Advanced Placement (AP) Chemistry Practice Exam

Which intermolecular force is defined as the attraction between the positive end of one polar molecule and the negative end of another?

1. Hydrogen bonding

2. London dispersion forces

3. Dipole-dipole forces

4. Ion-dipole forces

The correct answer is: Dipole-dipole forces

The correct response pertains to dipole-dipole forces, which are the specific type of intermolecular force that occurs between polar molecules. These forces arise because polar molecules possess regions of partial positive and partial negative charges due to the unequal sharing of electrons between atoms with different electronegativities. In a polar molecule, the positive end, where there is a slight deficiency of electrons, will be attracted to the negative end of another polar molecule, which has an excess of electrons. This interaction is critical for understanding the behavior of many liquids and solids that exhibit polar characteristics, influencing their boiling points, melting points, and solubility properties. Hydrogen bonding, while a significant force in polar molecules, is a special case of dipole-dipole interaction that involves hydrogen atoms bonded to highly electronegative elements like fluorine, oxygen, or nitrogen. It is generally stronger than regular dipole-dipole forces but is a subset rather than a definition of them. London dispersion forces are present in all molecules, irrespective of polarity, and are fluctuations in electron distribution causing temporary dipoles, which leads to an attraction. Ion-dipole forces, on the other hand, describe the attractions between ions and polar molecules but do not fit the criteria of