Question

(a) Which type of intermolecular attractive force operates between all molecules? (b) Which type of intermolecular attractive force operates only between polar molecules? (c) Which type of intermolecular attractive force operates only between the hydrogen atom of a polar bond and a nearby small electronegative atom?

Short answer

(a) Dispersion forces operate between all molecules. (b) Dipole-dipole forces operate only between polar molecules. (c) Hydrogen bonding operates only between the hydrogen atom of a polar bond (e.g., H-F, H-O, or H-N) and a nearby small electronegative atom (F, O, or N).

Step-by-step solution

(a) Intermolecular attractive force operates between all molecules

The type of intermolecular attractive force that operates between all molecules is called dispersion forces, also known as London dispersion forces or Van der Waals forces. These forces result from the temporary movement of electrons within a molecule, which creates a temporary dipole in the molecule. The temporary dipole can then induce nearby dipoles in neighboring molecules, leading to an attraction between the molecules.

(b) Intermolecular attractive force operates only between polar molecules

The type of intermolecular attractive force that operates only between polar molecules is called dipole-dipole forces. Polar molecules have a positive and a negative end, or dipole, due to a difference in electronegativity between the atoms making up the molecule. In a polar molecule, the more electronegative atom pulls electron density towards itself, creating a partial negative charge while the other atom develops a partial positive charge. Dipole-dipole forces occur when the partial positive charge of one polar molecule attracts the partial negative charge of an adjacent polar molecule.

(c) Intermolecular attractive force operates only between the hydrogen atom of a polar bond and a nearby small electronegative atom

The type of intermolecular attractive force that operates only between the hydrogen atom of a polar bond (e.g., H-F, H-O, or H-N) and a nearby small electronegative atom (F, O, or N) is called hydrogen bonding. Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs specifically between a hydrogen atom bonded to an electronegative atom and an electronegative atom in a neighboring molecule. This strong interaction results from the small size of hydrogen and the highly electronegative atoms (F, O, or N), which allow for a close approach and strong attraction between the molecules.

Key concepts

Dispersion Forces

Dispersion forces, also known as London forces or Van der Waals forces, are the weakest form of intermolecular attractive forces. They operate universally between all molecules, regardless of their polarity.
These forces arise due to the movement of electrons within a molecule which creates temporary dipoles. These temporary dipoles occur because electrons are always in motion, and at any given moment, they may be unequally distributed in an atom or molecule. This temporary uneven distribution leads to a temporary dipole.

• Induced Dipoles: When the temporary dipole of one molecule is close to another molecule, it can cause a shift in the electron cloud of the neighboring molecules, inducing a similar dipole.
• Attractive Force: The temporary and induced dipoles attract each other, resulting in the dispersion force.
• Significance: Even though individual dispersion forces are weak, collectively, they can be significant, especially in larger molecules.

Dispersion forces are particularly important in nonpolar substances and significantly contribute to their physical properties such as boiling and melting points.

Dipole-Dipole Forces

Dipole-dipole forces occur exclusively between polar molecules. Polar molecules have a distinct dipole moment due to differences in electronegativity between the atoms, leading to a molecule with a positive and negative pole.
This variation in electronegativity causes one side of the molecule to possess a partial positive charge while the other side is partially negative.

• Polar Interaction: Because of these charges, the positive end of one polar molecule attracts the negative end of another, establishing dipole-dipole attractions.
• Influence on Properties: Dipole-dipole forces affect the physical properties of substances, such as boiling and melting points, causing them to be relatively higher than those of nonpolar molecules of similar size.
• Directionality: These forces are directional, meaning that they depend on the orientation of the molecules relative to each other.

Understanding dipole-dipole interactions helps explain why some substances are soluble in others. For example, polar solvents typically dissolve polar solutes due to these interactions.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that significantly influences the properties of many compounds. It occurs specifically in molecules where hydrogen is bonded to highly electronegative atoms like fluorine, oxygen, or nitrogen.
The small size of hydrogen, along with the high electronegativity of F, O, or N, makes hydrogen bonds specially strong among intermolecular forces.

• Strong Attraction: The hydrogen atom becomes highly positive when bonded to a highly electronegative atom. This positive hydrogen is strongly attracted to lone pairs of electrons on nearby electronegative atoms in other molecules.
• Impact on Properties: Hydrogen bonding impacts properties like water's high boiling point, ice's low density compared to liquid water, and the shape and function of biological molecules like DNA.
• Biological Significance: In proteins and DNA, hydrogen bonds are crucial in maintaining structures such as alpha helices and beta sheets in proteins or the double helix structure of DNA.

While not as strong as covalent bonds, hydrogen bonds are much stronger than regular dipole-dipole interactions, making them highly influential in chemical behavior.