Question

Evaluate Which of the molecules listed below can form hydrogen bonds? For which of the molecules would dispersion forces be the only intermolecular force? Give reasons for your answers. a. \(\mathrm{H}_{2}\) b. \(\mathrm{H}_{2} \mathrm{S}\) c. \(\mathrm{HCl}\) d. \( \mathrm{HF}\)

Short answer

Only molecule d (\(HF\)) can form hydrogen bonds. Dispersion forces are the only intermolecular force for molecule a (\(H_{2}\)).

Step-by-step solution

Identify the molecular structures and electronegativity values of the elements

In this step, let's list down the molecular structure and electronegativity values (in the Pauling scale) of the elements in the molecules: a. \(H_{2}\): H-H, Electronegativity: H (2.1) b. \(H_{2}S\): H-S-H, Electronegativity: H (2.1), S (2.5) c. \(HCl\): H-Cl, Electronegativity: H (2.1), Cl (3.0) d. \(HF\): H-F, Electronegativity: H (2.1), F (3.98)

Determine the ability to form hydrogen bonds

To form hydrogen bonds, a molecule must have a highly electronegative element (N, O, or F) bonded to hydrogen. a. \(H_{2}\): No highly electronegative element is present. b. \(H_{2}S\): No highly electronegative element is present. c. \(HCl\): No highly electronegative element is present. d. \(HF\): The molecule has H bonded to F (highly electronegative). Hence, only molecule d (\(HF\)) can form hydrogen bonds.

Determine if dispersion forces are the only intermolecular force

Dispersion forces are present in all molecules, but we must find out if they are the only intermolecular force present. a. \(H_{2}\): No hydrogen bonding or dipole-dipole interaction. So, dispersion forces are the only intermolecular force. b. \(H_{2}S\): No hydrogen bonding. Weak dipole-dipole interaction due to the difference in electronegativity between H and S. Thus, dispersion forces are not the only intermolecular force. c. \(HCl\): No hydrogen bonding. Strong dipole-dipole interaction due to the difference in electronegativity between H and Cl. Thus, dispersion forces are not the only intermolecular force. d. \(HF\): Hydrogen bonding is present. So, dispersion forces are not the only intermolecular force. Thus, dispersion forces are the only intermolecular force for molecule a (\(H_{2}\)).

Key concepts

Hydrogen Bonds

Hydrogen bonds are a special type of intermolecular force that occurs when a hydrogen atom is bonded to a highly electronegative atom such as nitrogen (N), oxygen (O), or fluorine (F), and this hydrogen atom is attracted to another electronegative atom in a different molecule. This kind of attraction is relatively strong compared to other intermolecular forces, contributing to unique properties of substances like water, where hydrogen bonding is responsible for its high boiling point. In the given exercise, out of the listed molecules, only hydrogen fluoride ((HF)) qualifies to form hydrogen bonds because it contains a hydrogen atom directly bonded to a highly electronegative fluorine atom.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are a type of intermolecular force that exist between all molecules, whether they are polar or nonpolar. These forces are the result of momentary changes in electron distribution within molecules, which create fleeting dipoles that induce corresponding dipoles in nearby molecules. The strength of dispersion forces increases with the size of the molecule and the number of electrons. In the exercise, (H_2) exhibits only dispersion forces since it is nonpolar and lacks any other type of stronger intermolecular forces like hydrogen bonding or dipole-dipole interactions.

Electronegativity

Electronegativity is a chemical property that describes how strongly an atom can attract electrons towards itself when forming a chemical bond. On the Pauling scale, higher values indicate greater electronegativity. The differences in electronegativity between bonded atoms determine the polarity of the bond. Polar bonds, where electrons are unequally shared due to a significant difference in electronegativity, lead to dipole-dipole interactions—a type of intermolecular force. In the given exercise, (HCl) exhibits a significant difference in electronegativity between hydrogen and chlorine, resulting in a polar covalent bond and, consequently, dipole-dipole interactions in addition to dispersion forces.

Molecular Structure

The molecular structure or geometry of a molecule plays a crucial role in determining the type and strength of the intermolecular forces it can exhibit. The spatial arrangement of atoms within a molecule affects its polarity and ability to interact with other molecules. For example, linear or symmetrical molecules like (H_2) tend to be nonpolar and only exhibit dispersion forces. In contrast, bent or asymmetrical molecules like (H_2S) can exhibit both dispersion and dipole-dipole forces due to the uneven distribution of electron density. Understanding the molecular structure is essential for predicting the intermolecular forces and properties of a substance.