toluene and to keep the rate of distillation relatively constant at 1 – 2 mL per minute. 10. The distillation process slows down or stops at about 7 – 9 mL collection. Transfer the collected fraction (about 7 – 8 mL) into a separate 25 mL Erlenmeyer flask and label as Fraction 1. 11. Continue collecting fractions in the receiver (10 mL graduated cylinder) and recording the temperature for each subsequent 1 mL of distillate by increasing the heat controller. 12. Collect about 2 - 3 mL of distillate (this is intermediate fraction) and discard in a separate 50 mL beaker). 13. Continue collecting fractions in the receiver (10 mL graduated cylinder) until there is only small portion of liquid remains in the round bottom flask or the distillation process completely stops. When the distillation has stopped or slowed significantly, you should have collected a total of 13 – 17 mL of distillate. Turn off and remove the heat source and as the boiling flask cools down, ~ 1 – 3 mL of liquid (“holdup”) should remain in the boiling flask. NOTE: The last fraction can range anywhere from fraction number 13 to 17 – the number of fractions collected depends on several factors, i.e. simple vs. fractional distillation, “holdup” volume, technique, etc. 14. Transfer the collected fraction in a separate 25 mL Erlenmeyer flask and label as Fraction 2. 15. Make a plot of temperature versus distillate volume for the fractional distillation. 16. Perform GC analysis with your instructor for both Fraction 1 and 2. 17. Calculate % composition of hexane and toluene in Fraction 1 and 2 by using triangulation. Chem 347, Lab Section Reviewed 8-27-18 By JJ Koh Example of Table to Record Distillation Data Fractional Distillation of 1:1 Hexane/Toluene Fraction Number (1 mL fractions) Distillation Temperature (°C) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Pot residue (~ 2 – 4 mL) Data to Record in Your Notebook: ? Your observation! ? Distillation temperature (temperature reading) at each subsequent 1 mL of distillate fraction in the distillation data table. ? Construct a distillation curve for the fractional distillation as below using MS Excel. Example of a distillation curve Notes for the Template: 1. Do not turn in this first page (which is this page you are looking at now). Remove all text inside of the parenthesis as well as the example paragraphs. Your final write-up should not contain any parenthesis except for the ones you choose to add. There will be point deduction if you still have them in your final, submitted draft. Name: CHEM XXX Section #: XXXX MACRO-SCALE FRACTIONAL DISTILLATION Purpose: (Specifically state the purpose of the experiment and evaluation methods in a few sentences.) Procedure: (Write a paragraph of the detailed experimental procedure including observation in 3rd person past tense. In this report, do not copy your notebook procedure. See the following example for your understanding.) Example: 316 mg of ferrocene (1.7 mmol) and 2 mL of acetic anhydride were placed in a 10 mL of round-bottom flask, To this mixture, 10 drops of 85% phosphoric acid was added slowly with stirring. A water cooling condenser was attached, and the mixture was heated for 20 minutes in a 90 ?C water bath. After cooling down, 1.0 mL of ice cold water was added dropwise to the reaction mixture. The diluted mixture was poured into a 50 mL beaker containing about 10 g of ice. The mixture was neutralized by adding approximately 5 g of solid sodium bicarbonate (about 0.25 g at a time) until the CO2 stops bubbling off and pH = 5 – 6 with the pH paper testing. The crude solid product was collected by vacuum filtration and washed thoroughly with cold water. After determining the mobile phase by TLC, the crude product was purified by column chromatography using ethyl acetate/hexane (1:9, v/v). Data/Results: (Record all collected and calculated data from the experiment. If possible, a tabular form is preferred.) (For online group, use the following temperatures written in the table. For hybrid group, you should record temperatures from the experiment) Fractional Distillation of 1:1 Hexane/Toluene Fraction Number (1 mL fractions) Distillation Temperature (°C) 1 56 2 57 3 58 4 58 5 58 6 57 7 57 8 64 9 78 10 85 11 104 12 105 13 105 14 15 16 17 Pot residue (~ 2 – 4 mL) 105 105 98 - (Use MS Excel to create a graph as shown in the below example) (If a peak is invisible or unmeasurable, type “n/a”.) Components Retention Time (min) Height of the peak (mm) Width @ ½ height (mm) Area (mm3) Total Area (mm3) % Composition Hexane peak 1 Hexane peak 2 Hexane peak 3 Hexane peak 4 Toluene (If a peak is invisible or unmeasurable, type “n/a”.) Components Hexane peak 1 Retention Time (min) Height of the peak (mm) Width @ ½ height (mm) Area (mm3) Total Area (mm3) % Composition Hexane peak 2 Hexane peak 3 Hexane peak 4 Toluene Conclusion: Summarize your results and draw conclusions from the experiment using complete sentences in a narrative form. Did you prepare the proper substances or perform the experiment properly? Always include your yield data (grams and percent) and any other pertinent data collected (melting point, TLC Rf values, etc.). If your yield was low, discuss any possible sources of error. Can you suggest any improvements in the procedure? If you were given an unknown, always include the unknown code with your analysis. Example: "Acetylsalicylic acid was synthesized from salicylic acid. The yield was 5.32 grams, which represents a 76% yield. The melting point of the acetylsalicylic acid was determined to be 131-134 °C which is slightly below the literature data of 135-136 °C, indicating the presence of impurities. The crystals were white and powdery in appearance, but did give a slight purple color on testing with ferric chloride, indicating the presence of some unreacted salicylic acid in the product. (For this experiment, include the following specifics) - State % composition of each component. - Which component was distilled first from the mixture? Why? - How many peaks did you observe on each of the provided chromatograms? What compound does each peak correspond to? - Explain how you identified the peaks on the provided GC chromatograms. - Based on the results of the experiment, evaluate the effectiveness of the fractional distillation for hexane and toluene separation. - Suggest any possible improvement in this experiment for the future. Post Lab Questions: (20 points) 1. Why fractional distillation was used to separate a mixture of hexane and toluene rather than simple distillation? (5 pts) 2. What is the most important physical property of a compound to determine the retention time in Gas Chromatography in this experiment? (5 pts) 3. Why is it important to heat the mixture slowly in distillation process? (5 pts) 4. What is the role of the fractionating column in fractional distillation? Explain how it works. (5 pts) C:\LabSolutions\Data\Distillation\fractional distillation 1 demo1.gcd Macro-scale fractional distillation – Fraction 1 Chromatogram uV 1 FID1 1.696 17500000 15000000 12500000 10000000 7500000 1.1 1.2 1.3 1.4 1.5 1.990 2.742 1.605 0 1.537 1.454 2500000 1.0 1.818 1.645 5000000 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 min C:\LabSolutions\Data\Distillation\fractional distillation 2 3.gcd Macro-scale fractional distillation – Fraction 2 Chromatogram 2.858 uV 13000000 1 FID1 12000000 11000000 10000000 9000000 8000000 7000000 6000000 5000000 4000000 3000000 2000000 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2.011 1.836 1.623 0 1.0 1.709 1000000 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 min h © Cengage Learning 2013 area = h ¥ W1/2 W112 22.12 Quantitative The area under a gas-chromatograph peak is proportional to the amount (moles) of Analysis compound eluted. Hence, the molar percentage composition of a mixture can be ap- proximated by comparing relative peak areas. This method of analysis assumes that the detector is equally sensitive to all compounds eluted and that it gives a linear response with respect to amount. Nevertheless, it gives reasonably accurate results. The simplest method of measuring the area of a peak is by geometrical approximation, or triangulation. In this method, you multiply the height h of the peak above the Approximate baseline of the chromatogram by the width of the peak at half of its height W1/2* This is illustrated in Figure 22.13. The baseline is approximated by drawing a line between the two side arms of the peak. This method works well only if the peak is symmetrical. If the peak has tailed or is un- Figure 22.13 symmetrical, it is best to cut out the peaks with scissors and Triangulation of a peak. weigh the pieces of paper on an analytical balance. Because the weight per area of a piece of good chart paper is reason- ably constant from place to place, the ratio of the areas is the same as the ratio of the weights. To obtain a percentage composition for the mixture, first add all the peak areas (weights). Then, to calculate the percentage of any component in the mixture, divide its individual area by the total area and multiply the result by 100. A sample calculation is illustrated in Figure 22.14. If peaks overlap (see Figure 22.8), either the gas-chromatographic conditions must be readjusted to achieve better resolu- tion of the peaks or the peak shape must be estimated. There are various instrumental means, which are built into recorders, of detect- ing the amounts of each sample automatically. One method uses a separate pen that produces a trace that integrates the area under each peak. Another method employs an electronic device that automatically prints out the area under each peak and the percentage composition of the sample. Area Peak B = 19 x 122 = 2320 mm2 Area Peak A = 17 x 40 = 680 mm2 Total area = 3000 mm2 h = 122 mm 680 %A x 100 = 22.7% 3000 Composition 2320 %B = x 100 = 77.3% Jof mixture 3000 Total 100.0% B Ratio A 2320 680 3.35 1 W12 = 19 mm h = 40 mm W = 17 mm ?, © Cengage Learning 2013 Air peak B Figure 22.14 Sample percentage composition calculation.