Posted: March 3rd, 2014

Intermolecular Forces in Liquids and Gases.

 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.

 

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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?

 

 

 

1. 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

 

1. 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.

 

 

 

 

 

1. For each type of compound below, indicate how the boiling point changes as the molecular weight of the compound changes.

 

1. Alkane

 

 

 

1. Ketone

 

 

 

1. Alcohol

 

 

 

1. Based on you answers to question 4 above, how do the intermolecular forces between molecules change as the molecular weight increases?

 

 

 

 

1. Find an alkane, a ketone, and an alcohol with roughly the same mw (within 5). Rank these compounds in terms of relative boiling points.

 

 

 

 

 

1. 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.

 

 

 

 

 

 

1. Rank the three types of compounds in terms of their relative strength of intermolecular interaction, for molecules of roughly equal mw.

 

 

 

 

 

 

1. 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.

 

 

 

 

 

 

1. 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

 

1. What is the difference between intramolecular bonds and intermolecular forces?

 

 

 

 

 

1. Rank these forces in terms of their typical relative strengths” hydrogen bonding; dipole-dipole; induced dipole-induced dipole.

 

 

 

 

 

1. In the alkanes:

 

1. What type(s) of intermolecular forces is (are) present?

 

 

 

 

1. What is the strongest intermolecular force present?

 

 

 

 

1. In the ketones:

 

1. What type(s) of intermolecular forces is (are) present?

 

 

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1. What is the strongest intermolecular force present?

 

 

 

 

1. In the alcohols:

 

1. What type(s) of intermolecular forces is (are) present?

 

 

 

 

1. What is the strongest intermolecular force present?

 

 

 

 

1. 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?

 

 

 

1. 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:

 

1. Heptane, CH₃(CH₂)₅CH₃

 

1. Ethanol, CH₃CH₂OH

 

1. 2-Octanone, CH₃CO(CH₂)₅CH₃

 

 

1. 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?

 

 

 

 

1.  Rank each of the following groups of substances in order of increasing boiling points, and explain your reasoning:

 

1. NH₃, He, CH₃F, CH₄

 

1. CH₃Br, Ne, CH₃OH, CH₃CN

 

1. CH₄, SiH₄, GeH₄, SnH₄

 

1. 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.

 

 

 

 

1. 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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