Experiment #1
Evaporative Cooling as it Relates to Intermolecular Forces
Adapted by Tom Schultz
from Laboratory Experiments in Inorganic Chemistry
by Seattle U. Chemistry Faculty
Introduction
Intermolecular forces are responsible for substances being in the
solid, liquid, or gaseous state. This means that molecules with weak
intermolecular forces are going to exist in the gaseous state and
molecules with strong intermolecular forces are going to be solids.
Recall from the kinetic molecular theory that absolute temperature is
directly proportional to the average the average kinetic energy. If the
kinetic energy of a molecule is greater than the strength of the
intermolecular force then that molecule can evaporate thus leaving
the liquid with a lower average kinetic energy or temperature. This is
where we get the concept of evaporative cooling. By measuring the
rate or amount of cooling between a series of different organic
(carbon containing) molecules we can determine relative strengths of
intermolecular forces between them.
Part of your pre-lab exercise will be to develop a hypothesis relating
the strength of intermolecular forces to some structural or physical
property of organic molecules that are provided below. You will then
test your hypothesis by measuring the evaporative cooling of four
different organic molecules to see if they prove or disprove you
hypothesis. Some ideas you might use for a hypothesis would be to
organize them by molecular weight, density, number of hydrogen
atoms. Some physical properties of the organic molecules that you
will be analyzing are given below. Part of the scientific method is to
revise hypotheses after experimentation to support the experimental
data, which you will do, as part of this experiment. Certainly
consulting with others is part of this process, but not copying
someone else’s idea verbatim. You will submit your cooling data on
the classroom computer, which will be posted on the course web site,
so that you can again change and revise your hypothesis to explain
the entire class set of data on a larger group of organic substances in
the discussion (conclusion) portion of your lab report.
Learning
Objectives
1. Gain practical “hands on” experience of generating and modifying
a hypothesis.
2. Use laboratory experiments to support and/or revise a hypothesis.
3. Organizing and correlating laboratory data relative to support a
proposed hypothesis.
Pre Lab
On Tuesday you and your lab partner will be assigned a set of organic
molecules to formulate a hypothesis regarding intermolecular forces.
Your hypothesis should be submitted to your instructor at the
beginning of the laboratory session on a sheet of paper and also
recorded in your laboratory notebook. Since temperature is directly
proportional to the average kinetic energy of molecules it can be used
indirectly to determine the relative strengths of intermolecular
forces. During evaporation molecules with the highest kinetic energy
evaporate thus lowering the average kinetic energy of the remaining
molecules, or lowering the temperature of the liquid. Another way of
looking at this is that the molecules with the weakest intermolecular
forces evaporate faster than those with stronger intermolecular
forces thus causing more cooling. This means that the change in
temperature, ΔT, is inversely proportional to relative strengths of
intermolecular forces. Your task is to formulate a hypothesis to
predict the relative strength of intermolecular forces. For example,
perhaps the molecular weight is directly, or indirectly proportional to
the strength of intermolecular forces. Or maybe density could be
used to predict strength. Graphing ΔT vs. a physical constant of your
choice would give credence to your hypothesis. A second part is to
change your hypothesis to fit the entire set of class data listed on the
course website.
Safety
1. Organic molecules are always a fire hazard and should not be used
around open flames, such as a Bunsen burner.
2. Liquids used in today’s lab are poisonous.
3. Organic molecules often have unpleasant orders and should be
used under the laboratory hoods.
4. Most organic substances can be absorbed through the skin or
eyes.
5. Keep lids (stoppers) on the organic liquids under your hood.
Waste
Disposal
Place liquid soaked filter paper rolls in the dish under the waste hood.
Experimental
Procedure
Under the hood
 Four test tubes containing four different organic liquids
 Two ring stands equipped with clamps for holding test tubes and
temperature probes.
 2 Vernier temperature probes and the calculator
 Eight pieces of filter paper (2.5 cm by 2.5 cm)
 Eight small pieces of copper wire to hold the rolled filter paper
1.
2.
3.
4.
5.
6.
7.
8.
9.
Work in the hood that containing the compounds you were
assigned.
Using the temperature probe as a guide wrap the provided
filter paper around the probe and secure it by wrapping the
copper wires around the filter paper to prevent unraveling of
the filter paper. You will need 8 rolls of filter paper.
Clamp the test tubes containing two liquids to be measured to
the ring stands in a vertical position.
Place one the rolled filter paper cylinders on to the tip of each
temperature probe being sure that the bottom of the filter
paper is even with the tip of the temperature probe.
Dip both temperature probes with installed filter paper into
the liquid and start recording the temperature. Remove the
temperature probe after about 30 seconds and clamp it to the
ring stand.
Continue to monitor the temperature until the process is
complete (temp. begins to increase). Sketch the curve from
the computer into your lab notebook.
To analyze the data press the soft key below the plot. Select
Plot 1 or plot 2 using the up/down keys to get the plot from
probe 1 or probe 2 and press enter to see the temperature vs.
time plot
The curve may show the temperature rising when the probe is
put into the solution as it equilibrates with the liquid (or it
may stay relatively constant) and then dropping as the probe
is removed from the liquid and evaporation takes place. After
reaching a minimum it may again begin to rise as evaporation
ceases and the probe is warmed by room air. ΔT is obtained
from the differences between the maximum temperature and
the minimum temperature of this curve.
You will use the right/left keys to move the curser to the
maximum to read Tmax and then to the minimum to read Tmin.
and ΔT. Be sure to do this process for both curves.
10. Install new pieces of filter paper on each probe and replace
the two clamped liquids with the next tow liquids. Carry out
three trials of the last two liquids in the same manner as
before.
11. Be sure to save the data to your flash drive and record your
data on to the classroom computer
12. These curves should be sketched into your laboratory
notebook showing the minimum, maximum points.
Compound
Acetone
1-butanol
butanone
heptane
2-heptanone
hexane
1-hexanol
methanol
nonane
pentane
1-pentanol
3-pentanone
1-propanol
Molecular
Weight
g/mol
58.08
74.12
72.12
100.21
114.19
86.18
102.18
32.04
128.26
72.15
88.15
86.14
60.11
Boiling Pt
°C
56
117
79
98
151
69
158
64.6
150
36
137
101
97
Density
g/mL
0.790
0.810
0.805
0.684
0.811
0.660
0.814
0.791
0.718
0.626
0.814
0.814
0.804
Dipole
Moment
(debetes)
2.88
1.66
2.70
0 (very small)
?
0.08
?
1.69
0 (very small)
0 (very small)
1.8
?
1.68
Viscosity
(millipoise)
3.16
22.7
?
3.90
?
2.92
?
?
?
2.15
33.5
?
20.0
Compounds
Group #1
H2
C
H2
C
H2
C
C
H2
H3C
H2
C
C
H2
H3C
CH3
Pentane
H2
C
H2
C
H2
C
C
H2
H2
C
C
H2
H3C
CH3
Group #2
C
H2
H3C
CH3
H3C
H2
C
C
H2
CH3 H3C
H3C
H2
C
C
H2
C
H2
H2
C
C
H2
C
H2
CH3
2-Heptanone
CH3
H2
C
C
CH3
C
H2
H3C
3-Pentanone
OH
C
H2
C
H2
H3C
CH3
Hexane
H2
C
H2
C
O
H2
C
Pentane
H2
C
O
C
3-Pentanone
H2
C
H3C
1-Pentanol
H2
C
C
H2
H3C
Butanone
Acetone
Group #4
CH3
C
C
Group #3
O
C
OH
C
H2
C
H2
H3C
1-butanol
O
H2
C
H2
C
OH
C
H2
H3C
1-Propanol
O
H2
C
H2
C
OH
C
H2
H3C
Methanol
CH3
C
H2
Octane
H2
C
CH3
H2
C
C
H2
Heptane
HO
CH3
Hexane
H2
C
C
H2
H3C
C
H2
1-Pentanol
O
Group #5
C
H3C
O
H2
C
CH3
Acetone
H3C
OH
C
H2
1-Propanol
H3C
C
H2
3-Pentanone
H2
C
H2
C
H2
C
C
CH3
H3C
C
H2
1-Pentanol
OH
C
H2
Group #6
H2
C
H2
C
C
H2
H3C
C
H2
CH3
H2
C
H3C
1-Pentanol
H2
C
C
H2
CH3
Heptane
O
H2
C
C
H2
C
H2
H3C
Hexane
H2
C
H2
C
H2
C
C
OH
C
H2
H3C
C
H2
H2
C
C
H2
2-Heptanone
CH3