Question

"Ethylamine" and propane have about the same molecular mass. Explain why the boiling point of "ethylamine" \(\left(17^{\circ} \mathrm{C}\right)\) is much higher than propane \(\left(-44^{\circ} \mathrm{C}\right)\).

Short answer

Ethylamine has a higher boiling point due to hydrogen bonding, unlike propane, which only has weaker dispersion forces.

Step-by-step solution

Compare Intermolecular Forces

Ethylamine (C2H5NH2) has a higher boiling point than propane (C3H8) due to the type of intermolecular forces present. Ethylamine can participate in hydrogen bonding because of the presence of a hydrogen atom bonded to nitrogen (N-H bond). This makes hydrogen bonds possible, which are stronger intermolecular forces compared to the London dispersion forces present in propane.

Recognize Role of Hydrogen Bonds

Hydrogen bonds significantly increase the boiling point of a substance because they require more energy to break during the phase transition from liquid to gas. In ethylamine, the nitrogen atom creates a polar molecule capable of hydrogen bonding, while propane, being nonpolar, relies only on weaker dispersion forces.

Analyze Molecular Structure

Both ethylamine and propane have similar molecular masses, but the presence of nitrogen in ethylamine brings about polarity and the ability to form hydrogen bonds. The molecular structure of propane lacks areas for significant electron density distribution differences, hindering stronger intermolecular attractions.

Evaluate Boiling Points in Context

Boiling point differences can be attributed to the strengths of the intermolecular forces. In this case, ethylamine's stronger hydrogen bonds result in a boiling point of 17°C, much higher than that of propane at -44°C, which relies solely on weak dispersion forces typical of nonpolar molecules.

Key concepts

Hydrogen Bonding

Hydrogen bonding plays a crucial role in determining the boiling points of substances. It is a specific type of dipole-dipole interaction occurring when hydrogen is covalently bonded to electronegative atoms like nitrogen, oxygen, or fluorine.

• Increased Boiling Point: Hydrogen bonds are relatively strong compared to other intermolecular forces, such as dispersion forces. This means that compounds capable of hydrogen bonding, like ethylamine, will generally require more energy to transition from liquid to gas, resulting in higher boiling points.
• Role in Ethylamine: In ethylamine, the N-H bond facilitates hydrogen bonding, contributing significantly to its higher boiling point compared to substances that cannot hydrogen bond, such as propane.

Understanding hydrogen bonds can help explain why molecules with similar masses can have vastly different boiling points.

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. This physical property can reveal much about the nature of the substance's intermolecular forces.

• Intermolecular Forces Influence: Stronger intermolecular forces result in higher boiling points because more energy is needed to overcome these attractions.
• Comparison of Ethylamine and Propane: Ethylamine's boiling point of 17°C is significantly higher than propane's -44°C. This difference illustrates how hydrogen bonding in ethylamine leads to a higher energy requirement for its molecules to escape the liquid phase.

Boiling points are a key indicator of the nature of intermolecular forces present in a substance.

Molecular Structure

The shape and composition of a molecule are pivotal in determining its intermolecular forces and, subsequently, its boiling point.

• Ethylamine: Ethylamine contains a nitrogen atom that allows the molecule to form hydrogen bonds due to asymmetric electron density. This results in a polar molecule with significant intermolecular attractions.
• Propane: Propane lacks such structural features. Its lack of electronegative atoms and resultant nonpolarity mean it can only participate in dispersion forces, which are relatively weak. This structural difference largely contributes to the significantly lower boiling point of propane.

Molecular structure dictates the types of intermolecular forces that are possible, influencing physical properties like boiling point.

Polarity

Polarity refers to the distribution of electrical charge around a molecule, deeply influencing its physical and chemical properties.

• Understanding Polarity: A polar molecule has a significant difference in electronegativity between elements, resulting in partial charges on different sides of the molecule. Ethylamine's nitrogen atom causes such an effect, leading to polarity.
• Impact on Boiling Point: Polar molecules tend to have higher boiling points because their partial charges result in stronger dipole-dipole interactions (like hydrogen bonds), increasing the energy needed for phase transition.
• Comparison with Nonpolar Molecules: In contrast, nonpolar molecules like propane have an even distribution of charge, meaning only weaker dispersion forces are present. As a result, these molecules have lower boiling points.

Polarity can thus be seen as a key factor in understanding a molecule's behavior and its physical characteristics.