Which compound would you expect to have greater surface tension: acetone \(\left[\left[\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\right]\) or water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) ? Explain.

Intermolecular forces are the attractive forces between molecules that determine many of their physical properties, including surface tension. Surface tension is a measure of the elastic tendency of a fluid surface which makes it acquire the least surface area possible. This property occurs because molecules at the surface experience a net inward force due to the lack of neighboring molecules above, which causes the surface to contract.

There are several types of intermolecular forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding. For example, in compounds like acetone, the primary intermolecular forces are dipole-dipole interactions that occur between its polar carbonyl groups. While these forces contribute to acetone's surface tension, they are significantly weaker compared to hydrogen bonding, as witnessed in water.

The molecular structure of a compound is closely tied to the type and strength of the intermolecular forces it can exhibit. The arrangement and types of atoms within a molecule dictate its polarity and the possible intermolecular forces it may participate in. Acetone's structure \(\left[\left[\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\right]\) includes a polar carbonyl group bonded to nonpolar methyl groups, giving it a polar profile but limited to dipole-dipole interactions. On the other hand, the bent molecular structure of water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) leads to a highly polar molecule capable of forming strong hydrogen bonds between the oxygen and hydrogen atoms of nearby water molecules, which greatly enhances its surface tension.

Hydrogen bonding is a strong type of dipole-dipole interaction, but it's specific to molecules containing hydrogen atoms bound to highly electronegative atoms such as oxygen, nitrogen, or fluorine. This is seen in water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\), where the hydrogen atoms of one molecule are attracted to the lone pairs of the oxygen atom in another molecule, resulting in a hydrogen bond.

Why is Hydrogen Bonding Special?

Hydrogen bonding is responsible for many of water's unique properties, including its high boiling point and high surface tension. The strong attraction between water molecules creates a 'network' of bonds, giving water more structure and cohesiveness at its surface, compared to acetone, which cannot form hydrogen bonds and therefore has lower surface tension.