1.12 : 分子間力と物性

Electrostatic interactions that exist between molecules are called intermolecular forces. These attractive forces influence various physical properties, such as melting point, boiling point, and solubility.

The three major types of intermolecular forces include strong ion–dipole interactions between ions and polar molecules, dipole–dipole interactions between polar molecules, and finally, the weakest of all — dispersion forces — present in all molecules, polar and nonpolar.

Ion–dipole interactions are common to solutions. When an ionic compound like sodium chloride is dissolved in a polar solvent like water, the ions align with the oppositely charged ends of the water molecules, allowing maximum electrostatic attraction.

Alternatively, dipole–dipole interactions are exhibited by polar molecules with permanent dipoles, where the positive end of one molecule interacts electrostatically with the negative end of the neighboring molecule.

When a hydrogen atom bonded to either oxygen, nitrogen, or fluorine is attracted to the electronegative atom of a neighboring molecule, a hydrogen bond — a special type of dipole–dipole interaction — is formed. Notably, compounds capable of hydrogen bonding exhibit high melting and boiling points.

Dispersion forces are a result of temporary dipoles and tend to increase with the molar mass. Atoms with higher masses have more electrons and larger electron clouds, leading to increased dispersion forces. As a result, more energy is required to overcome the attractive forces between neighboring atoms, resulting in higher boiling and melting points.

This explains why alkane boiling points increase with their molar mass.

However, molar mass is not the only criterion. Despite having the same masses, n-pentane has a higher boiling point than neopentane due to the increased surface area available for dispersion forces.

Intermolecular forces are also crucial in determining solubility. Liquids of similar type and magnitude of intermolecular forces, such as water and ethanol, are entirely soluble in all proportions or are miscible.

In contrast, liquids such as hexane and water, with different types and magnitudes of intermolecular forces, are insoluble in most proportions or immiscible.

Overall, intermolecular forces are relatively weak because small or partial charges interact over large distances.