Question

Explain why \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) has a boiling point of \(78^{\circ} \mathrm{C}\), but \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) has a boiling point of \(-42^{\circ} \mathrm{C}\).

Short answer

Ethanol (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}\)) has a higher boiling point than Propane (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_3\)) because it has stronger intermolecular forces, including hydrogen bonding, which require more energy to break.

Step-by-step solution

Understand the role of intermolecular forces in boiling points

The differences in boiling points between different substances can often be explained by looking at the types of intermolecular forces present in the substance. Boiling point is defined as the temperature at which the vapor pressure of a liquid equals the external pressure. Therefore, substances with stronger intermolecular forces require more energy (higher temperature) to break these forces, resulting in a higher boiling point.

Identify intermolecular forces in Ethanol and Propane

In Ethanol (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}\)), there are London dispersion forces, dipole-dipole forces, and hydrogen bonding (exists because of the \(\mathrm{OH}\) group). The latter is the strongest intermolecular force that makes ethanol's boiling point high. In contrast, Propane (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_3\)) only has London dispersion forces, which are significantly weaker.

Conclude

Due to stronger intermolecular forces (including hydrogen bonding) in Ethanol as compared to weaker forces (London dispersion forces) in Propane, Ethanol exhibits a higher boiling point.

Key concepts

Intermolecular Forces

Intermolecular forces are crucial in determining the physical properties of substances, such as boiling points. These forces occur between molecules and influence how much energy is needed to change a substance from liquid to gas. The stronger the intermolecular forces, the more energy required to break them apart, which in turn raises the boiling point.

There are several types of intermolecular forces:

• London Dispersion Forces: These are the weakest forces and arise from temporary dipoles in molecules. All molecules experience these forces regardless of their polarity.
• Dipole-Dipole Forces: Present in polar molecules, these forces occur due to the attraction of oppositely charged ends of molecules.
• Hydrogen Bonding: A special, stronger type of dipole-dipole interaction, hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine.

In the case of boiling points, substances with stronger intermolecular forces, such as hydrogen bonds, have higher boiling points compared to those with only dispersion forces.

Ethanol

Ethanol (chemical formula: \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}\)) is an organic compound known for its presence in alcoholic beverages. Its structure includes a hydroxyl (\(\mathrm{OH}\)) group attached to a carbon chain, which heavily influences its physical properties.

This hydroxyl group makes ethanol a polar molecule and also a candidate for hydrogen bonding, a strong type of intermolecular force. As a result, ethanol’s boiling point is notably higher than many other organic compounds, standing at \(78^{\circ} \mathrm{C}\).

The presence of hydrogen bonding in ethanol significantly contributes to its higher boiling point compared to non-polar molecules like propane. Because hydrogen bonds require more energy to break, ethanol stays in a liquid state until a higher temperature is reached. This property makes ethanol useful not only as a beverage but also as a solvent and fuel.

Hydrogen Bonding

Hydrogen bonding is an essential concept when discussing the boiling points of certain compounds. It is a particularly strong intermolecular force that occurs in molecules where a hydrogen atom is bound to a highly electronegative atom such as oxygen, nitrogen, or fluorine.

In ethanol, the hydrogen bonding arises due to the presence of the \(\mathrm{OH}\) group. This group enables ethanol molecules to form hydrogen bonds with one another, creating a network of strong intermolecular attractions. These bonds are significantly stronger than London dispersion forces, thus requiring more energy to break them.

Consequently, the hydrogen bonding in ethanol sets its boiling point much higher than compounds lacking such interactions, like propane. In simple terms, a substance with hydrogen bonding among its molecules will generally have a higher boiling point compared with one that does not, because of the additional energy needed to overcome these bonds.