Question

Which of the following statements are false? (a) Dipoledipole interactions between molecules are greatest if the molecules possess only temporary dipole moments. (b) All compounds containing hydrogen atoms can participate in hydrogen-bond formation. (c) Dispersion forces exist between all atoms, molecules, and ions.

Short answer

Statements (a) and (b) are false.

Step-by-step solution

Analyze Statement (a)

Statement (a) claims that dipole-dipole interactions are greatest when molecules possess only temporary dipole moments. We know that dipole-dipole interactions occur between permanently polar molecules. Temporary dipoles are associated with dispersion forces. Therefore, statement (a) is false because molecules with permanent dipole moments engage in stronger dipole-dipole interactions.

Analyze Statement (b)

Statement (b) suggests that all compounds containing hydrogen atoms can participate in hydrogen bonding. Hydrogen bonding requires not just the presence of hydrogen atoms but also a very electronegative atom, like F, O, or N, which is absent in all compounds containing hydrogen. Thus, not all hydrogen-containing compounds can form hydrogen bonds, making this statement false.

Analyze Statement (c)

Statement (c) asserts that dispersion forces exist between all atoms, molecules, and ions. Dispersion forces, also known as London dispersion forces, arise due to momentary fluctuations in electron density. These forces exist between all particles, regardless of their nature. Therefore, statement (c) is true.

Key concepts

Dipole-Dipole Interactions

Dipole-dipole interactions are attractive forces occurring between polar molecules. Such molecules have permanent dipole moments due to an uneven distribution of electrons between atoms of differing electronegativities.
For example, in a water molecule, the high electronegativity of the oxygen atom pulls the shared electrons closer to itself, making oxygen slightly negative and hydrogen slightly positive. This occurs in polar molecules, resulting in molecules aligning so that opposite charges attract.

• These interactions are strongest in molecules with greater polarity.
• These interactions are directional, meaning that the way molecules align affects the strength of these bonds.

To sum up, dipole-dipole interactions are significant because they influence the physical properties of compounds, like boiling and melting points. The stronger the dipole-dipole interaction, the higher the energy required to break them apart.

Hydrogen Bonding

Hydrogen bonding is a specific, strong type of dipole-dipole interaction. Contrary to the name, it's not an actual "bond" but a strong attraction.
For hydrogen bonding to occur, hydrogen must be bonded to highly electronegative atoms like fluorine, oxygen, or nitrogen. These atoms pull the hydrogen’s electron towards themselves intensely, creating a significant charge difference. For instance, in water (H₂O), the hydrogen and oxygen are involved in hydrogen bonding, contributing to water's unique properties.

• Hydrogen bonds are crucial in biological systems, helping to stabilize structures like DNA and proteins.
• Despite being strong, hydrogen bonds are weaker than true covalent or ionic bonds, but stronger than most other dipole-dipole interactions.

This bonding has a profound impact on the properties of substances, affecting solubility, boiling, and melting points.

Dispersion Forces

Dispersion forces, or London forces, are weak interactions present in all molecules, whether they are polar or nonpolar. They arise due to momentary fluctuations in electron density. Because electrons are always moving, there can be a temporary imbalance, leading to a transient dipole.
These forces are typically the weakest intermolecular force but become significant in large molecules or atoms where there's a greater chance of electron imbalance.

• Dispersion forces explain why even noble gases can be liquefied under certain conditions.
• The strength of dispersion forces increases with the size of atoms or molecules.

Overall, although dispersion forces are the weakest, they play a critical role in the behavior of molecules, particularly in gases and nonpolar substances. Their universality makes them fundamental in understanding all kinds of molecular interactions.