Intermolecular Forces
Davis
The question: How do intermolecular forces determine the boiling point of a liquid?
The model: INTRAmolecular vs INTERmolecular forces.
Examine the sample of the diatomic molecule represented in the figure.
To break INTERmolecular forces:
Br2(l)  Br2(g)
To break INTRAmolecular forces:
Br2(g)  2 Br(g)
Figure 1. A liquid of diatomic bromine
in Equilibrium with its vapor.
Answer the following:
1. Referring to the diagram of liquid bromine above:
a. Shade in a covalently bound molecule
b. Draw a circle around a pair of molecules that are held together in the liquid phase.
2. What is an INTRAmolecular force?
3. What is an INTERmolecular force?
4. According to the figure, when a substance is converted from the liquid to the gaseous phase,
what type of force is broken: INTERmolecular or INTRAmolecular? Circle your answer.
5. Are the other types of forces broken? Why or why not?
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Intermolecular Forces
Davis
The Model: London Dispersion Forces (van Der Waals forces)
The questions:
1. Is the bromine in Figure 1 a polar or non-polar molecule?
2. Based on your answer to question 1, can you explain why bromine can be liquefied?
3. Electrons have a (negative/positive) charge and (attract/repel) each other.
Regardless of polarity, all gases can be converted into a liquid or a solid if enough heat is removed and
the molecules or atoms are moved close enough to each other. Consider Figure 2 – a model of Neon.
In neon’s valence, there are 8 electrons, 2s22p6.
1. Describe the distribution of electrons in each
section of the model (top, middle, bottom).
While this model is of neon, it can represent any atom
or molecule, polar or nonpolar. As you look at the top
of the model, it is clear that the valence electrons
(charge cloud or electron cloud) are equally distributed
(symmetrical). Electrons are (moving/stationary – circle the answer) and because of this the charge
cloud can become asymmetrical. This condition creates a temporary dipole in that atom (or molecule).
The electron cloud can resume its usual symmetry, however, if other atoms (or molecules) are nearby,
the temporary dipole may induce a dipole in the next atom (or molecule). The induced dipoles are
temporary and they are constantly inducing dipoles in other neighboring atoms (or molecules). It is
important to note that this is an average picture over time – constant movement of electrons resulting
in a constant condition of forming and reforming dipoles. This type of Intermolecular Force is called
London Dispersion force or Instantaneous Dipoles or Van Der Waals forces.
2. What can you say about the temporary dipoles in the bottom section of the model with respect
to their attraction to each other?
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Intermolecular Forces
Davis
Application Questions for London Forces
3. Complete the table:
Gas
b.p K
Ne
Ar
Kr
Xe
a. Rank the gases in order of increasing strength of their intermolecular forces.
b. Place the gases in order of increasing atomic radius.
c. As the valence cloud gets larger and further from the nucleus, does it become easier or
harder for the charge cloud to become asymmetrical and create a temporary dipole? Circle
your answer.
4. The alkane hydrocarbon family (general formula CnH2n+2) are usually considered to be non-polar
compounds. Methane, CH4, is a gas at room temperature. Butane (C4H10 or CH3-CH2-CH2CH3) is a
gas at 25oC but can easily be liquefied with a little added pressure. Hexane (C6H14 or CH3- CH2CH2- CH2-CH2-CH3) is a liquid at 25oC. Isocane [CH3(CH2)18CH3) is a solid at 25oC.
a. What happens to the surface are of the charge (electron) cloud that molecules have as
the size of the alkane increases? What happens to the contact area the molecules have
with each other as the molecules increase in size?
b. What happens to the strength of the intermolecular (London Force) forces as the length
of the alkane increases?
c. What is the relationship between the size of the molecule’s charge (electron) cloud and
the strength of the intermolecular forces?
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Intermolecular Forces
Davis
The Model: Dipole-Dipole Forces
A polar molecule is one in which there is a separation of the charge within the molecule. This is due to
differences in electronegativity. For example, is the C—O bond polar? Find the difference in
electronegativity and make the determination. You will find that it is, in fact, a polar bond. Close
examination of a large sample of a polar compound would show that the molecules arrange themselves
in order to maximize attractive forces and minimize repulsive forces. (In a liquid, the molecules still
have enough energy to be constantly moving past each other – but in the big, over time picture, the
polarity and resulting attractions are very important) These overall, big picture attractive forces
between polar molecules are called dipole-dipole forces. Dipole-dipole forces are typically only about
1% as strong as a covalent bond. This means they are strong enough to provide an attraction (stickiness)
between molecules but the molecules can be fairly easily separated.
Consider the molecules CH4 and CH3F.
1. Draw each.
2. Determine their polarity and draw the appropriate arrow to show direction of polarity.
3. For the polar molecule(s), draw three of them below and show how the polarity and
resulting dipole demonstrates dipole-dipole interactions.
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Intermolecular Forces
Davis
The Model: Hydrogen Bonding
A very special type of dipole-dipole force occurs in samples of molecules that contain O-H, N-H, and H-F.
Consider the following drawing of hydrogen fluoride:
The intermolecular force between the H of one molecule and the F of the other molecule is quite strong.
Calculate the difference in electronegativity between:
H—F
H—O
H—N
This is the largest difference in electronegativity for the covalent molecules. The intermolecular force
that exists between molecules that contain these three types of bonds is called a hydrogen bond. These
are many times stronger than ordinary dipole-dipole forces and this is why they get a special name and
consideration.
Consider the following:
Describe the intermolecular forces that exist in a sample of each compound. Which intermolecular
forces dominate (are the strongest)? Explain why H2O has the highest boiling point. Explain the trend
you see in boiling points for the rest of the compounds.
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Intermolecular Forces
Davis
Practice Exercises
1. Identify the strongest intermolecular force in a sample of each of the following.
a. SO2
b. CF4
c. CH3OH
d. CH3NH
2. Circle the molecule in each pair of compounds that will form hydrogen bonds and then draw and
label how those hydrogen bonds would be arranged.
a. (CH3)2NH or (CH3)3N
b. HOCH2CH2OH or FCH2CH2F
3. Which member of each pair has the greater polarizability – that is, which member will form
stronger dispersion forces? Give a brief explanation.
a. Ca+2 or Ca
b. CH3CH3 or CH3CH2CH3
c. CCl4 or CF4
4. Circle the member of each pair that has the greater boiling point. Briefly explain your reasoning.
Your reasoning must include the nature and type of intermolecular forces involved.
a. CH3CH2OH or CH3CH2CH3
b. NO or N2
c. H2S or H2Te
5. Which would you expect to have the LOWER boiling point, butane, CH3CH2CH2CH3 or hexane,
CH3CH2CH2CH2CH2CH3? Briefly explain your reasoning. Your reasoning must include the nature
and type of intermolecular forces involved.
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