Question

Which forces are intramolecular and which intermolecular? (a) Those allowing fog to form on a cool, humid evening (b) Those allowing water to form when \(\mathrm{H}_{2}\) is sparked (c) Those allowing liquid benzene to crystallize when cooled (d) Those responsible for the low boiling point of hexane

Short answer

Intermolecular forces: (a), (c), (d). Intramolecular forces: (b).

Step-by-step solution

- Define Intramolecular and Intermolecular Forces

Intramolecular forces are the forces that hold atoms together within a molecule. Intermolecular forces are forces that occur between molecules. These forces are generally weaker than intramolecular forces.

- Analyze Part (a)

Fog formation is related to the condensation of water vapor into tiny liquid droplets. This process is driven by intermolecular forces, primarily hydrogen bonding between water molecules.

- Analyze Part (b)

When \(\text{H}_2\) is sparked, it reacts with \(\text{O}_2\) to form water. This involves the breaking and forming of covalent bonds within the molecules, which are intramolecular forces.

- Analyze Part (c)

When liquid benzene crystallizes upon cooling, the molecules are arranging into a solid structure due to intermolecular forces. These forces between benzene molecules are primarily van der Waals forces (dispersion forces).

- Analyze Part (d)

The low boiling point of hexane is due to relatively weak intermolecular forces, specifically dispersion forces, between the hexane molecules.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole intermolecular force. It occurs when hydrogen is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. When this happens, the hydrogen atom becomes partially positive, allowing it to attract lone pairs of electrons from other electronegative atoms in nearby molecules. This force plays a key role in many biological processes and contributes to the unique properties of water.
For instance, hydrogen bonding is what makes ice less dense than liquid water and gives water its high surface tension and boiling point.

Covalent Bonds

Covalent bonds are intramolecular forces formed by the sharing of electrons between atoms within a molecule. They are strong compared to intermolecular forces and are essential for creating stable molecules.
In a covalent bond, each atom contributes one or more electrons to create an electron pair that is shared between them. This bond can be single, double, or triple depending on how many pairs of electrons are involved.
Covalent bonds are responsible for the formation of most of the molecules that make up living organisms, including DNA, proteins, and carbohydrates.

Van der Waals Forces

Van der Waals forces are weak intermolecular forces that arise from interactions between uncharged molecules. Unlike ionic or covalent bonds, these forces do not involve the sharing or transfer of electrons.
They include attractions between permanent dipoles (dipole-dipole interactions), attractions between an ion and a permanent dipole (ion-dipole interactions), and the weak interactions between induced and instantaneous dipoles (dispersion forces).
Van der Waals forces are significant in situations where molecules are close together, such as in liquids and solids, and contribute to phenomena like boiling points and melting points.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are a type of van der Waals force. They are the weakest intermolecular forces and arise from temporary fluctuations in electron density in all atoms and molecules.
These temporary dipoles induce similar dipoles in neighboring molecules, leading to a transient attraction between them.
Dispersion forces are an essential factor in nonpolar molecules like hexane, affecting their physical properties, such as boiling and melting points. Despite their weak nature, they are significant in large nonpolar molecules where the cumulative effect can be considerable.