Question

Select the substance in each pair that should have the higher boiling point. In each case identify the principal intermolecular forces involved and account briefly for your choice: (a) \(\mathrm{K}_{2} \mathrm{~S}\) or \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N},\) (b) \(\mathrm{Br}_{2}\) -or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} .\)

Short answer

(a) K2S; strong ionic bonds. (b) C4H10; stronger dispersion forces.

Step-by-step solution

Determine the Boiling Point Factors

The boiling point of a substance is determined by the strength of the intermolecular forces. The stronger the intermolecular forces, the higher the boiling point. Common intermolecular forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

Analyze Pair (a) - K2S and (CH3)3N

For potassium sulfide (\( \text{K}_2\text{S} \)), the principal intermolecular force is ionic bonding, which is very strong due to the attraction between charged ions. On the other hand, for trimethylamine (\((\text{CH}_3)_3\text{N}\)), the intermolecular forces are weaker dipole-dipole interactions due to the polar nitrogen atom bonded to carbon atoms. Hence, \( \text{K}_2\text{S} \) with its ionic bonds should have a higher boiling point.

Analyze Pair (b) - Br2 and C4H10

For \( \text{Br}_2 \), the main intermolecular forces are London dispersion forces as it is a non-polar molecule. For \( \text{C}_4\text{H}_{10} \) (butane), the forces involved are also London dispersion forces but with more mass and surface area compared to \( \text{Br}_2 \), resulting in stronger dispersion forces. Hence, \( \text{C}_4\text{H}_{10} \) should have a higher boiling point due to its larger size and greater surface area.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractive forces that occur between molecules. They play a crucial role in determining the physical properties of substances, such as the boiling and melting points. These forces can be categorized into different types based on the molecular characteristics they arise from.

Unlike covalent bonds within molecules, intermolecular forces are usually weaker. However, their strength can vary widely depending on the type of force at play. The stronger these forces are, the more energy (in the form of heat) is required to separate the molecules from each other, which results in a higher boiling point. Understanding these forces helps us to predict and compare the boiling points of different substances based on the nature of interactions between their molecules.

Ionic Bonding

Ionic bonding is a type of strong intermolecular force resulting from the electrostatic attraction between oppositely charged ions. This force is typical in ionic compounds, such as salts.

When atoms transfer electrons to achieve a full outer shell, they form ions: cations (+ charge) and anions (- charge). The attraction between these charged particles results in an ionic bond. This bond requires a significant amount of energy to break, leading to very high boiling points for ionic compounds.

• Example: In the compound potassium sulfide (\( ext{K}_2 ext{S}\)), potassium donates electrons to sulfur, resulting in strong ionic bonds.
• Key Feature: High melting and boiling points due to strong attraction between ions.

This is why potassium sulfide has a much higher boiling point compared to substances held together by weaker intermolecular forces like dipole-dipole interactions.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where one end of the molecule has a slight positive charge and the other end has a slight negative charge. These partial charges arise from differences in electronegativity between the atoms involved, causing a dipole.

These interactions are stronger than London dispersion forces but weaker than ionic bonds and are crucial in molecules with polar covalent bonds.

• Example: Trimethylamine (\(( ext{CH}_3)_3 ext{N}\)) has a polar C-N bond resulting in dipole-dipole interactions among molecules.
• Key Feature: Moderate strength; higher boiling points than non-polar molecules.

Even though dipole-dipole interactions increase the boiling point compared to nonpolar substances, they are not as strong as ionic bonds, which is why trimethylamine has a lower boiling point than potassium sulfide.

London Dispersion Forces

London dispersion forces exist between non-polar molecules and are the weakest type of intermolecular force. Despite their relative weakness, they are present in all molecular interactions and arise from temporary fluctuations in electron distribution.

These forces become more significant in larger atoms and heavier molecules because they have more electrons that can form temporary dipoles.

• Example: Both \( ext{Br}_2\) and \( ext{C}_4 ext{H}_{10}\) experience London dispersion forces.
• Key Feature: Dependence on molecular size and shape; more pronounced in larger and heavier molecules.

In the case of butane (\( ext{C}_4 ext{H}_{10}\)), its larger molecular size and surface area compared to \( ext{Br}_2\) result in stronger dispersion forces, thus giving butane a higher boiling point.