Intermolecular forces: describe hydrogen bonding, dipole-dipole forces and weak dispersion forces, and use them to explain trends in boiling point and the properties of water.

What this topic is asking

The Core Curriculum asks you to describe the forces between molecules (as distinct from the bonds within them) and to use them to explain physical properties, especially boiling point and the unusual behavior of water. The Regents tests the relative strengths of hydrogen bonding, dipole-dipole forces and weak dispersion forces, and asks you to rank substances by boiling point on that basis.

Forces between molecules versus bonds within them

This distinction is important: boiling water separates water molecules from one another by overcoming hydrogen bonding, but it does not break the O-H bonds inside the molecules. That is why water vapor is still $\text{H}_2\text{O}$.

The three types, strongest to weakest

For molecules of similar size, the order of attraction strength is hydrogen bonding $>$ dipole-dipole $>$ dispersion. Dispersion forces also grow with molecular size, so larger nonpolar molecules can have higher boiling points than smaller ones even without polarity.

Intermolecular forces and boiling point

This is the logic behind most Regents intermolecular-force questions: identify the strongest force present in each substance, then rank by boiling point. A molecule is hydrogen-bonded only if it has H directly bonded to N, O or F.

Why water is unusual

Water's small size would suggest a low boiling point, yet it boils at $100\ ^\circ\text{C}$. Hydrogen bonding between water molecules explains this: each molecule can hydrogen-bond to several neighbors, and breaking that network takes a lot of energy. The same hydrogen bonding gives water its high surface tension and causes ice to be less dense than liquid water (the molecules lock into an open, lower-density arrangement on freezing), which is why ice floats. These are favorite Part C "explain" prompts.

Try this

Q1. State the only intermolecular force present between nonpolar molecules. [1 point]

• Cue. Weak dispersion (van der Waals) forces.

Q2. Explain why ammonia, $\text{NH}_3$, has a higher boiling point than methane, $\text{CH}_4$, of similar size. [1 point]

• Cue. Ammonia has hydrogen bonding (H bonded to N), which is stronger than the dispersion forces in methane, so it needs more energy to boil.