Experiment 6: Simple and Fractional Distillation

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)

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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 days distillation will be
  analyzed by gas chromatography.

3
Key Point!

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

4
Simple Distillation Apparatus
Put in boiling stone!
5
Correct Thermometer Placement
Thermometer must be below this level
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.
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Temperature Behavior During Distillation

1. Single pure component
2. Two components of similar boiling points
3. Two components with widely different boiling
   points

8
Phase Diagram Two Component Mixture of Liquids
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.

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

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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).

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Raoults Law
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Raoults law calculations
See Figure 15.6 on page 720 for example
calculations.
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Fractional Distillation Apparatus
Put in boiling stone
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Vaporization-Condensation
bp of pure A 51 bp of pure B 87
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Temperature vs. Volume Fractional Distillation
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Fractional Distillation Phase Diagram
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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)!

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Fractional Distillation Phase Diagram. The
arrows indicate a theoretical plate!
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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
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Microscale distillation Hickman Head
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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.

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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

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Maximum Boiling-Point Azeotrope
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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
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Minimum Boiling-Point Azeotrope
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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
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Dean-Stark Water Separator
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The Gas Chromatograph
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Gas Chromatography Separation of a Mixture
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Gas Chromatogram
Highest b.p.
Retention time
Lowest b.p.
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Triangulation of a Peak
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Sample Percentage Composition Calculation
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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.
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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!
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Gas Chromatography Standards
Retention time
solvents
The x axis is in min.
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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.
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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.
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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!

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Retention Times and Response Factors
Component Retention Time (min) Response Factor
Hexanes (mixture of isomers) 3.054 1.022
Cyclohexane 3.491 1.133
Heptane 3.812 1.000
Toluene 4.331 1.381
NOTE These values are for illustration
purposes. Your actual values will be different!
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First Fraction Cyclohexane/TolueneChromatogram
cyclohexane
Solvents
toluene
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Data Cyclohexane/Toluene First Fraction
solvents
?
cyclohexane
toluene
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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
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Second Fraction Cyclohexane/TolueneChromatogram
toluene
solvents
cyclohexane
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Data Cyclohexane/Toluene Second Fraction
solvents
?
cyclohexane
toluene
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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
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First Fraction Hexane/HeptaneChromatogram
solvents
heptane
hexanes
?
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Data Hexane/Heptane First Fraction
solvents
?
Three peaks for hexanes
heptane
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
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Second Fraction Hexane/HeptaneChromatogram
heptane
solvents
hexanes
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Data Hexane/Heptane Second Fraction