CHEM 211-2008
3
Experiment 1
How is the
Boiling Point
of a Molecule
Related to Its Structure?
G. Procedure:
Boiling Point and Distillation: (See also Basic Laboratory Techniques, Padais pp. 16-21, Simple
Distillation, Padais pp. 129-134, Boiling Points, Padais pp. 51-52 and Heating Methods, Padais pp. 22-24)
The process for determining a liquid's boiling point, distillation, is also a method for
purifying the liquid. As a liquid compound is heated gently at constant pressure (in a
container that is open to the atmosphere), its temperature will rise slowly. The molecules in
the liquid move faster and faster as they absorb more and more energy until the individual
molecules attain sufficient energy to begin to escape from the molecular interactions in the
liquid and enter the gas phase. The boiling point (bp) is the temperature at which the
molecules are in equilibrium between the liquid and gas phases at standard atmospheric
pressure, 760 torr. At a compound's bp, bubbles of the gas phase spontaneously form within
the liquid. The temperature at which boiling occurs is generally measured during the
distillation of a liquid and is a characteristic of the pure compound. Thus, the boiling point is
an important physical characteristic that can be used in determining if liquid compounds are
the same or different.
You will use a standard-scale simple distillation to purify your assigned liquid and determine
its bp. A description of a simple distillation apparatus and an outline of a simple distillation
procedure are presented in Padais pp. 131 and the procedure is outlined below. Prepare your
laboratory notebook for Experiment 1 as described on pp. 16-18 of the lab manual. The
following procedure must be written in your notebook as you work in the laboratory.
Specific Instructions for Bp Determination
1. Assemble the simple distillation apparatus (See model apparatus on the front bench in the
lab) being careful to properly position the thermometer bulb. Note that the thermometer
placement is different from that shown in Padais, we will not use a drying tube on the
apparatus and we will use a graduated cylinder as a receiving container with no ice bath.
2. Obtain an ~15 mL sample of your assigned compound.
3. Remove the thermometer and adapter from the still head and add a boiling chip to the
flask through the open still head.
4. Insert a long stem funnel into the still head. (Be sure that the funnel stem extends past the
side arm of the still head.)
5. Pour all of your sample into the distillation pot, remove the funnel and replace the
thermometer and adapter. Check the position of the thermometer bulb and adjust it if
necessary.
6. Attach the heating mantle to the current regulator and plug the regulator into an electrical
outlet.
7. Begin heating your compound by setting the current regulator dial to 5.
While the liquid is heating, draw a picture of your distillation apparatus in the results section
of your notebook opposite the procedure step # 1. Be sure to include the positions of clamps
and accurate thermometer placement as indicated in the pre-lab questions.
CHEM 211 Experiment 1
4
Boiling Points vs. Structure
8. As the liquid begins to boil watch the condensing vapor as it moves up toward the
thermometer.
9. Observe the thermometer reading when the condensing vapor reaches the thermometer
bulb.
10. Observe the condensing vapor as it reaches the still head sidearm and passes into the
condenser. Record the temperature reading when the first drop of liquid drips from the
end of the condenser into the receiving graduated cylinder.
In your results section opposite procedure step # 10 (see below) prepare a Table of Vapor
Temperatures to record the vapor temperature when the first drop of distillate is collected and
after every mL is collected until the distillation is discontinued. (See model table below.)
Table of Vapor Temperatures for Distillation of “Compound Name”
Distillate
Volume
Vapor Temp. (˚C)
(mL)
First Drop
1
2
Etc.
11. Adjust the current regulator to attain a rate of distillation of ~1 drop per second.
12. Record the temperature after each mL of liquid is collected.
13. Continue distilling until the about 1 mL of liquid remains in the distillation pot. Be sure
to STOP THE DISTILLATION BEFORE THE DISTILLATION POT IS DRY.
14. To discontinue the distillation, unplug the current regulator and lower the heating mantle.
15. Pour your distilled liquid back into the original labeled test tube and submit it to your
instructor.
16. When the apparatus is cool, dismantle it and discard any liquid remaining in the pot into
the appropriate waste container.