Advanced Placement (AP) Chemistry Practice Exam

Which of the following substances would most likely have the highest melting point due to its intermolecular forces?

1. A molecule with only London dispersion forces

2. A polar molecule exhibiting dipole-dipole forces

3. A molecule capable of hydrogen bonding

4. A nonpolar molecule

The correct answer is: A molecule capable of hydrogen bonding

The substance most likely to have the highest melting point is the one capable of hydrogen bonding. Hydrogen bonds are a specific type of strong intermolecular force that occurs when hydrogen is bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. These bonds are considerably stronger than both dipole-dipole interactions and London dispersion forces. When comparing different types of intermolecular forces, it is essential to recognize their strengths. London dispersion forces, found in nonpolar molecules, are generally the weakest because they arise from momentary dipoles formed when electron distributions fluctuate. Polar molecules exhibiting dipole-dipole interactions experience moderate interactions, which are stronger than London dispersion forces but weaker than hydrogen bonds. Since hydrogen bonding involves partial charges and the highly polar nature of compounds, it typically results in much higher melting points than either dipole-dipole or dispersion forces. Therefore, a molecule capable of hydrogen bonding will often require significantly more energy to disrupt these interactions during the melting process compared to substances that rely on weaker intermolecular forces.