8.4 Exploring Intermolecular Forces Lab Activity
Name ___________________________________
Objective:
Intramolecular forces are forces acting on atoms WITHIN molecules. Intramolecular forces are responsible
for many macroscopic properties such as electrical conductivity, hardness, and luster. Other properties of
matter such as boiling point, vapor pressure and surface tension are best explained by the forces acting
BETWEEN separate molecules (intermolecular forces). In this experiment six liquids (acetone, ethanol,
glycerin, hexane, isopropyl alcohol, and water) will be compared in order to assess the strength of their
intermolecular forces.
Materials:
3 250 mL Erlenmeyer flasks with stoppers
6 plastic pipets
6 Watch glasses
stop-watches or clock with second hand
1 mL each of: acetone, ethanol, glycerin (a.k.a. glycerol), hexane, isopropyl alcohol
100 mL each of: glycerin(a.k.a. glycerol), isopropyl alcohol and water
Prelab Questions:
How can VSEPR models help predict intermolecular bonding? Answer these pre-lab questions BEFORE
beginning the experiment.
1. Predict whether the following molecules are polar or nonpolar. Justify your answer using VSEPR models.
Draw the molecules, showing polar bonds and drawing in an arrow for molecular polarity.
a) oxygen difluoride, OF2
Polar?
b) methane, CH4
Polar?
c) carbon disulfide, CS2
Polar?
d) fluoromethane, CH3F
Polar?
e) hydrogen peroxide, H2O2
Polar?
f) ammonia, NH3
Polar?
2. What two conditions are necessary for molecules to be polar?
3. If water had a linear molecular shape, would the molecule be polar or nonpolar? Explain your answer.
Intermolecular Forces Lab -- Page 2
Procedure:
Part 1: Surface tension and vortex When a liquid is swirled, a vortex is developed in which the surface level
of the center of the liquid is substantially below the surface level of the perimeter. The greater the surface
tension, the longer the vortex will remain after you have stopped swirling the container.
Put 100 mL of isopropyl alcohol in one flask, 100 mL of water in a second flask, and 100 mL of
glycerin in yet another flask. Stopper the flasks to prevent vapors from polluting the room. Try to swirl each
flask with the same intensity; once a vortex has formed at that intensity, set the flask down and start the
stopwatch; stop the stopwatch when the vortex disappears. Record the time it takes for the vortex to disappear
in the table below.
Substance:
Isopropyl
alcohol
Water
Glycerin
Time:
Part 2: Vaporization Using an eyedropper or pipet, transfer 1.0 mL of acetone to a watch glass and place it
under a fume hood. Record the time it takes for the acetone to evaporate. Make sure the watch glass is clean
and dry and repeat with each of the remaining five liquids. Record each value in the table below.
Acetone
Ethanol
Glycerin
Hexane
Isopropyl
alcohol
Water
Part 3: Consider the following structural formulas and dipole moments (a measure of the amount of molecular
polarity reported in units of Debye).
Acetone: 2.91 Debye
Isopropyl alcohol: 1.66 D
Ethanol: 1.69 D
Glycerin: 2.7 D
Water: 1.85 D
Hexane: 0.0 D
Intermolecular Forces Lab -- Page 3
Analysis:
1. Describe the similarities and differences in the structural features of the six molecules.
2. Rank these chemicals from least polar to most polar and justify your answer based on the given
models.
3. Describe what types of intermolecular forces (London dispersion forces, dipole-dipole, H-bonding) you
would expect for each molecule and justify your answer based on the given models.
Further Questions:
4. Which of the liquids you tested displayed the greatest surface tension (greatest intermolecular forces)?
5. Which of the liquids you tested do you think will boil at the lowest temperature? Why? Be specific.
6. You may have noticed mosquitoes, water striders, and other insects walking on the surface of a pond.
Why don't they sink?
Parts of this lab have been taken and adapted from the original source at:
www.csd509j.net/cvhs/kirscha/Intermolecular%2520Forces%