Problem 31 - Chemistry

Question

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

Step-by-Step Solution

Answer

The liquid with greater surface tension is ethanol (\(C_2H_5OH\)).

Step 1: Identify Intermolecular Forces in Each Liquid

For ethanol (\(C_2H_5OH\)), a polar molecule, there is a strong form of dipole-dipole interaction known as hydrogen bonding due to the presence of a hydroxyl group (\(OH\)). On the other hand, for dimethyl ether (\(CH_3OCH_3\)), although it is a polar molecule, there are no hydrogen atoms bonded to fluorine, oxygen, or nitrogen, hence it lacks hydrogen bonding and only exhibits the weaker London dispersion forces and dipole-dipole interactions.

Step 2: Understand the Impact of Intermolecular Forces on Surface Tension

Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid. The stronger the intermolecular forces, the greater the surface tension is. Hydrogen bonding is a stronger intermolecular force compared to dipole-dipole interactions and London dispersion forces.

Step 3: Predict the Liquid with Greater Surface Tension

Since the hydrogen bonds in ethanol represent a stronger form of intermolecular attraction compared to the forces in dimethyl ether, it is expected that ethanol would have a higher surface tension.

Key Concepts

Hydrogen Bonding

The phenomenon of hydrogen bonding plays a crucial role in determining the physicochemical properties of compounds, including surface tension. This type of interaction occurs when the hydrogen atom from one molecule is attracted to an electronegative atom, mainly oxygen, fluorine, or nitrogen, in another molecule. Imagine this attraction like a tiny magnet pulling the molecules together.

In the case of ethanol \(C_2H_5OH\), the hydroxyl (OH) group exhibits a significant polar character due to the difference in electronegativity between the oxygen and hydrogen atoms. This creates a strong hydrogen bond whenever the hydrogen is near another molecule's electronegative atom. Because of these strong intermolecular attractions, ethanol molecules 'stick' together more strongly than those in liquids without hydrogen bonding, leading to a higher surface tension.

Hydrogen bonding is not only the reason behind ethanol's high surface tension but also contributes to other properties such as higher boiling points and solubility in water, making it a vital concept in understanding the behavior of molecules.

Dipole-Dipole Interactions

Dipole-dipole interactions are another type of intermolecular force that significantly influences the surface tension of liquids. These interactions occur between polar molecules that have permanent dipoles, meaning one end of the molecule has a slight positive charge while the other end has a slight negative charge.

Such is the case for both ethanol and dimethyl ether \(CH_3OCH_3\). The presence of a dipole means that the positive end of one molecule can attract the negative end of another, similar to how magnets attract one another at opposite poles. Although these interactions are present in many polar substances, they are generally weaker compared to hydrogen bonding because there is no actual bond but just an attraction between oppositely charged regions of molecules.

For dimethyl ether, which lacks hydrogen bonding, dipole-dipole interactions are one of the primary contributors to its surface tension, even though it ultimately results in a lower surface tension than that of ethanol, which benefits from the additional strength of hydrogen bonds.

London Dispersion Forces

Lastly, let's discuss London dispersion forces, which are the weakest as compared to hydrogen bonding and dipole-dipole interactions but are present in all molecular compounds, polar or nonpolar. These forces arise due to the temporary fluctuations in the electron distribution within molecules that create an instantaneous dipole, leading to a momentary attraction between neighboring molecules.

Back to our comparison, while dimethyl ether does exhibit some dipole-dipole interaction, it also relies on London dispersion forces. This results because even when the electrons are distributed uniformly, at any given moment, the distribution can become lopsided, causing a temporary dipole that induces another in a neighboring molecule.

Although these forces are fleeting and weak on a per-interaction basis, in significant numbers, like in a liquid, they can influence the surface tension. However, given their relatively weaker nature, liquids whose intermolecular forces are dominated by London dispersion forces, such as dimethyl ether, generally have a lower surface tension compared to substances like ethanol with stronger intermolecular forces.