Question

For each of the following substances describe the importance of dispersion (London) forces, dipoledipole interactions, and hydrogen bonding: (a) \(HCl;\) (b) \(\mathrm{Br}_{2} ;\) (c) ICl; (d) \(\mathrm{HF} ;\)\ (e) \(\mathrm{CH}_{4}\)

Short answer

HCl exhibits dispersion forces and dipole-dipole interactions. Br2 and CH4 only exhibit dispersion forces. ICl exhibits dispersion forces and dipole-dipole interactions. HF exhibits all three forces: dispersion, dipole-dipole, and hydrogen bonding.

Step-by-step solution

Identify the Type of Substance in HCl

HCl is a polar covalent compound and hence, it exhibits dipole-dipole forces and dispersion forces. Hydrogen bonding is not observed in HCl, as it does not contain a hydrogen atom bonded to F, O, or N.

Identify the Type of Substance in Br2

Br2 is a nonpolar covalent compound. In nonpolar compounds, only dispersion forces exist. Thus, Br2 exhibits dispersion (London) forces.

Identify the Type of Substance in ICl

ICl is a polar covalent compound, thus it exhibits dipole-dipole forces and dispersion forces. Just like HCl, ICl does not present hydrogen bonding since it lacks a hydrogen atom that is directly bonded to F, O or N.

Identify the Type of Substance in HF

HF is a polar covalent compound that contains a hydrogen atom bonded to F. Thus, apart from dispersion forces and dipole-dipole forces, HF also exhibits hydrogen bonding.

Identify the Type of Substance in CH4

Finally, CH4 is a nonpolar covalent compound, just as Br2. Therefore, it only exhibits dispersion (London) forces.

Key concepts

Dispersion Forces

Dispersion forces, also known as London forces, are the weakest intermolecular forces present in all molecules, regardless of their polarity. These forces arise due to temporary fluctuations in electron distribution within atoms and molecules, leading to a fleeting dipole effect. This temporary dipole can induce a similar dipole in neighboring atoms, resulting in a weak attraction between them.

Key points about dispersion forces include:

• Present in all molecules, polar and nonpolar.
• Strength increases with the size and number of electrons in a molecule.
• More significant in larger, heavier atoms like bromine (\( ext{Br}_2\)) or methane (\( ext{CH}_4\)).

These forces are crucial in nonpolar substances like \( ext{Br}_2\) where they are the primary type of intermolecular force.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules that have a permanent dipole moment. A dipole moment arises due to the difference in electronegativity between atoms forming a molecule, resulting in partial positive and negative charges at opposite ends. These interactions are stronger than dispersion forces but weaker than hydrogen bonds.

Some central aspects of dipole-dipole interactions include:

• Only occur in polar molecules, like HCl and ICl.
• The strength of the interaction depends on the magnitude of the dipole moment.
• Higher boiling points in polar substances indicate stronger dipole-dipole interactions.

In substances like \( ext{ICl}\) and HCl, these forces lead to a greater attraction than what dispersion forces alone would offer.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when a hydrogen atom is directly bonded to a highly electronegative element such as fluorine, oxygen, or nitrogen. This bond plays a crucial role in determining the structure and properties of compounds.

Key features of hydrogen bonding are:

• Strongest type of intermolecular force after ionic and covalent bonds.
• Significantly affects boiling and melting points, solubility, and viscosity.
• Commonly seen in water, alcohols, and HF molecules.

Hydrogen fluoride (\( ext{HF}\)) is a prime example where hydrogen bonding leads to high boiling points and unique properties when compared to other hydrogen halides.