Download Intermolecular Forces: Applying What You Know

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Intermolecular Forces: Applying What You Know
PART 1: INTERMOLECULAR FORCES IN LIQUIDS AND GASES.
Molecules attract each other, and 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 the 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.
CRITICAL THINKING QUESTIONS:
1) When water evaporates, are any bonds between the H atoms and O atoms within a molecule broken?
2) On average, are the intermolecular forces stronger in liquid water or in gaseous water? Explain.
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PART 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 b y the structure of individual molecules.
Alkane
CH3CH2CH3
propane
CH3(CH2)2CH3
butane
CH3(CH2)3CH3
pentane
CH3(CH2)4CH3
hexane
CH3(CH2)8CH3
decane
MW
(amu)
44.1
bp (°C)
Ketone
-42.1
58.1
-0.5
72.2
36.1
86.2
69.0
142
174
CH3CCH3O
acetone
CH3CCH2CH3O
2-butanone
CH3C(CH2)2CH30
2-pentanone
CH3C(CH2)3CH3O
2-hexanone
CH3C(CH2)7CH3O
2-decanone
MW
(amu)
58.1
bp (°C)
Alcohol
56.2
72.1
79.6
86.1
102.0
100
128.0
156
210.0
CH3CH2CH2OH
1-propanol
CH3(CH2)2CH2OH
1-butanol
CH3(CH2)3CH2OH
1-pentanol
CH3(CH2)4CH2OH
1-hexanol
CH3(CH2)8CH2OH
1-decanol
MW
(amu)
60.1
bp
(°C)
97.4
74.1
117.0
88.2
137.0
74.0
158.0
158
229.0
MW= Molecular Weight
Alkanes are hydrocarbons containing only C and H and have all single bonds.
Ketones 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 that O has a significantly higher
electronegativity. For each type of compound (alkane, ketone, 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
increases:
a) alkane
b) ketone
c) alcohol
5) Based on your answers to question 4, 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 size. Rank these compounds in terms of relative boiling
points.
7) Repeat question 6 with a different set of compounds.
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8) Using complete sentences, describe any general pattern that you can identify about the relative boiling points of
alkanes, ketones and alcohols of roughly equal size.
9) Rank the three types of compounds in terms of their relative strength of intermolecular interaction, for molecules of
roughly the same size.
10) Based on the data provided on the pervious page, does the presence of a dipole moment in a molecule tend to
increase or decrease the strength of intermolecular interactions? Explain your reasoning.
11) What is the difference between intramolecular bonds and intermolecular forces?
12) Rank these intermolecular forces in terms of the typical relative strengths: hydrogen bonding; dipole-dipole, and
London dispersion forces.
13) In the alkanes: what types of intermolecular forces are present? Which is the strongest?
14) In the Ketone: what types of intermolecular forces are present? Which is the strongest?
15) In the alcohols: what types of intermolecular forces are present? Which is the strongest?
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?
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17) Both cis-1,2,- dichloroethylene and trans-1,2,-dichloroethylene pictured below have the same molecular formula:
C2H2Cl2. However, the cis- compound is polar, while the trans- compound is non polar. One of these chemicals has a
boiling point of 60.3°C and the other has a boiling point of 47.5°C. Which compound has which boiling point?
18) Rank each of the following groups of substances in order of increasing boiling point, and explain your reasoning:
a) NH3, He, CH3F, CH4
b) CH3Br, Ne, CH3OH, CH3CN
c) CH4, SiH4, GeH4, SnH4
19) Fluoromethane, CH3F, and methanol,CH3OH, have approximately the same molecular weight. However, the boiling
point of CH3OH is 65.15°C, whereas the boiling point of CH3F is almost 100 degrees lower, -78.4°C. Explain.
20) Which has a higher boiling point, ethylene glycol or ethanol? Why? Both are pictured below.
Ethylene glycol
Ethanol