Experiment 1J: Simple and
Fractional Distillation of a Binary Mixture Chemistry II
1)
Aim of the Experiment
In
this experiment you will perform simple and fractional distillation in order to
separate a mixture of two liquids, namely cyclohexane
and toluene.
Cyclohexane bp: 81 °C
Toluene bp: 111 °C
2)
Introduction
Distillation
is the process in which a liquid is vaporized, the vapor is condensed by
cooling it, and the condensate or distillate is collected. This method of
purification is extremely useful for separating a liquid mixture based on
boiling point differences.
The
boiling point of a liquid is defined as the temperature at which its vapor
pressure equals atmospheric pressure. The boiling point is a physical
characteristic that can be used as important evidence to a liquid’s identity as
well as its purity.
Distillation techniques are often used
to separate two or more components on basis of vapor pressures. Vapor pressure
results from molecules in motion as they leave the liquid surface and become
vapor. Separation is usually then accomplished by taking advantage of the fact
that the component with a higher vapor pressure, lower boiling point, will be
in higher concentration in the vapor phrase. Therefore, the lower boiling
liquid will be collected first.
There are two distillation techniques
you need to be familiar with in the organic chemistry lab. Simple distillation
works well for most routine separations and purification of organic compounds
when the boiling points of the two components are large. However, when the
boiling point difference of the components is small, fractional distillation
makes use of a fractional column which allows for repeated vaporizations and
condensations thus providing a better separation of a liquid mixture.
3) Pre-Lab
Questions
1.
Why is a
fractionating column packed with small glass beads or short pieces of glass
tubing? How does this help improve a distillation?
2.
Some mixtures of volatile liquids cannot be
completely separated by fractional distillation because of the formation of
what are called azeotrope among the liquids. Use
a chemical encyclopedia to find the definition of an azeotrope, and why azeotropes such as 95% ethyl alcohol cannot be separated by
distillation.
3.
In distillation,
one needs to condense the vapor being produced; therefore a condenser is used.
Cold water is passed through the condenser. There two ways to allow the water
the flow, concurrent and countercurrent. What is the difference between concurrent exchange and
countercurrent exchange? Which method is more efficient? Explain.
4)
Experimental Procedure
a.
Simple
Distillation of a Mixture of Cyclohexane and Toluene
Place 20 mL of the stock solutioin
containing a mixture of cyclohexane and toluene (1:1)
into the smallest round bottom flask with a boiling stone. Set up the apparatus
for simple distillation and use a 10 mL graduated
cylinder for the receiver. Clamp the apparatus firmly in place. Bury the flask
in the heating coil. Carefully, align the top of the thermometer bulb below the
side arm of the distillation head. Note: it is crucial that the thermometer be
positioned in the appropriate location for accurate boiling point measurements.
Before beginning the distillation, be sure that you can remove the distillate
as it collects with disturbing the distillation. Also, have your instructor
check your set-up. Initiate the distillation and heat at a reasonable rate,
which may be around level 4. Once the liquid in the vial begins to boil, reduce
the heating rate to 2-3°C per minute to allow for proper establishment of equilibrium
between the liquid and vapor phases. If
you heat the flask too quickly, both liquids will distill over together,
resulting in no separation. The rate of distillation should be no faster
than two drops per minute!
Monitor
the distillation rate appropriately, and make adjustments accordingly. Do not
let the temperature rise too rapidly.
As the
distillation progresses, you may note a ring of liquid moving up the flask into
the distillation head. The temperature should rise as well at this point, and
you should observe the walls of the setup become wet at this point, and you
should observe the walls of the setup become wet as the vapor condenses.
Continue a slow, steady heating until the temperature of the inside thermometer
rises rapidly and then reaches a more or less steady value of (~80°C). Then,
conduct the distillation, collecting the major fraction without interruption.
Note and record the temperature range and volume of distillate for the fraction
collected at regular time intervals. Record the volume of the distillate by
reading the graduations on the graduated cylinder. Make estimations if
necessary. When the first fraction has more or less completely distilled (~10 mL), you will notice that the temperature will either
decrease or begin to increase. At this point, change to a new graduated
cylinder. Set aside the first distillate. At this point, turn up the heating
coil to distill the higher boiling component (toluene), and collect the second
steady boiling fraction (~110 °C) as before. Insulation may be necessary.
Continue the distillation until the second component is nearly all collected. Note and record the temperature range and
volume of distillate for this fraction collected as well. However, never distill to dryness!
Discontinue the distillation, and shut
off the heating coil. Allow the apparatus to cool, and disassemble. Graph the
distillation curve of your results using either excel or graphical analysis.
Label the y-axis as temperature and x-axis as volume of distillate.
b.
Fractional
Distillation of a Mixture of Cyclohexane and Toluene
Place 20 mL of the stock solution
containing a mixture of cyclohexane and toluene (1:1)
into the smallest round bottom flask with a boiling stone. Set up the apparatus
as in simple distillation; however, insert the small fractionating column
containing glass beads, between the flask and the distillation head. Do not
over load the fractionating column, which will hold back the higher boiling
material to be distilled. The packing will allow for efficient separation of cyclohexane and toluene. Insulate the column by wrapping
with aluminum foil to keep the temperature of the column constant. Use a 10 mL graduated cylinder for collecting the distillate.
Assemble the thermometer as above, and mount the apparatus in the heating coil
using a clamp. Again heat steadily, using a heating coil. Do not allow the temperature to rise too rapidly, and distill the
mixture at a rate of no faster than two drops per minute!
Once constant but gentle boiling begins, lower the
heating rate and heat slowly at a constant rate to permit a good
separation. Record the temperature and volume at regular increments in your
notebook. Remove what you regard to be the pure cyclohexane
fraction as previously done in simple distillation. After you have distilled
off the cyclohexane, change the receiver to another
10 mL graduated cylinder and turn the hot plate up
and collect the toluene as done previously above. Record
temperature and volumes accordingly at regular intervals.
Discontinue the distillation Allow the apparatus to
cool and disassemble the distillation set up. Graph your results in the same
fashion as above.
Interpret and explain your results in your report.
Include an appropriate discussion of the difference between the two graphs.
5) Post-Lab
Report
1.
How do simple and
fractional distillation differ? Under
what circumstances is one method likely to be used in preference to the other
method?
2.
In spite of such
a major portion of the earth’s surface being covered by water, surprisingly
little use has been made of distillation of sea water as a source of drinking
water in arid areas. Why do you suppose this is so?
Example setup apparatuses for
simple and fractional distillation:
Fractionating Column
Simple Distillation Fractional Distillation