experiment 6: fractional distillation reading assignment –experiment 6 (pp. 58...

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

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  • Slide 1
  • Experiment 6: Fractional Distillation Reading Assignment Experiment 6 (pp. 58 -64) Operation 29
  • Slide 2
  • Key Point! When conducting a distillation, the vapor should be richer in the lower boiling component than what you started with.
  • Slide 3
  • Simple Distillation: Apparatus Put in boiling stone!
  • Slide 4
  • Temperature Behavior During Distillation A.Single pure component B.Two components of similar boiling points C.Two components with widely different boiling points
  • Slide 5
  • Phase Diagram: Two Component Mixture of Liquids
  • Slide 6
  • Questions based upon the previous slide: a)What is the bp of pure A? b)What is the bp of pure B? c)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 o C.
  • Slide 7
  • Vapor-Liquid Composition Curve (Benzene vs. Toluene) Vapor liquid
  • Slide 8
  • Questions based upon the previous slide: a)What is the bp of pure toluene? b)What is the bp of pure benzene? c)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? e)How many theoretical plates would be necessary for a fractional distillation starting with a 50 % benzene solution?
  • Slide 9
  • When will simple distillation do a reasonable job of separating a mixture? 1)When the difference in boiling points is over 100 o 2)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).
  • Slide 10
  • Raoults Law
  • Slide 11
  • Fractional Distillation: Apparatus Put in boiling stone
  • Slide 12
  • Temperature vs. Volume: Fractional Distillation
  • Slide 13
  • Fractional Distillation Phase Diagram
  • Slide 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)!
  • Slide 15
  • Fractional Distillation Phase Diagram. The arrows indicate a theoretical plate!
  • Slide 16
  • Theoretical Plates Required to Separate Mixtures based on BP Boiling Point Difference Theoretical Plates 1081 722 543 434 365 2010 1020 730 450 2100
  • Slide 17
  • Azeotrope Some mixtures of liquids, because of attractions or repulsions between the molecules, do not behave ideally These mixtures do not obey Raoults 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.
  • Slide 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
  • Slide 19
  • Maximum Boiling-Point Azeotrope
  • Slide 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
  • Slide 21
  • Minimum Boiling-Point Azeotrope
  • Slide 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
  • Slide 23
  • The Gas Chromatograph
  • Slide 24
  • Gas Chromatography: Separation of a Mixture
  • Slide 25
  • Gas Chromatogram Lowest b.p. Highest b.p. Retention time
  • Slide 26
  • Triangulation of a Peak
  • Slide 27
  • Sample Percentage Composition Calculation
  • Slide 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.
  • Slide 29
  • Compounds in mixture: boiling points. Cyclohexane80 o C Toluene110 o C 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!
  • Slide 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. Dont rely on the distillation plot to determine the composition of your mixture!
  • Slide 31
  • First Fraction: Cyclohexane/Toluene Chromatogram Solvents cyclohexane toluene
  • Slide 32
  • Data: Cyclohexane/Toluene First Fraction solvents cyclohexane toluene ?
  • Slide 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%
  • Slide 34
  • Second Fraction: Cyclohexane/Toluene Chromatogram solvents cyclohexane toluene
  • Slide 35
  • Data: Cyclohexane/Toluene Second Fraction solvents cyclohexane toluene ?
  • Slide 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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