Intermolecular Forces Reference Cheat Sheet

Key Facts

• London dispersion forces occur in all particles and generally increase as molar mass and polarizability increase.
• Dipole-dipole forces occur between polar molecules with permanent partial charges, such as attraction between $\delta^+$ and $\delta^-$ ends.
• Hydrogen bonding occurs when $H$ is bonded to $N$, $O$, or $F$ and is attracted to a lone pair on $N$, $O$, or $F$ nearby.
• Ion-dipole forces occur between an ion and a polar molecule, and they are important when ionic compounds dissolve in water.
• A useful strength trend is London dispersion $<$ dipole-dipole $<$ hydrogen bonding $<$ ion-dipole, but size and shape can change comparisons.
• For similar molecules, stronger intermolecular forces usually mean a higher boiling point because more energy is needed to separate particles.
• Like dissolves like means polar solutes usually dissolve best in polar solvents, while nonpolar solutes usually dissolve best in nonpolar solvents.
• Coulombic attraction can be summarized by $F \propto \frac{q_1q_2}{r^2}$, so greater charge and shorter distance increase attraction.

Vocabulary

Intermolecular force

An attraction between separate particles or molecules that affects physical properties such as boiling point and solubility.

London dispersion force

A temporary attraction caused by shifting electrons that create instantaneous and induced dipoles.

Dipole-dipole force

An attraction between the positive end of one polar molecule and the negative end of another polar molecule.

Hydrogen bond

A strong dipole-dipole attraction involving $H$ bonded to $N$, $O$, or $F$ and a lone pair on $N$, $O$, or $F$.

Ion-dipole force

An attraction between an ion and the partial charge on a polar molecule.

Polarizability

The ease with which an electron cloud can be distorted to form temporary dipoles.