Problem 28 - Chemistry The Science in Context

Question

Explain why the boiling point of \(\mathrm{Br}_{2}\left(59^{\circ} \mathrm{C}\right)\) is lower than that of iodine monochloride, ICl \(\left(97^{\circ} \mathrm{C}\right)\), even though they have nearly the same molar mass.

Step-by-Step Solution

Answer

Answer: The boiling point of Br2 is lower than that of ICl because ICl has stronger overall intermolecular forces due to the presence of both dipole-dipole and London dispersion forces, while Br2 only has weaker London dispersion forces. This difference in intermolecular forces results in a higher boiling point for ICl as compared to Br2.

Step 1: Identify the types of intermolecular forces present in Br2 and ICl molecules

In Br2, the molecule is nonpolar, so the main intermolecular force present is London dispersion forces. In ICl, there is a significant difference in electronegativity between iodine and chlorine atoms, creating a polar bond and a dipole moment in the molecule. Therefore, the intermolecular forces present in ICl are dipole-dipole forces and London dispersion forces.

Step 2: Compare the strength of intermolecular forces in Br2 and ICl

Dipole-dipole forces are generally stronger than London dispersion forces. Since ICl has both dipole-dipole and London dispersion forces, while Br2 only has London dispersion forces, the overall intermolecular forces in ICl are expected to be stronger than those in Br2.

Step 3: Relate the strength of intermolecular forces to boiling points

Boiling points are directly proportional to the strength of intermolecular forces in a substance. A higher boiling point indicates stronger intermolecular forces, while a lower boiling point suggests weaker intermolecular forces.

Step 4: Explain the difference in boiling points between Br2 and ICl

Since ICl has stronger overall intermolecular forces due to the presence of dipole-dipole forces, in addition to London dispersion forces, it has a higher boiling point than Br2, which only has weaker London dispersion forces. This difference in intermolecular forces explains why the boiling point of Br2 (\(59^{\circ}\mathrm{C}\)) is lower than that of ICl (\(97^{\circ}\mathrm{C}\)), even though they have nearly the same molar mass.

Key Concepts

London dispersion forces

London dispersion forces are a type of intermolecular force that are present in all molecules, whether they are polar or nonpolar. These forces arise due to temporary fluctuations in the electron distribution within atoms and molecules. This can create an instantaneous dipole moment, which induces a similar dipole in neighboring molecules, leading to an attraction between them.

• They are the only type of intermolecular force present in nonpolar molecules like \(\mathrm{Br}_2\).
• These forces are often weaker compared to other types of intermolecular forces.

The strength of London dispersion forces increases with the size and mass of the molecules. Larger atoms have more electrons, leading to increased polarizability and thus stronger dispersion forces. However, in the case of \(\mathrm{Br}_2\), these forces are still not strong enough to compete with stronger intermolecular forces such as dipole-dipole interactions present in other molecules like \(\mathrm{ICl}\).

dipole-dipole forces

Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. These forces occur when there is a significant difference in electronegativity between the atoms in a molecule, creating a permanent dipole moment.

• Example: In \(\mathrm{ICl}\), there is a notable difference in electronegativity between iodine and chlorine, resulting in a polar molecule.
• Because of the polarity, \(\mathrm{ICl}\) molecules exhibit dipole-dipole interactions in addition to London dispersion forces.

Dipole-dipole interactions are generally stronger than London dispersion forces. This means that molecules like \(\mathrm{ICl}\) with significant dipole-dipole forces will typically exhibit higher boiling points. Such interactions require more energy to overcome, which results in a higher boiling point compared to molecules that only have London forces, such as \(\mathrm{Br}_2\).

boiling point

The boiling point of a substance is the temperature at which its liquid phase transitions into a gas. This transition requires breaking the intermolecular forces holding the molecules in the liquid state. Hence, boiling points are a reflection of the strength of these intermolecular forces.

• Higher boiling points imply stronger intermolecular forces.
• Lower boiling points suggest weaker intermolecular interactions.

For example, \(\mathrm{Br}_2\) has a lower boiling point of \(59^{\circ}\mathrm{C}\) because the only forces at play are London dispersion forces. Conversely, \(\mathrm{ICl}\) has a higher boiling point of \(97^{\circ}\mathrm{C}\) because it experiences stronger dipole-dipole forces alongside London dispersion forces. Therefore, understanding the types and strengths of intermolecular forces in a substance can predict its boiling point and explain differences observed in similar-sized molecules with differing properties.