ChemActivity#4
Intermolecular Forces
(What Determines the Boiling
Point?)
Model
1: Intermolecular Forces in Liquids and Gases.
Molecules attract each other, an the
force of attraction increases rapidly as the intermolecular distance decreases.
In a liquid, the molecules are very close to one another and are constantly
moving and colliding. When a liquid evaporates, molecules in the liquid must
overcome these intermolecular attractive forces and break free into the gas
phase, where on average molecules are very far apart. For example, when water
evaporates, rapidly moving H2O molecules at the surface of the
liquid pull away from neighboring H2O molecules and enter the gas
phase, as shown in figure 1.
Figure
1. H2O molecules in the solid, liquid, and gas phases.
Critical
Thinking Questions
1. When
water evaporates, are any bonds between H atoms and O atoms within a molecule
broken?
2. On
average, are the intermolecular forces stronger in H2O (l)
or in H2O (g)? Explain.
Model
2: Intermolecular Forces and Boiling Points.
To a large extent, the boiling point of
a liquid is determined by the strength of the intermolecular interactions in
the liquid. These interactions are largely determined by the structure of the
individual molecules.
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Table
1. Boiling points of selected compounds.
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Alkane
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mw
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bp
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Ketone
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mw
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bp
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Alcohol
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mw
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bp
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propane
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44.1
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-42.1
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acetone
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58.1
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56.2
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1-propanol
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60.1
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97.4
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CH3CH2CH3
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CH3COCH3
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CH3CH2CH2OH
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butane
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58.1
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-0.5
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2-butanone
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72.1
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79.6
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1-butanol
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74.1
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117
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CH3(CH2)2CH3
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CH3COCH2CH3
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CH3(CH2)2CH2OH
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pentane
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72.2
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36.1
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2-pentanone
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86.1
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102
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1-pentanol
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88.2
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137
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CH3(CH2)3CH3
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CH3CO(CH2)2CH3
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CH3(CH2)3CH2OH
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hexane
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86.2
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69
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2-hexanone
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100
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128
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1-hexanol
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100
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128
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CH3(CH2)4CH3
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CH3CO(CH2)3CH3
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CH3(CH2)4CH2OH
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decane
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142
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174
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2-decanone
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156
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210
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1-decanol
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128
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229
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CH3(CH2)8CH3
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CH3CO(CH2)7CH3
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CH3(CH2)8CH2OH
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mw = molecular weight
Alkanes are hydrocarbons containing only
C and H and have all single bonds.
Ketone contain a C=O group.
Alcohols contain an O-H group.
Critical
Thinking Questions
3. Recall
that the electronegativity of C and H are roughly the same, but O has a
significantly higher electronegativity. Or each compound (alkane, ketone, and
alcohol) predict whether or not the compound is expected to be polar or
nonpolar.
4. For
each type of compound below, indicate how the boiling point changes as the
molecular weight of the compound changes.
a.
Alkane
b.
Ketone
c.
Alcohol
5. Based
on you answers to question 4 above, how do the intermolecular forces between
molecules change as the molecular weight increases?
6. Find
an alkane, a ketone, and an alcohol with roughly the same mw (within 5). Rank
these compounds in terms of relative boiling points.
7. Repeat
question 6 with two more sets of compounds.
Using
grammatically correct English sentences, describe an general pattern that you
can identify abou the relative boiling points of alkanes, ketones, and alcohols
of roughly equal mw.
8. Rank
the three types of compounds in terms of their relative strength of
intermolecular interaction, for molecules of roughly equal mw.
9. Based
on the data in Table 1, does the presence of a dipole moment in a molecule tend
to increase or decrease the strength of intermolecular interactions? Explain
your reasoning.
10. Is
the strength of intermolecular forces determined by the bond strengths within
the individual molecules? Explain your reasoning.
Model
3: Intermolecular Forces are Weaker than Covalent Bonds.
The intermolecular forces that attract
molecules to each other are much weaker than the bonds that hold molecules
together. For example, 463 kJ/mole are required to break one mole of O-H bonds
in H2O molecules, but only44 kJ/mole are needed to separate one mole
of water molecules in liquid water.
Critical
Thinking Questions
11. What
is the difference between intramolecular bonds and intermolecular forces?
12. Rank
these forces in terms of their typical relative strengths” hydrogen bonding;
dipole-dipole; induced dipole-induced dipole.
13. In
the alkanes:
a.
What type(s) of intermolecular
forces is (are) present?
b.
What is the strongest
intermolecular force present?
14. In
the ketones:
a.
What type(s) of intermolecular
forces is (are) present?
b.
What is the strongest
intermolecular force present?
15. In
the alcohols:
a.
What type(s) of intermolecular
forces is (are) present?
b.
What is the strongest
intermolecular force present?
16. In
terms of intermolecular forces, why does the boiling point of a particular type
of compound (for example, an alkane) increase as the molecular weight
increases?
17. In
terms of intermolecular forces, explain the general trend that you described in
question 8.
Exercises
1. Based
on the data in Table 1, predict the boiling points of:
a.
Heptane, CH3(CH2)5CH3
b.
Ethanol, CH3CH2OH
c.
2-Octanone, CH3CO(CH2)5CH3
2. Both
cis-1,2-dichloroethylene and trans-1,2-dichloroethylene have the same
molecular formula: C2H2Cl2. However, the cis compound has a dipole moment, while
the trans compound does not. One of
these species has a boiling point of 60.3 oC and the other has a
boiling point of 47.5 oC. Which compound has which boiling point?
3. Rank
each of the following groups of substances in order of increasing boiling
points, and explain your reasoning:
a.
NH3, He, CH3F,
CH4
b.
CH3Br, Ne, CH3OH,
CH3CN
c.
CH4, SiH4,
GeH4, SnH4
4. Using
grammatically correct English sentences, describe the difference between the
hydrogen bond between two water molecules and the O-H bond in a particular
water molecule.
5. Fluoromethane,
CH3F, and methanol, CH3OH, have approximately the same
molecular weight. However, the boiling point of CH3OH is 65.15 oC,
whereas the boiling point of CH3F is almost 100 degrees lower, -78.4
oC. Explain.
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