Question

Boilingpoint of hydrogen fluoride is highest amongst HF, HCl, HBr and HI. Which type of intermolecular forces are present in hydrogen fluoride? (a) \(\mathrm{H}-\mathrm{F}\) has highest van der Waals' forces and dipole moment. (b) \(\mathrm{H}-\mathrm{F}\) has highest London forces. (c) H-F has highest dipole moment hence has dipole-dipole, London forces and hydrogen bonding. (d) H-F has strong intermolecular interactions like dipole-induced dipole.

Short answer

The strongest intermolecular forces present in hydrogen fluoride are due to the high dipole moment leading to hydrogen bonding, along with London forces and dipole-dipole interactions (option c).

Step-by-step solution

Identify the Strongest Intermolecular Forces in HF

First, identify the types of intermolecular forces present in the molecules. Hydrogen fluoride (HF) can exhibit London dispersion forces, dipole-dipole interactions, and hydrogen bonding due to the highly electronegative fluorine atom bonded to hydrogen.

Compare Intermolecular Forces

Among HF, HCl, HBr, and HI, HF has the strongest intermolecular forces because hydrogen bonding is present along with dipole-dipole interactions. Hydrogen bonding is a particularly strong form of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms such as fluorine, oxygen, or nitrogen.

Analyze the Options

Option (a) highlights van der Waals forces and dipole moment but doesn't explicitly mention hydrogen bonding. Option (b) only mentions London forces, which are present in all molecules but are not the strongest in HF. Option (c) correctly identifies that HF has a high dipole moment and includes all relevant intermolecular forces: hydrogen bonding, London forces, and dipole-dipole interactions. Option (d) mentions dipole-induced dipole interactions, which are not the primary reason for HF's high boiling point.

Key concepts

Hydrogen Bonding

Hydrogen bonding is an especially strong type of intermolecular force that has a crucial impact on the physical properties of substances, including their boiling points. It involves a hydrogen atom that is covalently bonded to a highly electronegative atom, such as fluorine, oxygen, or nitrogen, interacting with another electronegative atom in a neighboring molecule.

For instance, in the case of hydrogen fluoride (HF), the hydrogen atom forms a bond with a fluorine atom. Fluorine's high electronegativity creates a large dipole moment, where the electron density is pulled closer to fluorine, leaving the hydrogen with a partial positive charge. This positive charge can then attract the negative regions of neighboring fluoride atoms, creating a strong intermolecular bond known as hydrogen bonding.

This strong attraction results in a higher boiling point for HF compared to other hydrogen halides because more energy is needed to break these intermolecular bonds during the transition from a liquid to a gas. In essence, hydrogen bonding can be considered as a supercharged dipole-dipole interaction that greatly influences substance properties.

van der Waals Forces

van der Waals forces are a collection of intermolecular forces that include attractions between molecules. These forces include London dispersion forces, Keesom forces (dipole-dipole attractions), and Debye forces (dipole-induced dipole attractions). London forces, specifically, are a type of van der Waals force resulting from the correlated movements of the electrons in interacting molecules.

Even though these forces are weaker than chemical bonds, they play a significant role in the physical properties of molecules, such as their boiling points. London dispersion forces arise due to temporary dipoles formed when the electron distribution within a molecule becomes asymmetrical. They are present in all molecules, whether polar or nonpolar, but are relatively weaker than other types of van der Waals forces.

In the case of hydrogen fluoride, while London dispersion forces are present, they are overshadowed by the much stronger hydrogen bonds. However, it's important to understand that London dispersion forces are ubiquitous and contribute to the overall intermolecular forces holding a substance together.

Dipole-dipole Interactions

Dipole-dipole interactions occur between polar molecules, where the positive end of one polar molecule attracts the negative end of another. This phenomenon arises due to the uneven distribution of electrons within the molecule, which creates a dipole moment—a measure of the separation of positive and negative charges within the molecule.

In hydrogen fluoride, the difference in electronegativity between the hydrogen and fluorine atoms leads to a significant dipole moment. The positive hydrogen end of an HF molecule is attracted to the negative fluorine end of another HF molecule, leading to dipole-dipole interactions. These interactions are stronger than London dispersion forces but generally weaker than hydrogen bonds.

However, in the case of HF, the dipole-dipole interaction is part of the hydrogen bonding phenomenon and contributes to the overall strong intermolecular forces, which are responsible for HF's high boiling point. Understanding dipole-dipole interactions is essential for grasping why certain substances, like HF, exhibit such high boiling points compared to others in their group.