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
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
Key Point!
• When conducting a distillation, the
vapor should be richer in the lower
boiling component than what you
started with.
WWU -- Chemistry
Simple Distillation: Apparatus
Put in boiling
stone!
WWU -- Chemistry
Correct Thermometer Placement
Thermometer
must be below
this level
WWU -- Chemistry
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
Temperature Behavior During
Distillation
A. Single pure component
B. Two components of similar boiling points
C. Two components with widely different boiling points
WWU -- Chemistry
Phase Diagram: Two Component
Mixture of Liquids
WWU -- Chemistry
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 oC.”
WWU -- Chemistry
Vapor-Liquid Composition Curve
(Benzene vs. Toluene)
liquid
Vapor
WWU -- Chemistry
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?
WWU -- Chemistry
When will simple distillation do a
reasonable job of separating a
mixture?
1) When the difference in boiling points is over 100o
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).
WWU -- Chemistry
Raoult’s Law
PTOTAL = PANA + PBNB
NA = Mole Fraction of A =
Moles A
Moles A + Moles B
WWU -- Chemistry
Raoult’s law calculations
See Figure 15.6 on page 720 for example calculations.
WWU -- Chemistry
Fractional Distillation:
Apparatus
Put in boiling stone
WWU -- Chemistry
Vaporization-Condensation
bp of pure A = 51°
bp of pure B = 87°
WWU -- Chemistry
Temperature vs. Volume:
Fractional Distillation
WWU -- Chemistry
Fractional Distillation Phase
Diagram
WWU -- Chemistry
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
Fractional Distillation Phase
Diagram. The arrows indicate a
theoretical plate!
WWU -- Chemistry
Theoretical Plates Required to
Separate Mixtures based on BP
Boiling Point Difference
108
72
54
43
36
20
10
7
4
2
Theoretical Plates
1
2
3
4
5
10
20
30
50
100
WWU -- Chemistry
Microscale distillation:
Hickman Head
WWU -- Chemistry
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
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
Maximum Boiling-Point
Azeotrope
WWU -- Chemistry
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
Minimum Boiling-Point
Azeotrope
WWU -- Chemistry
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
Dean-Stark Water Separator
WWU -- Chemistry
The Gas Chromatograph
WWU -- Chemistry
Gas Chromatography: Separation of a
Mixture
WWU -- Chemistry
Gas Chromatogram
Highest
b.p.
Retention
time
Lowest
b.p.
WWU -- Chemistry
Triangulation of a Peak
WWU -- Chemistry
Sample Percentage Composition
Calculation
WWU -- Chemistry
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
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
Gas Chromatography: Standards
solvents
Retention
time
The x axis
is in min.
WWU -- Chemistry
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”
CH3
hexane
cyclohexane
heptane
toluene
Increasing b.p.
WWU -- Chemistry
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
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
Retention Times and Response
Factors
Component
Retention
Time (min)
Response
Factor
3.054
1.022
Cyclohexane
3.491
1.133
Heptane
3.812
1.000
Toluene
4.331
1.381
Hexanes
(mixture of isomers)
NOTE: These values are for illustration purposes. Your
actual values will be different!
WWU -- Chemistry
First Fraction: Cyclohexane/Toluene
Chromatogram
cyclohexane
Solvents
toluene
WWU -- Chemistry
Data: Cyclohexane/Toluene First Fraction
solvents
cyclohexane
toluene
?
WWU -- Chemistry
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
Second Fraction: Cyclohexane/Toluene
Chromatogram
toluene
solvents
cyclohexane
WWU -- Chemistry
Data: Cyclohexane/Toluene Second
Fraction
solvents
cyclohexane
toluene
?
WWU -- Chemistry
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
First Fraction: Hexane/Heptane
Chromatogram
solvents
heptane
hexanes
?
WWU -- Chemistry
Data: Hexane/Heptane First Fraction
solvents
?
Three peaks for hexanes
heptane
WWU -- Chemistry
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
Second Fraction: Hexane/Heptane
Chromatogram
heptane
solvents
hexanes
WWU -- Chemistry
Data: Hexane/Heptane Second Fraction
WWU -- Chemistry