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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.

Table 1. Boiling points of selected compounds.

Alkane

mw

bp

Ketone

mw

bp

Alcohol

mw

bp

propane

44.1

-42.1

acetone

58.1

56.2

1-propanol

60.1

97.4

CH3CH2CH3

CH3COCH3

CH3CH2CH2OH

butane

58.1

-0.5

2-butanone

72.1

79.6

1-butanol

74.1

117

CH3(CH2)2CH3

CH3COCH2CH3

CH3(CH2)2CH2OH

pentane

72.2

36.1

2-pentanone

86.1

102

1-pentanol

88.2

137

CH3(CH2)3CH3

CH3CO(CH2)2CH3

CH3(CH2)3CH2OH

hexane

86.2

69

2-hexanone

100

128

1-hexanol

100

128

CH3(CH2)4CH3

CH3CO(CH2)3CH3

CH3(CH2)4CH2OH

decane

142

174

2-decanone

156

210

1-decanol

128

229

CH3(CH2)8CH3

CH3CO(CH2)7CH3

CH3(CH2)8CH2OH

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.