AP Chemistry Topic 3.1: Intermolecular Forces

Vocabulary practice flashcards covering AP Chemistry Topic 3.1: Intermolecular Forces, including London dispersion, dipole-dipole, hydrogen bonding, ion-dipole interactions, induced dipoles, and their influences on physical properties.

Intermolecular Forces

The attractive electrostatic forces that exist between different molecules, which are generally much weaker than intramolecular bonds.

Intramolecular Forces

The chemical bonds (including ionic, covalent, and metallic bonds) that hold atoms together within an individual molecule or compound.

Electrostatic Interactions

Forces governed by Coulomb's law ($F_e = \frac{k q_1 q_2}{r^2}$) that involve attractions between positive and negative species; they strengthen as the magnitude of charges increases and weaken as distance increases.

London Dispersion Forces

The intermolecular attractive forces present in all atoms and molecules, caused by the momentary unsymmetrical distribution of electrons that produces an instantaneous dipole, which in turn induces a dipole in a neighboring atom or molecule.

Polarizability

The ease or ability with which the electron cloud of an atom or molecule can be distorted by an external electric field to form a temporary or induced dipole.

Surface Area Effect on Dispersion

The principle stating that molecules with larger, more elongated surface areas provide more contact points, resulting in stronger dispersion forces and higher boiling points compared to compact, spherical isomers.

$\pi$ Bonding Effect on Dispersion

The enhancement of London dispersion forces that occurs in the presence of double or triple bonds due to the higher polarizability of loosely held $\pi$ electrons.

Dipole-Dipole Forces

Attractive intermolecular forces that originate from electrostatic attractions between the partially positive ($\delta+$) pole of one polar molecule and the partially negative ($\delta-$) pole of an adjacent polar molecule.

Permanent Dipole Moment

A lasting separation of electric charge within a molecule, resulting from polar covalent bonds whose individual bond dipole vectors do not cancel out due to molecular geometry.

Hydrogen Bonding

A strong type of dipole–dipole attraction occurring between a hydrogen atom covalently bonded to a highly electronegative atom ($\text{N}$, $\text{O}$, or $\text{F}$) and a nonbonding lone pair of electrons on an adjacent $\text{N}$, $\text{O}$, or $\text{F}$ atom.

Direct Bonding Requirement

The specific chemical structural rule that hydrogen must be directly bonded to fluorine, oxygen, or nitrogen ($\text{H-F}$, $\text{H-O}$, or $\text{H-N}$) for that hydrogen atom to participate as a hydrogen-bond donor.

Additive Property of Hydrogen Bonds

The cumulative increase in intermolecular attraction that occurs when a molecule forms multiple hydrogen bonds per unit (such as acetic acid forming two bonds versus 1-propanol forming one), substantially raising the boiling point.

Ion-Dipole Forces

The attractive forces between an ion (cation or anion) and the oppositely charged pole of a polar molecule, commonly responsible for dissolving ionic substances in polar solvents such as $\text{NaCl}$ in water.

Dipole-Induced Dipole Forces

Intermolecular attractions that occur when the permanent dipole of a polar molecule disturbs the electron arrangement of a neighboring nonpolar molecule or neutral atom, inducing a temporary dipole.

Ion-Induced Dipole Forces

Intermolecular attractive forces that occur when the charge of an ion perturbs the electron cloud of a neighboring nonpolar molecule or atom, generating a temporary dipole.

IMF Comparison: Comparable Molecular Weights

The rule establishing that when two substances possess similar molecular weights and shapes, their dispersion forces are approximately equal, meaning differences in boiling points are determined primarily by dipole–dipole attractions and hydrogen bonding.

IMF Comparison: Significantly Different Molecular Weights

The rule establishing that when two substances differ widely in molecular weight, dispersion forces tend to determine which substance has the stronger overall intermolecular forces, with the heavier, more polarizable substance having the higher boiling point.

Physical Change (IMF Perspective)

A process, such as melting or boiling, that alters physical state by overcoming or forming intermolecular forces between molecules without breaking or forming chemical covalent bonds.