Intermolecular Forces of Attraction

Intermolecular Forces (IMFA)

Forces of attraction that exist between molecules, also known as van der Waals forces.

London Dispersion Forces (LDF)

Weak intermolecular forces present in all molecules and the only intermolecular force in nonpolar molecules. They arise from temporary (instantaneous) dipoles caused by the movement of electrons.

London Dispersion Force Strength

Larger, heavier, and more polarizable molecules have stronger London dispersion forces.

Dipole-Dipole Forces

Weak attractions between the partially positive end of one polar molecule and the partially negative end of another polar molecule.

Dipole-Dipole Force Strength

More polar molecules have stronger dipole-dipole forces than less polar molecules.

Hydrogen Bonding

A strong type of dipole-dipole interaction that occurs when hydrogen is directly bonded to fluorine (F), oxygen (O), or nitrogen (N).

Water (H₂O) Hydrogen Bonding

The hydrogen atom of one water molecule is attracted to the oxygen atom of another water molecule.

Phase at Room Temperature (Properties Dependent on IMFA)

Strong intermolecular forces usually result in solids or liquids, while weak intermolecular forces usually result in gases.

Solubility (Properties Dependent on IMFA)

The ability of a substance to dissolve in a solvent at a given temperature.

Like Dissolves Like (Properties Dependent on IMFA)

Substances with similar intermolecular forces tend to dissolve in each other.

Water and Ethanol (Properties Dependent on IMFA)

Both are polar substances that exhibit London dispersion forces, dipole-dipole forces, and hydrogen bonding, making them completely miscible.

Hexane and Water (Properties Dependent on IMFA)

Hexane is nonpolar and exhibits only London dispersion forces, while water exhibits hydrogen bonding, so they do not mix and form a heterogeneous mixture.

Melting Point (Properties Dependent on IMFA)

The temperature at which a substance changes from solid to liquid. Stronger intermolecular forces result in higher melting points.

Boiling Point (Properties Dependent on IMFA)

The temperature at which a substance changes from liquid to gas. Stronger intermolecular forces result in higher boiling points.

Surface Tension (Properties Dependent on IMFA)

The tendency of a liquid to minimize its surface area. Stronger intermolecular forces produce higher surface tension.

Viscosity (Properties Dependent on IMFA)

A fluid's resistance to flow. Stronger intermolecular forces result in higher viscosity.

Vapor Pressure (Properties Dependent on IMFA)

The pressure exerted by a vapor in equilibrium with its liquid. Stronger intermolecular forces produce lower vapor pressure because fewer molecules escape into the gas phase.

Molecular Weight Rule

If the molecular mass difference is large (more than about 1000 g/mol), the heavier molecule generally has stronger intermolecular forces.

Hydrogen Bonding Rule

Molecules capable of hydrogen bonding have stronger intermolecular forces than those that cannot.

Polarity Rule

Polar molecules have stronger intermolecular forces than nonpolar molecules (when hydrogen bonding is absent).

London Dispersion Rule

Among nonpolar molecules, the more massive molecule has stronger London dispersion forces.