Topic: 2. Intermolecular forces, Liquids, and Solids
Concept: E. Evaluating Intermolecular Forces (Problem strategies)
Concept Overview:
There are typically two kinds of problems you'll encounter that deal with intermolecular forces:
Given a list of compounds, rank them in order of increasing boiling point.
Given a graph of boiling point values, explain the trend or explain the exception to the trend.
Ranking boiling points is simply a matter of ranking intermolecular forces--the stronger the intermolecular forces, the higher the boiling point. For the first several attempts, it's probably best to have your text open to Table 13.1 on page 596:
Look for ions--do you have an ionic compound such as NaCl dissolved in water? Is there a charge associated with any of your compounds? Any compounds in this category have the strongest intermolecular forces, and, therefore, the highest boiling points.
Look for polar molecules. Finding electronegative atoms such as oxygen, nitrogen, fluorine, chlorine indicates the possibility of dipoles, but you have to make sure the dipoles within the molecule don't cancel each other out. You need to understand the molecule's shape to evaluate molecular polarity. Don't forget to look for hydrogen bonding!
Rank the remaining items by molecular weight.Higher molecular weights mean more induced dipole-induced dipole interactions, and therefore, a higher boiling point.
