Question

Predict which liquid in each pair has the higher surface tension: (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) or \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\)

Short answer

(a) \( \text{CH}_3 \text{COOH} \); (b) \( \text{C}_2 \text{H}_5 \text{OH} \).

Step-by-step solution

Compare Intermolecular Forces for Pair (a)

Acetic acid \( \text{(CH}_3 \text{COOH)} \) is capable of hydrogen bonding due to the presence of both an oxygen atom bonded to a hydrogen (\(-OH\)) and a carbonyl group (\(=O\)). In contrast, ethyl chloride (\( \text{C}_2 \text{H}_5 \text{Cl} \)) primarily exhibits dipole-dipole interactions due to the presence of the polar C-Cl bond and some London dispersion forces. Hydrogen bonding is stronger than dipole-dipole interactions, thus \( \text{CH}_3 \text{COOH} \) has a higher surface tension than \( \text{C}_2 \text{H}_5 \text{Cl} \).

Compare Intermolecular Forces for Pair (b)

Ethanol \( \text{(C}_2 \text{H}_5 \text{OH)} \) can form hydrogen bonds because of the \( \text{-OH} \) group. Dimethyl ether \( \text{(CH}_3 \text{OCH}_3) \), however, lacks an \( \text{-OH} \) group and can only engage in dipole-dipole interactions and London dispersion forces. Hydrogen bonding is a much stronger intermolecular force than dipole-dipole interactions and dispersion forces, giving \( \text{C}_2 \text{H}_5 \text{OH} \) a higher surface tension than \( \text{CH}_3 \text{OCH}_3 \).

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles. These forces are crucial because they determine many physical properties of substances, such as boiling points, melting points, and surface tension. Understanding these forces can help us predict the behavior of different substances in various states. There are several types of intermolecular forces, each varying in strength and effect:

• London dispersion forces: Also known as van der Waals forces, these are the weakest intermolecular forces and arise due to temporary dipoles that occur when electron clouds within molecules move. All molecules experience these forces, but they are especially significant in nonpolar molecules.
• Dipole-dipole interactions: These occur in polar molecules where the positive end of one molecule is attracted to the negative end of another molecule. They are stronger than dispersion forces.
• Hydrogen bonding: A particularly strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine, leading to a significant intermolecular attraction.

Intermolecular forces play a vital role in determining a liquid's surface tension. Stronger forces result in higher surface tension because the molecules hold together more tightly.

Hydrogen Bonding

Hydrogen bonding is an essential concept in chemistry, known for its significant effect on the physical and chemical properties of substances. It happens when hydrogen is directly attached to an electronegative atom such as oxygen, nitrogen, or fluorine. This bonding is much stronger than most other intermolecular forces.

• Structure and Influence: In a hydrogen bond, the hydrogen atom carries a partial positive charge, while the electronegative atom to which it is attached, and the one it is attracted to, carry partial negative charges. This results in a strong attraction that holds molecules together closely.
• Example in Water: Water is a classic example that showcases hydrogen bonding beautifully. The hydrogen atoms in one water molecule bond with oxygen atoms of nearby water molecules, creating a robust network. This accounts for water's high boiling point and surface tension.
• Impact on Surface Tension: Hydrogen bonding contributes significantly to surface tension by keeping the surface molecules closely knit, making it harder for objects to penetrate the liquid surface.

Substances capable of hydrogen bonding often exhibit higher surface tension compared to those that can only engage in weaker intermolecular forces.

Dipole-Dipole Interactions

Dipole-dipole interactions are a moderate type of intermolecular force that occurs between polar molecules. These interactions result from the attraction between the positive end of one dipole and the negative end of another. They are stronger than London dispersion forces but weaker than hydrogen bonds.

• Polar Molecules: For dipole-dipole interactions to occur, molecules must have polar bonds where there is a difference in electronegativity between the bonded atoms, leading to partial positive and negative charges.
• Influence on Properties: These interactions can significantly influence the boiling and melting points of substances. They help stabilize the structure of the liquid, but not as strongly as hydrogen bonds.
• Comparison with Hydrogen Bonding: While dipole-dipole interactions are meaningful, they are less effective in increasing surface tension compared to hydrogen bonding. This is because hydrogen bonds create much stronger attractions between molecules.

In substances like dimethyl ether, dipole-dipole interactions play a role, but their effect on properties such as surface tension is limited when compared to hydrogen-bonding capable substances.