Question

Predict which of these liquids has the greater surface tension: ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) or dimethyl ether \(\left(\mathrm{CH}_{3} \mathrm{OCH}_{3}\right)\)

Short answer

Ethanol will have a greater surface tension because it has stronger intermolecular forces due to the presence of hydrogen bonding.

Step-by-step solution

Identify the Intermolecular Forces in Ethanol

Ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) has two types of intermolecular forces: dispersion forces and hydrogen bonding. The dispersion forces occur between all molecules, these are the weakest intermolecular forces. The hydrogen bonding, on the other hand, occurs because of the -OH group in ethanol. Hydrogen bonding is a special type of dipole-dipole attraction between molecules, where a hydrogen atom is bonded to a highly electronegative atom (Oxygen in this case). This makes the hydrogen atom to acquire a partial positive charge and the oxygen atom a partial negative charge, thus creating an attraction between these molecules.

Identify the Intermolecular Forces in Dimethyl Ether

Dimethyl ether \(\left(\mathrm{CH}_{3} \mathrm{OCH}_{3}\right)\) also has two types of intermolecular forces: dispersion forces and dipole-dipole interaction. Here also, dispersion forces exist between all molecules. Despite having oxygen, dimethyl ether does not form hydrogen bonds. The reason for this is that the hydrogen atoms in dimethyl ether are not bonded to oxygen directly, thus a difference in electronegativities does not exist. Therefore, no hydrogen bonding takes place.

Compare the Strength of Intermolecular Forces

Hydrogen bonds are significantly stronger than both dispersion forces and dipole-dipole interactions. As ethanol has hydrogen bonds, and dimethyl ether does not, ethanol has stronger intermolecular forces.

Relate the Strength of Intermolecular Forces to Surface Tension

The greater the intermolecular forces, the greater the surface tension of the liquid. As ethanol has stronger intermolecular forces due to hydrogen bonding, it has a greater surface tension compared to dimethyl ether.

Key concepts

Surface Tension

Surface tension is a fascinating property of liquids that explains why some liquids form droplets while others spread into thin layers. It is the result of the cohesive forces between liquid molecules. When these forces are strong, the liquid exhibits high surface tension, making it resistant to external forces. Imagine surface tension as a thin, stretched membrane at the liquid's surface.
Surfact tension is often influenced by the type and strength of intermolecular forces present.

• High Intermolecular Forces: Substances such as water, which involve hydrogen bonding, have strong intermolecular forces leading to high surface tension.
• Low Intermolecular Forces: Liquids like oils, which rely on weaker dispersion forces, have lower surface tension.

Understanding surface tension helps us predict how liquids behave in different scenarios, such as when poured over a surface or when forming bubbles.

Hydrogen Bonding

Hydrogen bonding is a unique and powerful type of intermolecular force that occurs when a hydrogen atom is directly bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine. This bond results in a special kind of dipole-dipole attraction.

For example, in ethanol egin{align*} ( ext{C}_2 ext{H}_5 ext{OH}) ext, he oxygen atom is bonded to a hydrogen atom, enabling hydrogen bonds to form. These bonds significantly strengthen the intermolecular forces within the liquid by hereby increasing its surface tension.
Some key characteristics of hydrogen bonding include:

• It is stronger than most other van der Waals forces but weaker than covalent or ionic bonds.
• Hydrogen bonding greatly affects the properties of substances. For instance, it is responsible for the unique properties of water, such as its high boiling point.
• Presence of hydrogen bonding often leads to increased solubility in water.

Understanding hydrogen bonding is crucial for predicting the behavior of many liquids and solids.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest type of intermolecular forces. They exist between all molecules, whether polar or nonpolar, due to temporary fluctuations in electron density. These fluctuations create temporary dipoles that attract nearby molecules.

Despite being weak, dispersion forces still play a critical role, especially in nonpolar substances. For example, dimethyl ether egin{align*} ( ext{CH}_3 ext{OCH}_3) ext{ has }dispersion forces since here is no direct hydrogen bonding.
Some factors influencing the strength of dispersion forces include:

• Molecular Size: Larger molecules with more electrons exhibit stronger dispersion forces due to greater potential fluctuations in electron density.
• Shape of Molecules: Long, less compact molecules can experience more significant dispersion forces compared to spherical molecules.

While dispersion forces may be weak compared to hydrogen bonds, they are crucial in understanding the behaviors of various substances, particularly gases and nonpolar liquids.