Intermolecular Forces: Liquids and Solids

Vocabulary flashcards covering intermolecular forces, Van der Waals interactions, London dispersion forces, dipole-dipole attractions, polarizability, and various forms and effects of hydrogen bonding based on General Chemistry lecture notes.

Van der Waals Forces

A collection of weak attractive forces between groups of atoms or molecules, comprising dipole–dipole interactions and instantaneous/induced dipoles (London dispersion forces).

London Dispersion Forces

Weak attractive forces occurring in both polar and nonpolar molecules, caused by the displacement of electrons that creates temporary instantaneous dipoles, which then induce dipoles in neighboring molecules.

Van der Waals Interaction Distance Threshold

The distance of $5\,\text{nm}$ or less between two atoms required for an interaction between their positive nuclei and negative electron clouds to induce polarity and a slight attraction.

Polarizability

The relative tendency of a charge distribution in an atom or molecule to distort from its normal shape, which generally increases with increasing molecular mass (atomic or molecular size).

Polarizability Volume

A designation for polarizability expressed in volume units ($10^{-25}\,\text{cm}^3$), serving as an approximate measure of atomic or molecular volume.

Molecular Shape and Polarizability

The principle that electrons in elongated molecules are more easily displaced than those in compact, symmetrical molecules, giving elongated molecules higher polarizability and higher boiling points (e.g., pentane at $36.1^\circ\text{C}$ versus neopentane at $9.5^\circ\text{C}$).

Dipole–Dipole Interactions

Attractive forces between polar molecules possessing permanent dipole moments, where molecules align with the positive end ($\delta+$) of one dipole directed toward the negative end ($\delta-$) of a neighboring dipole.

Permanent Dipole Moment ($\mu$)

A measurement of net polarity in a molecule caused by electronegativity differences; for example, homonuclear diatomics $N_2$ and $O_2$ have $\mu = 0$ (nonpolar), whereas $NO$ has $\mu = 0.153\,\text{D}$ (polar).

Hydrogen Bond

An electrostatic attraction between two polar groups occurring when a hydrogen atom covalently bonded to a highly electronegative atom ($N$, $O$, or $F$) experiences the electrostatic field of another nearby electronegative atom ($N$, $O$, or $F$).

Hydride Boiling Point Anomaly

The unexpected observation that the lightest hydrides of groups 15, 16, and 17 ($NH_3$, $H_2O$, and $HF$) possess significantly higher normal boiling points than heavier hydrides in their respective groups due to strong intermolecular hydrogen bonding.

Intermolecular Hydrogen Bonding

Hydrogen bonding occurring between two or more separate molecules of the same or different compound, which elevates the compound's boiling point by requiring additional energy to dissociate molecules for vaporization.

Intramolecular Hydrogen Bonding (Chelation)

Hydrogen bonding that occurs within two atoms of the same molecule, frequently developing five- or six-membered rings in organic compounds (such as salicylic acid and o-nitrophenol) and leading to a decrease in boiling point.

Gaseous Hydrogen Fluoride Ring Structure

A cyclic aggregate formed by gaseous $HF$ molecules held together by hydrogen bonds, characterized by an $F-H\cdots F$ bond angle of $180^\circ$.

Diol

An alcohol molecule containing two $-OH$ groups (such as ethylene glycol / 1,2-ethanediol), offering greater capacity for hydrogen-bond formation than a comparable alcohol with a single group.

Viscosity in Polyhydroxy Alcohols

The property that increases markedly at $20^\circ\text{C}$ with an increasing number of $-OH$ groups due to hydrogen-bonding networks: ethyl alcohol ($\eta = 1.20\,\text{cP}$), ethylene glycol ($\eta = 19.9\,\text{cP}$), and glycerol ($\eta = 1490\,\text{cP}$).

Salicylic Acid vs. para-Hydroxybenzoic Acid Hydrogen Bonding

Salicylic acid demonstrates intramolecular hydrogen bonding between its adjacent ortho groups, whereas para-hydroxybenzoic acid lacks intramolecular hydrogen bonding due to the spatial separation of its substituents.

Guanine–Cytosine Base Pairing

The biological intermolecular hydrogen bonding interaction that links the nitrogenous bases guanine and cytosine within the structure of DNA.