Question

Identify the types of intermolecular forces present in each of the following substances, and select the substance in each pair that has the higher boiling point: (a) \(\mathrm{C}_{6} \mathrm{H}_{14}\) or \(\mathrm{C}_{8} \mathrm{H}_{18}\) (b) \(\mathrm{C}_{3} \mathrm{H}_{8}\) or \(\mathrm{CH}_{3} \mathrm{OCH}_{3},(\mathrm{c}) \mathrm{HOOH}\) or \(\mathrm{HSSH}\), (d) \(\mathrm{NH}_{2} \mathrm{NH}_{2}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{3}\)

Short answer

The substances with the higher boiling points are: (a) \(\mathrm{C}_{8} \mathrm{H}_{18}\) (dispersion forces), (b) \(\mathrm{CH}_{3}\mathrm{OCH}_{3}\) (dipole-dipole forces), (c) \(\mathrm{HOOH}\) (hydrogen bonding), and (d) \(\mathrm{NH}_{2} \mathrm{NH}_{2}\) (hydrogen bonding).

Step-by-step solution

Identify intermolecular forces of each substance

There are three major types of intermolecular forces: dispersion forces (also known as London dispersion forces), dipole-dipole forces, and hydrogen bonding. (a) For both \(\mathrm{C}_{6} \mathrm{H}_{14}\) and \(\mathrm{C}_{8} \mathrm{H}_{18}\), there are only nonpolar covalent bonds present, so the predominant intermolecular forces are dispersion forces. (b) \(\mathrm{C}_{3} \mathrm{H}_{8}\) has nonpolar covalent bonds, so its predominant intermolecular force is dispersion forces. \(\mathrm{CH}_{3}\mathrm{OCH}_{3}\) has a polar covalent bond (the C-O bond), so its predominant intermolecular force is dipole-dipole forces. (c) \(\mathrm{HOOH}\) has hydrogen bonding (between the O-H bonds), while \(\mathrm{HSSH}\) has dipole-dipole forces (due to polar S-H bonds). (d) \(\mathrm{NH}_{2} \mathrm{NH}_{2}\) has hydrogen bonding (N-H bonds), while \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) has dispersion forces (due to nonpolar C-H bonds).

Compare intermolecular forces and determine the higher boiling point

By comparing the intermolecular forces, we can determine which substance will have a higher boiling point in each pair. The stronger the intermolecular forces, the higher the boiling point. (a) \(\mathrm{C}_{8} \mathrm{H}_{18}\) has more carbon atoms than \(\mathrm{C}_{6} \mathrm{H}_{14}\), which leads to stronger dispersion forces, and thus a higher boiling point for \(\mathrm{C}_{8} \mathrm{H}_{18}\). (b) \(\mathrm{CH}_{3}\mathrm{OCH}_{3}\) has stronger dipole-dipole forces than the dispersion forces in \(\mathrm{C}_{3} \mathrm{H}_{8}\), thus the higher boiling point will be for \(\mathrm{CH}_{3}\mathrm{OCH}_{3}\). (c) \(\mathrm{HOOH}\) has hydrogen bonding, which is a stronger intermolecular force than the dipole-dipole forces in \(\mathrm{HSSH}\), leading to a higher boiling point for \(\mathrm{HOOH}\). (d) \(\mathrm{NH}_{2} \mathrm{NH}_{2}\) has hydrogen bonding which is stronger than the dispersion forces in \(\mathrm{CH}_{3} \mathrm{CH}_{3}\), so \(\mathrm{NH}_{2} \mathrm{NH}_{2}\) will have a higher boiling point. In conclusion, the substances with the higher boiling points are: (a) \(\mathrm{C}_{8} \mathrm{H}_{18}\), (b) \(\mathrm{CH}_{3}\mathrm{OCH}_{3}\), (c) \(\mathrm{HOOH}\), and (d) \(\mathrm{NH}_{2} \mathrm{NH}_{2}\).

Key concepts

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest type of intermolecular force. These forces are present in all molecules, whether polar or nonpolar. However, they are the only type of intermolecular force that occurs between nonpolar molecules like \( \mathrm{C}_{6} \mathrm{H}_{14} \) and \( \mathrm{C}_{8} \mathrm{H}_{18} \). Dispersion forces arise due to temporary fluctuations in electron density, which create instantaneous dipoles. These dipoles induce other dipoles in neighboring atoms or molecules, resulting in a weak attraction.
Although weak, dispersion forces increase with the size and number of electrons in a molecule. That’s why \( \mathrm{C}_{8} \mathrm{H}_{18} \), which is larger, has stronger dispersion forces than \( \mathrm{C}_{6} \mathrm{H}_{14} \), giving it a higher boiling point.
Dispersion forces become significant as the mass and surface area of a molecule increases, leading to stronger attractions. This explains why larger alkanes tend to have higher boiling points.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where there is a permanent separation of charge within the molecule. Molecules like \( \mathrm{CH}_{3}\mathrm{OCH}_{3} \) exhibit dipole-dipole forces due to their polar covalent bonds—the C-O bond being one. These forces arise from the positive pole of one molecule being attracted to the negative pole of another.
Polar molecules align themselves in a way that maximizes attraction and minimizes repulsion, enhancing the strength of these interactions. Because dipole-dipole forces are generally stronger than dispersion forces, substances like \( \mathrm{CH}_{3}\mathrm{OCH}_{3} \) often have higher boiling points than nonpolar substances like \( \mathrm{C}_{3} \mathrm{H}_{8} \), which only have dispersion forces.
These interactions are essential in determining the molecule's physical properties. They affect boiling and melting points, solubility, and the way molecules interact with one another. Understanding dipole-dipole interactions can explain the behavior and properties of many compounds.

Hydrogen Bonding

Hydrogen bonding is a special, potent type of dipole-dipole interaction. This occurs when hydrogen is covalently bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine, as seen in \( \mathrm{HOOH} \) and \( \mathrm{NH}_{2} \mathrm{NH}_{2} \). Despite its name, a hydrogen bond is a strong intermolecular force rather than a true bond.
Hydrogen bonds result from the attraction between the hydrogen atom—which gets a partial positive charge—and the more electronegative atom it is near, with its partial negative charge. Because of the strength of hydrogen bonds, they have a significant impact on the physical properties of substances, like raising boiling points. Hence, \( \mathrm{HOOH} \) has a higher boiling point than \( \mathrm{HSSH} \), which only has dipole-dipole forces.
One of hydrogen bonding's most key roles is its impact on the properties of water, influencing everything from surface tension to ice's unique structure. Understanding hydrogen bonds helps explain a vast range of phenomena in chemistry and biology.