Question

Rank the following in order of decreasing surface tension at a given temperature, and explain your ranking: (a) \(\mathrm{CH}_{3} \mathrm{OH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) (c) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{O}\)

Short answer

(a) > (c) > (b)

Step-by-step solution

Understand Surface Tension

Surface tension is the energy required to increase the surface area of a liquid. It arises due to intermolecular forces; stronger intermolecular forces result in higher surface tension.

Identify Intermolecular Forces for Each Molecule

Identify the type of intermolecular forces present in each molecule: (a) \(\text{CH}_3 \text{OH}\): Hydrogen bonding (very strong)(b) \(\text{CH}_3 \text{CH}_3\): Van der Waals forces (weak)(c) \( \text{H}_2 \text{C}=\text{O} \): Dipole-dipole interactions (moderate strength)

Determine the Strength of Intermolecular Forces

Rank the intermolecular forces by strength: 1. Hydrogen bonding (strongest)2. Dipole-dipole interactions3. Van der Waals forces (weakest)

Rank the Molecules by Surface Tension

Based on the strength of intermolecular forces: (a) \( \text{CH}_3 \text{OH} \) (strongest surface tension, due to strong hydrogen bonding)(c) \(\text{H}_2 \text{C}=\text{O}\) (moderate surface tension, due to dipole-dipole interactions)(b) \(\text{CH}_3 \text{CH}_3\) (weakest surface tension, due to weak van der Waals forces)

Key concepts

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion that act between neighboring particles. These particles can be atoms, ions, or molecules. The strength and type of intermolecular forces significantly impact various physical properties of substances, such as boiling points, melting points, and surface tension.
Strong intermolecular forces lead to higher surface tension because molecules at the surface are pulled inward, requiring more energy to increase surface area.
There are several types of intermolecular forces, including hydrogen bonding, dipole-dipole interactions, and Van der Waals forces.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. The large difference in electronegativity creates a partial positive charge on the hydrogen and a partial negative charge on the electronegative atom.
This bond is very strong compared to other intermolecular forces. Substances that exhibit hydrogen bonding generally have higher boiling points, melting points, and surface tensions compared to those that do not. For example, in the ranking exercise, \(\text{CH}_3\text{OH}\) demonstrates strong hydrogen bonding, resulting in high surface tension.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent dipoles. These dipoles arise from differences in electronegativity between bonded atoms, creating partial positive and negative charges. Unlike hydrogen bonds, dipole-dipole interactions do not require the presence of a hydrogen atom bonded to an electronegative atom.
While not as strong as hydrogen bonding, dipole-dipole interactions are still stronger than Van der Waals forces. In terms of surface tension, substances with dipole-dipole interactions, like \(\text{H}_2\text{C}=\text{O}\), will have moderate surface tension.

Van der Waals Forces

Van der Waals forces, also known as London dispersion forces, are the weakest type of intermolecular interaction. They arise from temporary dipoles that occur when electrons move around in atoms and molecules. These forces are present in all molecules, but they are the only type of intermolecular force in nonpolar molecules.
Given their weak nature, substances that rely solely on Van der Waals forces, like \(\text{CH}_3\text{CH}_3\), have the lowest surface tension. This is because the temporary dipoles provide minimal attraction between molecules, resulting in lower energy requirements to increase the surface area.