Intermolecular Forces

Why can two molecules with similar molar masses have dramatically different boiling points?

The answer often lies in how their electron distributions allow neighboring particles to attract one another.

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Why can two molecules with similar molar masses have dramatically different boiling points?

The answer often lies in how their electron distributions allow neighboring particles to attract one another.

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All atoms and molecules experience dispersion interactions because their electron distributions fluctuate. Polar molecules can also align permanent partial charges, adding dipole–dipole attraction. Particular donor–acceptor arrangements involving strongly polar X–H bonds can produce hydrogen bonding.

A single intermolecular attraction is usually much weaker than a strong covalent bond, but a macroscopic liquid contains enormous numbers of interacting neighbors. Their collective effect can therefore dominate boiling point, viscosity, surface tension, crystal packing, and solvation.

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Intermolecular forces

Intermolecular forces are noncovalent attractive interactions among molecules or other particles. Common molecular categories include London dispersion forces, dipole–dipole interactions, and hydrogen bonding; ion–dipole interactions are important in many solutions.

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Why

Why it matters

Why do stronger intermolecular attractions often raise boiling points?

Boiling requires molecules to populate the vapor phase against the cohesive interactions that stabilize the liquid. When those attractions are stronger, more thermal energy is generally required for the vapor pressure to reach the external pressure.

The trend is not controlled by a single label alone. Molecular size, polarizability, shape, surface contact, dipole orientation, and hydrogen-bonding possibilities can all contribute.