Which compound would you expect to have greater surface tension: acetone [(CH3)2CO] or water (H2O)? Explain.

To get a grip on why substances like water and acetone differ in their surface tension, we need to delve into the science of intermolecular forces. These forces are the glue that holds molecules together within a substance. Elevated surface tension is a result of stronger attractions between molecules.

Intermolecular forces come in various strengths and types, primarily influenced by the molecular structure and the presence of electrical charges or temporary dipoles in molecules. They are much weaker than intramolecular forces, which are the bonds within a molecule.

• Ion-dipole interactions occur between ionic compounds and polar solvents.
• Dipole-dipole interactions are present in polar molecules.
• Hydrogen bonds are a special subset of dipole-dipole interactions occurring only with hydrogen atoms involved.
• London dispersion forces appear in all molecules, especially significant in nonpolar compounds.

Understanding these forces sets the stage for exploring how they give rise to surface tension in liquids like water and acetone.

Hydrogen bonding is the superstar when it comes to intermolecular forces. Occurring in molecules where hydrogen is directly bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine, these bonds are particularly strong due to the large difference in electronegativity.

In water (H2O), each hydrogen atom is bonded to the oxygen, forming a V-like structure. This enables the formation of a strong network of hydrogen bonds, as the hydrogen atoms of one water molecule are attracted to the electronegative oxygen atoms of neighboring water molecules.

Unique Properties

Such strong bonding is responsible for water's high boiling point, high heat capacity, and yes, its high surface tension. It's these hydrogen bonds that make water droplets bead up so satisfyingly on a smooth surface.

Let's touch base with dipole-dipole interactions. These interactions are significant when it comes to molecules with polar covalent bonds—where electrons are unequally shared, leading to a partial negative charge on one end and a partial positive charge on the other.

Acetone ((CH3)2CO) is a classic example of a molecule that exhibits dipole-dipole interactions. The central carbon atom is double-bonded to an oxygen atom, creating a significant dipole moment.

Closer Look at Acetone

Although these forces are strong enough to affect the physical properties of acetone, including its own surface tension, they are overshadowed by the might of hydrogen bonding found in water. As a result, acetone, though slightly adhesive, can't quite compete with water's surface-tightening effect.

Lastly, we zoom into the London dispersion forces, named after the scientist Fritz London who first explained them. They are the most common but weakest of the intermolecular forces and occur in all atom or molecule interactions, regardless of their polarity.

These forces originate from the momentary fluctuations in electron density within an atom or a molecule, which causes a temporary dipole. The temporary dipole induces a dipole in a neighboring atom or molecule, resulting in a weak attraction.

Everywhere and Ephemeral

While fleeting, London dispersion forces are ubiquitous. In acetone, these forces are combined with dipole-dipole interactions providing some extent of surface tension. However, compared to the robust hydrogen bonds in water, the London dispersion forces' contribution to surface tension is less significant, hence, lower surface tension in acetone.