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 \(\left(\mathrm{CH}_{3} \mathrm{OCH}_{3}\right)\).

Short answer

Ethanol has greater surface tension due to hydrogen bonding.

Step-by-step solution

Understand Surface Tension

Surface tension is a physical property of liquids related to the intermolecular forces present. It describes how much force is needed to stretch or increase the surface area of a liquid.

Analyze Molecular Structure

Ethanol \((C_2H_5OH)\) contains a hydroxyl group \((OH)\). This allows ethanol molecules to form hydrogen bonds with each other. Dimethyl ether \((CH_3OCH_3)\), on the other hand, lacks this hydroxyl group and cannot form hydrogen bonds as effectively as ethanol can.

Determine Hydrogen Bonding Capability

Ethanol can form strong hydrogen bonds due to the presence of the \(OH\) group, enhancing cohesion between molecules. Dimethyl ether, having only weak Van der Waals forces and no \(OH\) group, cannot form such strong bonds. Hence, ethanol's intermolecular forces are stronger.

Compare Surface Tension

Given that ethanol can engage in hydrogen bonding, it will have a higher surface tension compared to dimethyl ether, which lacks strong hydrogen bonding capability and only exhibits weaker intermolecular forces.

Key concepts

Ethanol

Ethanol, with the chemical formula \(C_2H_5OH\), is a simple alcohol widely known for its presence in alcoholic beverages. But what about its scientific properties? Ethanol is characterized by a prominent hydroxyl \(OH\) group in its structure. This feature is significant because it allows ethanol molecules to form hydrogen bonds with each other.

• Hydrogen bonds are a type of strong intermolecular force.
• They are especially important in determining the physical properties of liquids.

When it comes to surface tension, the ability of ethanol to form hydrogen bonds translates to a higher cohesive force between its molecules. This means that more energy is required to overcome these forces and increase the surface area of the liquid, thus imparting a higher surface tension to ethanol compared to substances without hydrogen bonding.

Dimethyl Ether

Dimethyl ether \((CH_3OCH_3)\) is an organic compound with a different structure from ethanol. It lacks the hydroxyl \(OH\) group present in ethanol, which means it cannot form hydrogen bonds like ethanol does. While dimethyl ether can interact through van der Waals forces, these interactions are generally weaker in comparison to the hydrogen bonds.

• Van der Waals forces are weak temporary attractions between molecules.
• They occur due to momentary positive and negative charges formed when electrons move around within molecules.

As a result, the surface tension of substances like dimethyl ether is lower. This is because these weaker forces mean less energy is required to stretch or increase the surface area of the liquid.

Hydrogen Bonding

Hydrogen bonding is a specific type of intermolecular force that occurs when a hydrogen atom is attracted to a highly electronegative atom, such as oxygen, nitrogen, or fluorine. This type of bonding is stronger than van der Waals forces but weaker than covalent bonds.
In the case of ethanol, the hydrogen of the \(OH\) group forms a bond with the oxygen of another ethanol molecule.

• This results in a network of strong attractions.
• Hydrogen bonds greatly influence the intermolecular cohesion of substances.

Such bonds are responsible for ethanol's higher surface tension, as they increase the energy needed for the molecule's surface area to expand. This strong cohesive force provides ethanol with unique physical characteristics compared to substances that cannot form hydrogen bonds.

Intermolecular Forces

Intermolecular forces are forces that act between molecules and influence various physical properties of substances. They are categorized into different types, such as hydrogen bonds, dipole-dipole interactions, and van der Waals forces.

• Hydrogen bonds – particularly strong in compounds with \(OH\) groups.
• Van der Waals forces – include dispersion forces and are typically weaker.

In liquids, these forces play a key role in defining surface tension. Strong intermolecular forces, like hydrogen bonds, lead to a higher surface tension. Liquid molecules are pulled together more tightly, resisting external forces that want to increase the surface area. On the other hand, weaker forces like van der Waals result in relatively lower surface tension, as seen in dimethyl ether. Understanding these forces helps explain why ethanol has a greater surface tension compared to dimethyl ether.