ANSWER KEYS FOR PRACTICE WORKSHEETS
Intermolecular Forces Practice #1
1. Why are the intermolecular attractions in H2O (g) much weaker than the attractions in
H2O (l) or H2O (s)?
The particles in a gas are further apart, so there would not be as much attraction
between the particles
2. Consider the following: Br2, Ne, HCl, and N2.
a. Which of the above would have the largest dispersion forces? Why?
Br2, because it has the largest mass.
b. Which of the above would have the largest dipole-dipole attractions? Why?
HCl, because it is the only molecule that is polar.
3. Which of the following molecules would exhibit hydrogen bonding: methane
(CH4), hydrazine (H2NNH2), methyl fluoride (CH3F), or hydrogen sulfide (H2S)?
Hydrazine would form hydrogen bonds.
Methyl fluoride does NOT form hydrogen bonds, because the H and F are not
bonded together when you draw the lewis structure.
4. List the type(s) of intermolecular forces that are present in each of the following
examples:
a. HBr dipole / LDF
d. H2Ohydrogen bonding /
dipole / LDF
b. I2
LDF (dispersion)
c. CS2 LDF
e. CH2Cl2
dipole / LDF
f. C2H6 dispersion
i. ClF3 dipole / dispersion
g. NH3 hydrogen bonding /
dipole / dispersion
h. H2S dipole / dispersion
j. CH3COOH hydrogen
bonding / dipole / dispersion
Intermolecular Forces Practice #2
1. Use your understanding of intermolecular forces to explain why:
a. ICl boils at 97 °C, while Br2 boils at 59 °C.
Br2 and ICl have similar masses (giving them similar dispersion forces). ICl is more
polar than Br2, resulting in dipole-dipole attractions. This gives ICl stronger IMF
and a higher boiling point than Br2.
b. CHCl3 boils at 61°C while CHBr3 boils at 150 °C.
CHBr3 has stronger IMF because it has greater dispersion forces than CHCl3 (higher
mass = more dispersion)
2. For each of the following, select the molecule that you would expect to have the
higher boiling point and explain your reasoning:
a. HF or HCl
HF would have the higher boiling point because it forms hydrogen bonds.
b. CH4 or C3H8
C3H8 would have the higher boiling point because it has a greater mass / more
dispersion.
3. List the substances BaCl2, H2, CO, HF, and Ne in order of increasing boiling points
and explain how you arrived at your answer.
BaCl2 > HF > CO > Ne > H2
Barium chloride is ionic (ionic bonding is stronger than any form of IMF), HF is
second because of its hydrogen bonding. CO > Ne >H2 because larger mass = more
dispersion = stronger attraction to overcome to reach boiling point.
4. Which of the following would you expect to have the highest boiling point:
CH3CH3, CH3OH, or CH3CH2OH? Explain your answer.
CH3CH2OH. It will be greater than CH3CH3 because it has hydrogen bonding. It
will be greater than CH3OH because while they both have hydrogen bonding,
CH3CH2OH has a greater mass / more dispersion.
5. How can we use intermolecular forces to explain the fact that chlorine is a gas at
room temperature, while bromine is a liquid, and iodine is a solid?
In terms of mass, I2 > Br2 > Cl2. With the largest mass, iodine has stronger
dispersion attractions and stronger IMF. This makes it a solid at room temperature.
Bromine has weaker dispersion giving it less IMF and making it a liquid. Chlorine
is the lightest of the three and exhibits the weakest attractions, making it a gas at
room temperature.
6. Using your knowledge of intermolecular forces, explain why the boiling point of
NO (–151 °C) is higher than the boiling point of either O2 (–183 °C) or N2 (–196°C)
All three have similar masses, but NO will be slightly polar. The dipole-dipole
attraction between NO molecules give it stronger IMF and a higher boiling point.
7. Arrange the following in order of increasing melting point: RbF, CO2, CH3OH,
CH3Br. Explain how you arrived at your answer.
RbF > CH3OH > CH3Br > CO2
Rubidium fluoride has ionic bonds, giving it the highest melting point. CH3OH has
hydrogen bonding making it have the second strongest IMF / second highest melting
point. CH3Br > CO2 because it has a much higher mass resulting in stronger
dispersion forces between its molecules.
8. Dimethyl Ether (CH3OCH3) boils at -25 °C whereas Ethanol (CH3CH2OH) boils at
79 °C. How can we account for this difference using intermolecular forces?
When drawing the structures, the oxygen is connected to a hydrogen atom in ethanol
but not in dimethyl ether. This means that ethanol is capable of forming hydrogen
bonds, but dimethyl ether is not. Hydrogen bonding in ethanol gives it much
stronger intermolecular attraction and a much higher boiling point as a result.
9. If you lived in Alaska, would it be better to keep methane (CH4), propane (C3H8), or
butane (C4H10) in an outdoor storage tank during the winter? Explain your answer.
You would want to select the fuel that would remain a gas at the lowest temperature
(to avoid freezing when outside). The fuel that has the weakest IMF would remain
as a gas at the lowest temperatures because it has very weak attractions to keep it’s
molecules together. With its lower molar mass, methane is the best choice because
it has weaker disperions attractions than either of the other two fuels.