BC Fractions and Distillation Chemistry Report

Separation Techniques:Fractional Distillation
Purpose
The goal of this experiment is to separate a mixture of two esters using fractional distillation.
Learning Objectives
Conduct a fractional distillation on a mixture of ethyl acetate and butyl acetate.
Describe the types of distillation (simple and fractional).
Define an azeotrope.
Laboratory Skills
Perform a macroscale fractional distillation.
Calculate percent composition.
Equipment
Chemicals
Macroscale fractional
Hot plate
Mixture of ethyl
distillation set-up
Test tubes
acetate and
butylacetate
Boiling chips
Background
Distillation
Many organic compounds are liquid at room temperature. If a reaction results in a mixture of miscible liquids,
the separation of the liquids requires a new technique known as distillation. Distillation uses the different boiling
points of liquids in a mixture to separate the components.
Many organic compounds are volatile, which means they have relatively high vapor pressures and low boiling
points. Distillation uses this volatility by boiling the liquid mixture in a single flask and condensing the vapors
that travel into the apparatus, allowing a liquid to be collected at the other end. As long as there is a difference
in boiling points between the liquids in the mixture, one component will distill over before the other.
Boiling point depends on atmospheric pressure, which is not likely to be 760 torr, and thus the observed boiling
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FD.1
Fractional Distillation
point is not the normal boiling point of the liquid. If the final boiling point does not exactly correspond to the
literature boiling point, it is most likely due to the pressure and/or the thermometer, and not to “operator/experimental error” or some impurity in the remaining mixture.
Azeotropes
Pure organic compounds will distill over a very narrow boiling point range. If the boiling point range or distilling
range is too wide, the collected compound is likely impure and a complete separation was not obtained. However,
not all compounds with narrow boiling point ranges are pure. An azeotrope is a mixture of two liquids which has
a constant boiling point and composition throughout distillation. An example azeotrope is a mixture of ethanol
and water; ethanol has a boiling point of 78.4 °C and water has a boiling point of 100 °C but a mixture of 95%
ethanol and 5% water has a boiling point of 78.1 °C. When distilling ethanol and water, the collected ethanol will
be 95% with 5% water due to the azeotrope.
Fractionating Column
There are many types of distillation that differ in the components of the distillation apparatus. All distillation
set-ups do have similarities, though. A simple distillation has the most basic set-up of the distillation options.
The flask with the mixture is attached to some form of condenser, which empties into a new container. The setups also include a thermometer to monitor the vapor temperature compared to the expected boiling points. All
set-ups must also be open to a gas environment in some way, whether that is open to air or another gas. A closed
distillation system could allow the pressure to build up and cause an explosion.
An important refinement of the basic technique is fractional distillation, which adds a fractionating column to
give better separation of liquids with similar boiling points. The fractionating column is included right above
the heated flask, as shown in Figure FD.1. The column contains extra surfaces on which the high boiling point
component can condense back to liquid into the flask, releasing heat that helps vaporize the low boiling point
component and move it through the system.
FD.2
Fractional Distillation
Figure FD.1: Macroscale fractional distillation set-up
Fractional distillation allows for a better separation of liquids with a boiling point difference of 40 degrees or less.
Consider the plot in Figure FD.2 following the distillation of two liquids with boiling points of 81 °C and 110 °C.
FD.3
Fractional Distillation
Figure FD.2: Comparison of simple and fractional distillation temperature behavior over a distillation
There is no sharp transition in the black line representing the temperature behavior during the simple distillation,
indicating that there is a wide range of collected distillate containing a mixture of both liquids. The red line has
a much sharper transition, which indicates a cleaner separation between the two liquids.
Analyzing the Fractional Distillation
The additive fraction volumes should approximately equal the volume of the mixture initially used. Remember, the pot residue is included because it was part of the initial mixture and you should be able to deduce its
composition.
Plot the line graph (not scatter plot) for volume of distillate (y-axis) collected in each temperature range versus
the temperature range (x-axis). Make sure to include your pot residue as part of your high boiling fraction. Begin
your temperature axis at about 4–5 °C prior to the actual distillation temperature. An example chart is shown in
Figure FD.3; your graph should resemble the red line from the figure. If you collected some distillate over some
irregular temperature intervals (< 5 °C or > 5 °C), please write correction notes directly on your graph. It is not
easy to put these non-regular intervals in a line graph.
FD.4
Fractional Distillation
Figure FD.3: A two-component mixture that has been separated by distillation. The boiling points
of the two pure components are 64 °C and 92 °C. The volumes collected between 70-85 °C are a
mixture of the two components.
After plotting the data, inspect the resulting graph. It should somewhat resemble a gas chromatogram. The
principle of separation on the basis of boiling point is the same in both the GC and distillation procedures. You
should be able to tell fairly clearly the areas of the graph that correspond to (mostly) ethyl acetate distilling and to
(mostly) butyl acetate distilling since you know their respective boiling points. On this basis, decide what parts
of the graph (fractions) are primarily ethyl acetate, primarily butyl acetate, and a mixture of the two.
If there is not a clear separation of the mixture, you will have to make some assumptions about the composition
of the volume that was not clearly separated. These assumptions will need to be explained and supported. On
the graph itself, clearly designate which part of the total [distillate fractions + pot residue] you will use for the
estimation of ethyl acetate and which part is used for the estimation of butyl acetate. All distillation fractions and
pot residue must be designated as ethyl acetate, butyl acetate, or a mixture of both. When you have decided how
much of the total recovered volume is ethyl acetate and how much is butyl acetate, calculate the volume percent
composition of each of the two components in the unknown mixture. The volume percentages should add up to
100% and must include all the recovered volumes in the calculation.
After you have determined the volume percent composition, use literature values for density and molar mass,
you can convert to mole percent composition.
FD.5
Fractional Distillation
Procedure
Safety Precautions
Safety goggles are required!
Ethyl acetate and butyl acetate are both flammable. There must be no flames or sparking sources present
in the laboratory during this experiment.
Add boiling chips before heating so that the liquid does not boil out of the apparatus.
Never heat a closed system and never distill to dryness.
Fractional Distillation
1.
Begin by securely clamping the 50 mL-round-bottom distilling flask to a ring stand.
2.
Place about 20 mL of the ethyl acetate/butyl acetate mixture of unknown volume composition and 2 or 3
small boiling chips in the round bottom flask. Make sure you record the volume in the graduated cylinder
precisely, e.g. 19.7 mL.
3.
Assemble the rest of the distillation apparatus. Insert the fractionating column between the distilling flask
and distilling head. The distillate fractions are collected in test tubes (”fraction collectors”) immersed in
an ice bath. When assembling the glass pieces of your distillation apparatus it is advisable to clamp the
glassware pieces to each other (using keck-clamps) as you proceed.
4.
Begin a gentle flow of water from the tap to the water inlet of the condenser. Connect another rubber hose
to the condenser outlet and run the water into the sink.
5.
Have your instructor inspect your set-up before you proceed.
6.
Slowly heat the distillation flask with a heating source as designated by the instructor. Regulate the heating
so that the condensate collects slowly and steadily, without interruption, approximately 1 drop/sec.
7.
During the distillation, collect and measure the volume that accumulates during a temperature change of
FD.6
Fractional Distillation
5 °C in each test tube. For example, the first test tube should hold the distillate collected between room
temperature and 70 °C, and the second test tube should hold the distillate collected between 70-75 °C. If
there is a dramatic change of the head temperature (1 degree per second), change the receiving test tube.
Record approximately how many drops were collected, and then begin your five-degree intervals again when
the temperature levels off.
8.
Measure the volume of each fraction using a small graduated cylinder. After measuring each volume in a test
tube, turn the graduated cylinder upside down on a paper towel that is crumpled in the bottom of a beaker
to allow the cylinder to drain between volume measurements. Measure as accurately as possible because
your final results will depend upon it.
9.
Continue the distillation until about 1-2 mL of liquid remains in the pot.
Caution
Do not distill to dryness. Remove the heat source and turn off the heat. Allow the liquid in the column to
drain into the distillation pot; measure and record the volume of this pot residue.
10. Record the distillation process in your notebook. Include temperature ranges and volumes for each fraction.
11. Dispose of waste into appropriate waste containers.
12. Wash all glassware.
FD.7
Fractional Distillation
FD.8
Name:
Report Sheet:
Section:
Fractional Distillation
Date:
Preparing for Distillation
Initial volume of ester mixture
Boiling Points of Esters
Literature reported boiling point of ethyl acetate
Literature reported boiling point of butyl acetate
Distillate Fractions
Record the temperatures and volumes of each fraction collected in your distillation in the table below.
Report Table FD.1: Fractions from Distillation
Fraction Number
Temperature Range (∘ C)
Volume (mL)
1
2
3
4
5
6
7
8
9
10
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FD.1
Fractional Distillation Report Sheet
Final Distillation Volumes
Pot residue volume (mL)
Total volume of distillate fractions (mL)
Percent recovery
Space for Calculations
Analyzing the Fractional Distillation
Complete the graph of your distillation data as described in the procedure. On the chart itself, clearly mark
which fractions you will use for the calculating the percent of ethyl acetate, which fractions you will use for the
calculating the percent of butyl acetate, and which fractions are a mixture of the two esters. The pot residue
(which would have distilled if you had let it), must be included.
Report Table FD.2: Combining Fractions
Fraction
Numbers
Temperature
Range (∘ C)
Volume (mL)
Fractions assumed
to be mostly ethyl
acetate
Fractions assumed
to be a mixture
Fractions assumed
to be mostly butyl
acetate
Total volume of
fractions
FD.2
Fractional Distillation Report Sheet
Calculating Percent Composition
Percent composition of ethyl acetate
Percent composition of butyl acetate
Space for Calculations
Calculating Mole % Composition
Report Table FD.3: Mole % Composition
Molar Mass
(g/mol)
Density (g/mL)
Mole %
Composition
ethyl acetate
butyl acetate
FD.3