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Experiment 6: Simple and Fractional Distillation
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  1. Experiment 6: Simple and Fractional Distillation • Reading Assignment • Experiment 6 (pp. 51 -57) • Technique 13, Parts A (pp. 694-702) • Technique 14 (pp. 703-715) • Technique 15 (pp. 715-732) • Technique 22 (pp. 797-818) WWU -- Chemistry

  2. Experiment 6: Simple and Fractional Distillation • Work in pairs.Each pair will conduct both the simple and fractional distillations. There are three unknowns, A, B, and C. Perform the experiment as follows: • Day One: Working in pairs, use simple distillation to separate the unknown (Experiment 6A). • Day Two: Again, working in pairs, repeat the experiment using fractional distillation on the same unknown (Experiment 6A) • Do not do Experiment 6B • The products from each day’s distillation will be analyzed by gas chromatography. WWU -- Chemistry

  3. Key Point! • When conducting a distillation, the vapor should be richer in the lower boiling component than what you started with. WWU -- Chemistry

  4. Simple Distillation: Apparatus Put in boiling stone! WWU -- Chemistry

  5. Correct Thermometer Placement Thermometer must be below this level WWU -- Chemistry

  6. Your equipment has a built-in thermometer adapter, so your equipment will look a bit different. Look at the setup in the hood before you start assembling the equipment. Ask your instructor if you will be attaching the vacuum adapter! Some instructors will ask you to leave off this piece of glassware! There are wooden blocks that can be used to raise the apparatus. The wooden blocks are in the cupboard under the hood. WWU -- Chemistry

  7. Temperature Behavior During Distillation • Single pure component • Two components of similar boiling points • Two components with widely different boiling points WWU -- Chemistry

  8. Phase Diagram: Two Component Mixture of Liquids WWU -- Chemistry

  9. Questions based upon the previous slide: • What is the bp of pure A? • What is the bp of pure B? • What is the bp of a solution with the composition • of 30 % B, assuming a simple distilllation apparatus? • d) What is the composition of the vapor assuming a simple distillation apparatus? • e) What is the composition of the distillate collected assuming a simple distillation apparatus? • f) What does the “tie-line,” x-y represent? Hint: the upper • curve is the vapor curve and the lower curve is the liquid curve. • “Composition of the vapor and liquid that are in equilibriuim with each other at 130 oC.” WWU -- Chemistry

  10. Vapor-Liquid Composition Curve (Benzene vs. Toluene) liquid Vapor WWU -- Chemistry

  11. Questions based upon the previous slide: • What is the bp of pure toluene? • What is the bp of pure benzene? • What is the bp of a solution with the composition • of 50 % benzene, assuming a simple distilllation apparatus? • d) What is the composition of the distillate assuming a • simple distillation apparatus? • How many theoretical plates would be necessary for a • fractional distillation starting with a 50 % benzene solution? WWU -- Chemistry

  12. When will simple distillation do a reasonable job of separating a mixture? • When the difference in boiling points is over 100o • When the there is a fairly small amount of impurity, say • less than 10 %. • 3) When one of the components will not distil because of • a lack of volatility (i.e. sugar dissolved in water). WWU -- Chemistry

  13. Raoult’s Law WWU -- Chemistry

  14. Raoult’s law calculations See Figure 15.6 on page 720 for example calculations. WWU -- Chemistry

  15. Fractional Distillation: Apparatus Put in boiling stone WWU -- Chemistry

  16. Vaporization-Condensation bp of pure A = 51° bp of pure B = 87° WWU -- Chemistry

  17. Temperature vs. Volume: Fractional Distillation WWU -- Chemistry

  18. Fractional Distillation Phase Diagram WWU -- Chemistry

  19. How many theoretical plates are need to separate a mixture starting at L? • Looks like about 5 plates are needed to separate the mixture on the previous slide! • Count the “tie-lines” (horizontal lines) to come up with the 5 plates (labelled with arrows on the next slide)! WWU -- Chemistry

  20. Fractional Distillation Phase Diagram. The arrows indicate a theoretical plate! WWU -- Chemistry

  21. Theoretical Plates Required to Separate Mixtures based on BP Boiling Point Difference Theoretical Plates 108 1 72 2 54 3 43 4 36 5 20 10 10 20 7 30 4 50 2 100 WWU -- Chemistry

  22. Microscale distillation: Hickman Head WWU -- Chemistry

  23. Azeotrope • Some mixtures of liquids, because of attractions or repulsions between the molecules, do not behave ideally • These mixtures do not obey Raoult’s Law • An azeotrope is a mixture with a fixed composition that cannot be altered by either simple or fractional distillation • An azeotrope behaves as if it were a pure compound, and it distills from beginning to end at a constant temperature. WWU -- Chemistry

  24. Types of Azeotropes • There are two types of non-ideal behavior: • Minimum-boiling-point • Boiling point of the mixture is lower than the boiling point of either pure component • Maximum-boiling-point • Boiling point of the mixture is higher than the boiling point of either pure component WWU -- Chemistry

  25. Maximum Boiling-Point Azeotrope WWU -- Chemistry

  26. Observations with maximum boiling azeotrope On the right side of the diagram: Compound B will distill (lowest bp). Once B has been removed, the azeotrope will distill (highest bp). On the left side of the diagram: Compound A will distill (lowest bp) Once A has been removed, the azeotrope will distill. (highest bp) The azeotrope acts like a pure “compound” WWU -- Chemistry

  27. Minimum Boiling-Point Azeotrope WWU -- Chemistry

  28. Observations with minimum boiling azeotrope On the right side of the diagram: The azeotrope is the lower boiling “compound,” and it will be removed first. Pure ethanol will distill once the azeotrope has distilled. On the left side of the diagram: the azeotrope is the lower boiling “compound,” and it will distill first. Once the azeotrope has been removed, then pure water will distill. The azeotrope acts like a pure “compound” WWU -- Chemistry

  29. Dean-Stark Water Separator WWU -- Chemistry

  30. The Gas Chromatograph WWU -- Chemistry

  31. Gas Chromatography: Separation of a Mixture WWU -- Chemistry

  32. Gas Chromatogram Highest b.p. Retention time Lowest b.p. WWU -- Chemistry

  33. Triangulation of a Peak WWU -- Chemistry

  34. Sample Percentage Composition Calculation WWU -- Chemistry

  35. Gas Chromatography: Results In a modern gas chromatography instrument, the results are displayed and analyzed using a computerized data station. It is no longer necessary to calculate peak areas by triangulation; this determination is made electronically. Our analysis will be conducted on a modern data station. WWU -- Chemistry

  36. Compounds in unknowns: boiling points. There will only be two components in each unknown Hexanes (mixture of isomers) 68-70 oC Cyclohexane 80 oC Heptane 98 oC Toluene 110 oC Mixture separates by distillation according to the boiling point. Compounds with the lower bp come off first! The same is true on the gas chromatographic column; the lower boiling compound comes off first! WWU -- Chemistry

  37. Gas Chromatography: Standards Retention time solvents The x axis is in min. WWU -- Chemistry

  38. Notice: 1) hexane has the lowest retention time 2) toluene has the highest retention time The four compounds come off in the order of increasing boiling point. Hexane is actually a mixture of three compounds. It is usually called “hexanes” Increasing b.p. WWU -- Chemistry

  39. Preparing distillation samples for gas chromatography After you have collected 1mL of distillate, then collect the next two drops in one of the special gas chromatography tubes. Add the solvent that is suggested by your instructor (methylene chloride or acetone). Screw on the cap and use a marking pen to put your initials on the tube. After 4.5 mL has been distilled, repeat the process indicated above. Charles Wandler will give a presentation in the lab on the instrument that we will use for gas chromatography. This includes a handout that tells you how to retrieve you data. The data will be available in the computer lab (CB 280). He will demonstrate where to put the tubes. He has a signup sheet and a carousel to put the samples in. WWU -- Chemistry

  40. How to identify the components in your unknown mixture Use the retention time information from your gas chromatograms to provide a positive identification of each of the components in the mixture. Don’t rely on the distillation plot to determine the composition of your mixture! WWU -- Chemistry

  41. Retention Times and Response Factors NOTE: These values are for illustration purposes. Your actual values will be different! WWU -- Chemistry

  42. First Fraction: Cyclohexane/TolueneChromatogram cyclohexane Solvents toluene WWU -- Chemistry

  43. Data: Cyclohexane/Toluene First Fraction solvents ? cyclohexane toluene WWU -- Chemistry

  44. Calculation of percentages from the data for fraction 1 area counts/response factor = adjusted area Cyclohexane area = 42795/1.133 = 32104 Toluene area = 18129/1.381 = 13127 Total area 45231 Note: this calculated area is different than that shown on the data sheet! Use this calculated area! Percent cyclohexane = 32104/45231 x 100 = 71.0% Percent toluene = 13127/45231 x 100 = 29.0 % Round off numbers so that the total equals 100% WWU -- Chemistry

  45. Second Fraction: Cyclohexane/TolueneChromatogram toluene solvents cyclohexane WWU -- Chemistry

  46. Data: Cyclohexane/Toluene Second Fraction solvents ? cyclohexane toluene WWU -- Chemistry

  47. Calculation of percentages from the data for fraction 2 area counts/ response factor = adjusted area Cyclohexane area = 57546/1.133 = 43170 Toluene area = 191934/1.381 = 138981 Total area 182151 Note: this calculated area is different than that shown on the data sheet! Percent cyclohexane = 43170/182151 x 100 = 23.7 % Percent toluene =138981/182151 x 100 = 76.3 % Round off numbers so percentage = 100% WWU -- Chemistry

  48. First Fraction: Hexane/HeptaneChromatogram solvents heptane hexanes ? WWU -- Chemistry

  49. Data: Hexane/Heptane First Fraction solvents ? Three peaks for hexanes heptane WWU -- Chemistry

  50. Calculation of percentages from the data for fraction 1 area counts/response factor = adjusted area Hexanes area = 1251 + 60375 + 8147 = 69773/1.022 = 68271 Heptane area = 26374/ 1.000 = 26374 Total area =94645 Note: this calculated area is different than that shown on the data sheet! Use this calculated area! Percent hexanes = 68271/94645 x 100 = 72.1 % Percent heptane = 26374/94645 x 100 = 27.9 % Round off numbers so that the total equals 100% WWU -- Chemistry