Topic 3.1 Intermolecular Forces: identify and rank the intermolecular forces (London dispersion, dipole-dipole, hydrogen bonding, ion-dipole) present in a substance and relate their strength to properties such as boiling point and vapor pressure.

What this topic is asking

The College Board (Topic 3.1) wants you to identify the intermolecular forces (IMFs) acting between the particles of a substance, rank their relative strength, and use that ranking to explain physical properties such as boiling point, melting point, viscosity and vapor pressure. The key distinction is between forces within a molecule (the covalent bonds, which are strong) and forces between molecules (the IMFs, which are far weaker but determine whether a substance is a gas, liquid or solid at room temperature).

The types of intermolecular force

London dispersion forces arise from instantaneous, fluctuating dipoles: at any moment the electron cloud of a molecule is slightly uneven, creating a temporary dipole that induces a matching dipole in a neighbor. They are present in every substance and grow stronger as the number of electrons (and therefore molar mass and polarisability) increases. They are the only force in nonpolar substances such as $\text{N}_2$, $\text{CH}_4$ and the noble gases.

Dipole-dipole forces act between polar molecules, where a permanent partial positive end ($\delta^+$) on one molecule attracts the partial negative end ($\delta^-$) on another. They add to the dispersion forces that are always present.

Hydrogen bonding is an especially strong dipole-dipole interaction that occurs when a hydrogen atom is bonded directly to a small, highly electronegative atom (N, O or F) and is attracted to a lone pair on an N, O or F of a neighboring molecule. It explains the unusually high boiling point of water, ammonia and hydrogen fluoride.

Ion-dipole forces act between an ion and a polar molecule, for example $\text{Na}^+$ surrounded by the negative ends of water molecules in a salt solution. They are central to why ionic compounds dissolve in water (Topic 3.10).

Ranking strength

Dispersion forces are not automatically the weakest in practice. A large nonpolar molecule such as $\text{I}_2$ has stronger total intermolecular forces than a small polar molecule, because it has so many electrons that its dispersion forces are large. This is why $\text{I}_2$ is a solid at room temperature while $\text{HCl}$ is a gas, even though $\text{HCl}$ is polar.

How IMFs set physical properties

Stronger intermolecular forces hold particles together more tightly, so the substance:

• has higher boiling and melting points, because more energy is needed to pull particles apart;
• has higher viscosity, because molecules resist flowing past one another;
• has lower vapor pressure, because fewer molecules can escape into the gas phase;
• has higher surface tension, for the same reason.

This is the central reasoning move of Unit 3: explain a macroscopic property from the particle-level forces. Notice that boiling does not break the covalent bonds inside molecules; it only overcomes the IMFs between them. That is why water vapor is still $\text{H}_2\text{O}$ molecules, not separated H and O atoms.

Try this

Q1. Identify the strongest intermolecular force in (a) $\text{NH}_3$, (b) $\text{CO}_2$, (c) $\text{CH}_3\text{Cl}$. [3 points]

• Cue. (a) hydrogen bonding (N-H); (b) dispersion only (nonpolar, linear, bonds cancel); (c) dipole-dipole.

Q2. Explain why $\text{Br}_2$ is a liquid at room temperature while $\text{Cl}_2$ is a gas, even though both are nonpolar. [2 points]

• Cue. $\text{Br}_2$ has more electrons, so stronger London dispersion forces, so a higher boiling point and a liquid state at room temperature.