# Intermolecular interactions

Physics I · Chemistry · https://tryals.app/learn/physics-i/intermolecular-interactions

## Intermolecular Forces

While **intramolecular** bonds hold atoms together inside a molecule, **intermolecular forces** act between molecules and govern macroscopic properties like boiling point and viscosity.

| Force | Origin | Strength |
| --- | --- | --- |
| **London dispersion** | Temporary dipoles in all molecules | Weak; scales with size |
| **Dipole-dipole** | Permanent dipoles align | Moderate |
| **Hydrogen bonding** | H bonded to N, O, or F | Strong |

**Van der Waals potential**: $V(r) \propto -\frac{1}{r^{6}}$, attractive at moderate $r$.

> **Common pitfall**: Hydrogen bonds are **between** molecules, not within them. They are attractions between an H bonded to N/O/F and a neighbouring lone pair, about 20 times weaker than covalent bonds.

## Energy, Phase and Boiling

- **Gibbs free energy**: $\Delta G = \Delta H - T\Delta S$. A process is **spontaneous** when $\Delta G < 0$.
- **Boltzmann distribution**: $P(\text{state}) \propto e^{-E/k_BT}$.

| Phase | Interaction Strength | Molecular Motion |
| --- | --- | --- |
| **Gases** | Weak | Move freely |
| **Liquids** | Moderate | Slide past each other |
| **Solids** | Strong | Locked in a lattice |

> **Physics link**: The depth of the intermolecular potential well determines the **boiling point**: a deeper well means more thermal energy is needed to vaporise.

## Practice questions

3 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Water boils at 100 °C while $H_2S$, a heavier molecule, boils at −60 °C. What explains the reversal?

A. Water molecules are lighter and so move faster at a given temperature
B. Sulfur is larger, so $H_2S$ molecules pack more loosely
C. Hydrogen bonding, which S–H bonds cannot form
D. Water is polar and $H_2S$ is completely nonpolar

**Answer:** C. Hydrogen bonding, which S–H bonds cannot form

**Why:** Boiling point tracks intermolecular forces, not molecular mass. Hydrogen bonding is strong enough to invert the trend entirely, and is why water is liquid at all on Earth’s surface.

Page: https://tryals.app/practice/physics-i/intermolecular-interactions/water-boils-at-100-c-while-h2s-a-heavier-molecule-boils-at-60-c

### 2. Pentane and neopentane have the **same formula** $C_5H_{12}$ and the same molar mass, yet pentane boils at 36 °C and neopentane at 9.5 °C. What accounts for the 26-degree gap?

A. Neopentane is lighter, and lighter molecules always boil first
B. Pentane is a long chain with more contact surface
C. Neopentane has weaker covalent bonds, so it breaks apart at a lower temperature
D. Pentane is polar and neopentane is not

**Answer:** B. Pentane is a long chain with more contact surface

**Why:** Boiling breaks intermolecular forces, not bonds, so the covalent option is off the point and the masses are identical. Both molecules are nonpolar, leaving dispersion, which grows with contact area, a chain can lie alongside its neighbours and a sphere touches at a point. This is why branching lowers boiling points right across organic chemistry.

Page: https://tryals.app/practice/physics-i/intermolecular-interactions/pentane-and-neopentane-have-the-same-formula-c5h12-and-the-same-molar

### 3. Which molecules are capable of forming hydrogen bonds with each other?

A. $CH_{4}$
B. $H_{2}$O
C. $NH_{3}$
D. HF

**Answer:** B. $H_{2}$O; C. $NH_{3}$; D. HF

**Why:** H-bonding requires H bonded to N, O, or F. $CH_{4}$ has H bonded only to C (not electronegative enough), so it cannot hydrogen bond.

Page: https://tryals.app/practice/physics-i/intermolecular-interactions/which-molecules-are-capable-of-forming-hydrogen-bonds-with-each-other