# Intermolecular forces

> IB Chemistry HL · IB Diploma Programme Chemistry HL
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u2-intermolecular-forces/

This sub-topic covers the different types of attractive forces between molecules, how their strength varies, and how intermolecular forces explain differences in physical properties like boiling point, solubility and viscosity.

**Prerequisites:** [Bond polarity and electronegativity](https://www.owlsprep.com/study/ib-chemistry-hl-u2-bond-polarity/); [Covalent bonding basics](https://www.owlsprep.com/study/ib-chemistry-hl-u2-covalent-bonding/)

## Learning objectives

- Distinguish between intermolecular and intramolecular forces
- Identify the four main types of intermolecular forces in pure substances and mixtures
- Relate intermolecular force strength to bulk physical properties of molecular compounds
- Predict relative boiling/melting points based on intermolecular force type and strength

## Intermolecular vs Intramolecular Forces

**Intermolecular Forces** — Attractive forces that act between separate molecules (or between atoms of monatomic substances like noble gases). These are much weaker than intramolecular chemical bonds.

*Example:* Forces holding separate H₂O molecules together in liquid water

**Intramolecular Forces** — Forces that hold atoms together within a single molecule or ionic lattice. This includes covalent, ionic, and metallic bonds.

*Example:* O-H covalent bonds within a single H₂O molecule

> **tip**
>
> Intermolecular forces are typically 10-100 times weaker than intramolecular covalent bonds.

**Worked example:** Identify whether the force broken when ethanol (C₂H₅OH) boils is intermolecular or intramolecular.

1. Boiling is a phase change that converts liquid ethanol to gaseous ethanol. No covalent bonds are broken during this process.
2. The forces broken are the attractions between separate ethanol molecules, not bonds within the molecules.
3. The force broken is therefore intermolecular.

## Types of Intermolecular Forces

**London Dispersion Forces (LDF)** — Temporary attractive forces caused by instantaneous dipoles formed from random electron movement. Strength increases with number of electrons (molar mass) and molecular surface area. Present between all atoms and molecules.

*Example:* Intermolecular forces between nonpolar methane (CH₄) molecules

- **Dipole-dipole forces**: Electrostatic attractions between permanent dipoles of polar molecules. Only present between polar molecules, stronger than LDF for similar molar mass.
- **Hydrogen bonding**: A special strong type of dipole-dipole attraction, occurring between a hydrogen covalently bonded to N/O/F and a lone pair on a N/O/F atom in another molecule.
- **Ion-dipole forces**: Attractions between an ion and a polar molecule, the strongest of the four common intermolecular forces. Found in mixtures of ionic compounds and polar solvents (e.g. salt water).

> **mnemonic**
>
> Remember: only H bonded to **N-O-F** can form hydrogen bonds. Mnemonic: *No Old Fire* = N, O, F.

**Worked example:** List all types of intermolecular forces present in a sample of pure methanamine (CH₃NH₂).

1. All molecular substances have London dispersion forces, so LDF are always present.
2. Methanamine has a net permanent dipole from polar N-H and C-N bonds, so it also has dipole-dipole forces.
3. Hydrogen is covalently bonded directly to nitrogen, one of N/O/F, so hydrogen bonding is also present.
4. Full list: London dispersion forces, dipole-dipole forces, hydrogen bonding.

## Relative Strength of Intermolecular Forces

For molecules of similar molar mass and size, the order of intermolecular force strength is consistent:

$$\text{Ion-dipole} > \text{Hydrogen bonding} > \text{Dipole-dipole} > \text{London dispersion}$$

When comparing London dispersion forces only, strength increases with: (1) higher number of electrons (higher molar mass), and (2) greater molecular surface area (straight chain > branched isomers).

**Worked example:** Which of the following has the highest boiling point: CH₄, CH₃CH₃, CH₃CH₂CH₃, CH₃CH₂CH₂CH₃? Justify your answer.

1. All four compounds are nonpolar alkanes, so the only intermolecular force present is London dispersion forces.
2. Boiling point increases with increasing intermolecular force strength. For nonpolar alkanes, LDF strength increases with increasing carbon chain length (higher molar mass = more electrons).
3. CH₃CH₂CH₂CH₃ (butane) has the longest chain, highest molar mass, and strongest LDF, so it has the highest boiling point.

**Check your understanding**

Test your understanding of strength order:

1. Which option correctly orders intermolecular forces from weakest to strongest for similar-sized molecules?

   - A: London dispersion < dipole-dipole < hydrogen bonding < ion-dipole
   - B: dipole-dipole < London dispersion < hydrogen bonding < ion-dipole
   - C: London dispersion < hydrogen bonding < dipole-dipole < ion-dipole
   - D: ion-dipole < London dispersion < dipole-dipole < hydrogen bonding

   *Why:* Correct! This matches the general strength order for similar-sized molecules.

## Intermolecular Forces and Physical Properties

Intermolecular force strength directly determines bulk physical properties of molecular substances: stronger intermolecular forces lead to higher boiling point, higher melting point, higher viscosity, higher surface tension, and lower volatility. Solubility follows the rule *like dissolves like*: polar/ionic substances dissolve in polar solvents, nonpolar dissolve in nonpolar solvents.

**Worked example:** Explain why ethanol (C₂H₅OH) has a higher boiling point than dimethyl ether (CH₃OCH₃), even though both have the same molecular formula C₂H₆O.

1. Both molecules have the same molar mass, so London dispersion forces are approximately equal in strength for both.
2. In ethanol, a hydrogen atom is covalently bonded directly to oxygen, so ethanol can form hydrogen bonds between molecules.
3. In dimethyl ether, oxygen is bonded to two carbon atoms, so there is no hydrogen directly bonded to oxygen. Dimethyl ether only has dipole-dipole forces, no hydrogen bonding.
4. Hydrogen bonds are stronger than dipole-dipole forces, so ethanol has stronger overall intermolecular forces, requiring more energy to separate molecules, leading to a higher boiling point.

> **exam_tip**
>
> When justifying boiling point differences in exams, always explicitly compare the strength of intermolecular forces, don't just name the types present.

> **Exam tip:** Always link structure to property explicitly in written answers

## Common pitfalls

- **Wrong:** Claiming covalent bonds are broken when a molecular substance boils
  - Why it fails: Students confuse intermolecular and intramolecular forces for phase changes
  - Correct: Phase changes of molecular compounds only separate molecules, so only intermolecular forces are broken; covalent bonds remain intact.
- **Wrong:** Forgetting London dispersion forces are present in all molecules, including polar molecules
  - Why it fails: Students only list the strongest intermolecular force, but all applicable types are present
  - Correct: Always include London dispersion forces when listing intermolecular forces for any molecular substance.
- **Wrong:** Claiming any hydrogen in a molecule can form hydrogen bonds
  - Why it fails: Hydrogen bonding only requires H to be bonded directly to N/O/F, not any H in the molecule
  - Correct: Check the bonding of hydrogen: only H covalently bonded to N/O/F can form hydrogen bonds.
- **Wrong:** Saying branched alkanes have stronger intermolecular forces than straight-chain isomers
  - Why it fails: Students confuse the effect of branching on molecular surface area
  - Correct: Straight-chain alkanes have more surface contact between molecules, so stronger LDF and higher boiling points than branched isomers of the same formula.
- **Wrong:** Claiming hydrogen bonding is stronger than covalent bonds
  - Why it fails: The name 'bond' leads to confusion between intermolecular and intramolecular forces
  - Correct: Hydrogen bonding is a strong intermolecular force, it is still 10-20x weaker than a covalent intramolecular bond.

## Cheatsheet

| Intermolecular force type | Occurrence | Relative strength (similar size) |
| --- | --- | --- |
| Ion-dipole | Between ions and polar molecules | 1 (strongest) |
| Hydrogen bonding | Between molecules with H bonded to N/O/F | 2 |
| Dipole-dipole | Between polar molecules with permanent dipoles | 3 |
| London dispersion | Between all atoms/molecules | 4 (weakest) |
| General property relation | Stronger intermolecular forces → | Higher bp/mp, higher viscosity, lower volatility |

## What's next

Understanding intermolecular forces is foundational for explaining the bulk properties of molecular compounds, and underpins advanced topics across IB Chemistry, including organic chemistry reactivity, solubility rules and separation techniques, and the structure of biological macromolecules like proteins and DNA. Intermolecular forces also connect bonding structure to observable physical properties, which is a core recurring assessment theme in IB Chemistry exams. Practice applying intermolecular force concepts to compare boiling points and explain solubility trends to master this frequently tested topic.

- [Metallic bonding and alloy structure](https://www.owlsprep.com/study/ib-chemistry-hl-u2-metallic-bonding-and-alloy-structure/)
- [Crystal lattice structures](https://www.owlsprep.com/study/ib-chemistry-hl-u2-crystal-lattice-structures/)
- [AHL: Covalent bond order and electron delocalization](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ahl-covalent-bond-order-and/)

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