Intermolecular forces
IB Chemistry HL· Topic 2: Models of bonding and structure, 2.4 Intermolecular forces· 25 min read
1. Intermolecular vs Intramolecular Forces★★☆☆☆⏱ 5 min
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
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.
2. Types of Intermolecular Forces★★★☆☆⏱ 8 min
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).
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.
3. Relative Strength of Intermolecular Forces★★★☆☆⏱ 7 min
For molecules of similar molar mass and size, the order of intermolecular force strength is consistent:
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).
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.
Test your understanding of strength order:
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
Reveal answer
A —Correct! This matches the general strength order for similar-sized molecules.
4. Intermolecular Forces and Physical Properties★★★★☆⏱ 7 min
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.
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:
Always link structure to property explicitly in written answers
5. Common Pitfalls
Wrong move:
Claiming covalent bonds are broken when a molecular substance boils
Why:
Students confuse intermolecular and intramolecular forces for phase changes
Correct move:
Phase changes of molecular compounds only separate molecules, so only intermolecular forces are broken; covalent bonds remain intact.
Wrong move:
Forgetting London dispersion forces are present in all molecules, including polar molecules
Why:
Students only list the strongest intermolecular force, but all applicable types are present
Correct move:
Always include London dispersion forces when listing intermolecular forces for any molecular substance.
Wrong move:
Claiming any hydrogen in a molecule can form hydrogen bonds
Why:
Hydrogen bonding only requires H to be bonded directly to N/O/F, not any H in the molecule
Correct move:
Check the bonding of hydrogen: only H covalently bonded to N/O/F can form hydrogen bonds.
Wrong move:
Saying branched alkanes have stronger intermolecular forces than straight-chain isomers
Why:
Students confuse the effect of branching on molecular surface area
Correct move:
Straight-chain alkanes have more surface contact between molecules, so stronger LDF and higher boiling points than branched isomers of the same formula.
Wrong move:
Claiming hydrogen bonding is stronger than covalent bonds
Why:
The name 'bond' leads to confusion between intermolecular and intramolecular forces
Correct move:
Hydrogen bonding is a strong intermolecular force, it is still 10-20x weaker than a covalent intramolecular bond.
6. Quick Reference 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 |
7. Frequently Asked
Is hydrogen bonding a real chemical bond?
No. Hydrogen bonding is a strong type of intermolecular attraction, not a covalent/ionic intramolecular bond. It is still ~10-20x weaker than a covalent bond.
Do nonpolar molecules have any intermolecular forces?
Yes. All atoms and molecules have London dispersion forces, even nonpolar molecules. No molecular substance can exist as a liquid/solid without intermolecular attractions.
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 · P1
Compare boiling points of isomers
- 2024 · P2
Identify all intermolecular forces present
- 2023 · P1
Order intermolecular forces by strength
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.