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Experiment 6: Fractional Distillation

Experiment 6: Fractional Distillation. Reading Assignment Experiment 6 (pp. 58 -64) Operation 29. Key Point!. When conducting a distillation, the vapor should be richer in the lower boiling component than what you started with. . Simple Distillation: Apparatus. Put in boiling stone!.

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Experiment 6: Fractional Distillation

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  1. Experiment 6: Fractional Distillation • Reading Assignment • Experiment 6 (pp. 58 -64) • Operation 29

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

  3. Simple Distillation: Apparatus Put in boiling stone!

  4. Temperature Behavior During Distillation • Single pure component • Two components of similar boiling points • Two components with widely different boiling points

  5. Phase Diagram: Two Component Mixture of Liquids

  6. 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.”

  7. Vapor-Liquid Composition Curve (Benzene vs. Toluene) liquid Vapor

  8. 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?

  9. 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).

  10. Raoult’s Law

  11. Fractional Distillation: Apparatus Put in boiling stone

  12. Temperature vs. Volume: Fractional Distillation

  13. Fractional Distillation Phase Diagram

  14. 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)!

  15. Fractional Distillation Phase Diagram. The arrows indicate a theoretical plate!

  16. 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

  17. 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.

  18. 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

  19. Maximum Boiling-Point Azeotrope

  20. 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”

  21. Minimum Boiling-Point Azeotrope

  22. 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”

  23. The Gas Chromatograph

  24. Gas Chromatography: Separation of a Mixture

  25. Gas Chromatogram Highest b.p. Retention time Lowest b.p.

  26. Triangulation of a Peak

  27. Sample Percentage Composition Calculation

  28. 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.

  29. Compounds in mixture: boiling points. Cyclohexane 80 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!

  30. 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!

  31. First Fraction: Cyclohexane/TolueneChromatogram cyclohexane Solvents toluene

  32. Data: Cyclohexane/Toluene First Fraction solvents ? cyclohexane toluene

  33. Calculation of percentages from the data for fraction 2 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%

  34. Second Fraction: Cyclohexane/TolueneChromatogram toluene solvents cyclohexane

  35. Data: Cyclohexane/Toluene Second Fraction solvents ? cyclohexane toluene

  36. Calculation of percentages from the data for fraction 4 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%

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