Intermolecular forces

IB Chemistry SL· 40 min read

1. Intermolecular vs Intramolecular Forces★★☆☆☆⏱ 10 min

📘 Definition

Intermolecular vs Intramolecular Forces

Intermolecular forces are electrostatic attractions between separate molecules, atoms, or ions. Intramolecular forces are forces that hold atoms together within a single molecule, such as covalent or ionic bonds.

Example:

In water: intermolecular forces attract separate $H_{2} O$ molecules; intramolecular forces hold O and H atoms within one $H_{2} O$ molecule.

📐 Worked Example

Identify whether the force broken when liquid ethanol boils is intermolecular or intramolecular.

1
When ethanol boils, liquid ethanol becomes gaseous ethanol. The ethanol molecules $C_{2} H_{5} O H$ remain fully intact, they do not split into individual atoms.
2
Only the attractions between separate ethanol molecules are overcome during the phase change. No covalent bonds inside the molecules are broken.
3
The force broken is therefore an intermolecular force.

2. Types of Intermolecular Forces★★★☆☆⏱ 15 min

IB SL Chemistry tests four main types of intermolecular forces, ordered from weakest to strongest for molecules of similar size.

London Dispersion (LD) Forces: Present in all substances; caused by temporary instantaneous dipoles. Strength increases with number of electrons (higher molar mass / larger molecular surface area).
Dipole-Dipole Forces: Occur between permanent dipoles of polar molecules; only present in polar molecules.
Hydrogen Bonding: Special strong dipole-dipole attraction; occurs when H bonded to N/O/F on one molecule is attracted to a lone pair on N/O/F on another molecule.
Ion-Dipole Forces: Occur between an ion and a polar molecule; critical for dissolving ionic compounds in polar solvents.

📐 Worked Example

List all intermolecular forces present in a sample of pure $C H_{3} O H$ (methanol).

1
All molecules have London dispersion forces, so this is always included.
2
Methanol is polar, so it has a permanent dipole, meaning dipole-dipole forces are also present.
3
Check for hydrogen bonding: Methanol has an H atom directly bonded to O, which meets the FON requirement. So hydrogen bonding is also present.
4
Final answer: London dispersion, dipole-dipole, and hydrogen bonding.

3. Intermolecular Forces and Physical Properties★★★☆☆⏱ 15 min

Stronger intermolecular forces require more energy to overcome, leading to higher boiling and melting points, lower volatility (higher boiling point), higher viscosity, and higher surface tension. For solubility, the rule "like dissolves like" applies: polar solutes dissolve in polar solvents, non-polar solutes dissolve in non-polar solvents.

📐 Worked Example

Predict which has a higher boiling point: $C H_{3} C H_{2} C H_{3}$ (propane, $M_{r} = 44$ ) or $C H_{3} O C H_{3}$ (dimethylether, $M_{r} = 46$ ). Justify your answer.

1
Propane is a non-polar molecule, so it only has London dispersion forces.
2
Dimethylether is polar due to the electronegative oxygen atom, so it has permanent dipoles and dipole-dipole forces, plus London dispersion.
3
Molar masses are almost identical, so London dispersion strengths are comparable. Dimethylether has extra dipole-dipole attractions, so total intermolecular forces are stronger.
4
Conclusion: Dimethylether has a higher boiling point than propane.

4. Common Trends and Exam Questions★★★★☆⏱ 10 min

IB exams frequently test unexpected boiling point trends, including for isomers. Straight-chain isomers have higher boiling points than branched-chain isomers of the same molar mass, because branching reduces the surface area available for London dispersion interactions.

📐 Worked Example

Explain why water ( $M_{r} = 18$ , $b p = 10 0^{e} x t o C$ ) has a much higher boiling point than hydrogen sulfide ( $H_{2} S$ , $M_{r} = 34$ , $b p = - 6 0^{e} x t o C$ ).

1
Water molecules have hydrogen bonded directly to oxygen, so water has strong hydrogen bonding between molecules.
2
$H_{2} S$ has hydrogen bonded to sulfur, which is not F/O/N, so it has no hydrogen bonding, only dipole-dipole and London dispersion forces.
3
Even though $H_{2} S$ has a higher molar mass (stronger London dispersion), hydrogen bonding in water is much stronger than the intermolecular forces in $H_{2} S$ , so more energy is needed to overcome attractions.
4
Conclusion: Water has a much higher boiling point as a result.

5. Common Pitfalls

Wrong move:

Claiming covalent bonds are broken when a molecular substance boils.

Why:

Students confuse intermolecular and intramolecular forces; phase changes only separate molecules, do not break bonds inside molecules.

Correct move:

Recognize that phase changes of molecular substances only overcome intermolecular forces, not intramolecular covalent bonds.

Wrong move:

Stating any molecule containing N/O/F has hydrogen bonding.

Why:

Hydrogen bonding requires hydrogen to be covalently bonded directly to F/O/N, not just the presence of these atoms in the molecule.

Correct move:

Always confirm that at least one H atom is bonded directly to F/O/N before stating hydrogen bonding is present.

Wrong move:

Assuming London dispersion forces are always the weakest force.

Why:

For very large molecules with many electrons, London dispersion forces can be stronger than dipole-dipole or even hydrogen bonding.

Correct move:

Compare molecular size/molar mass first before applying the general similar-size strength order.

Wrong move:

Classifying hydrogen bonding as an intramolecular covalent bond.

Why:

Hydrogen bonding is an intermolecular attraction between separate molecules, not a chemical bond.

Correct move:

Always classify hydrogen bonding as a strong type of intermolecular dipole-dipole attraction.

6. Quick Reference Cheatsheet

Force Type	Occurrence	Relative Strength (similar size)
London Dispersion	All molecules/atoms	Weakest
Dipole-Dipole	Polar molecules	Stronger than LD
Hydrogen Bonding	H directly bonded to F/O/N	Stronger than dipole-dipole
Ion-Dipole	Ions + polar molecules	Strongest

7. Frequently Asked

Are intermolecular forces considered chemical bonds?

No, they are electrostatic attractions between separate particles, not full intramolecular chemical bonds like covalent or ionic bonds. They are typically 1-10% the strength of a covalent bond.

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.

2022 · 1
Identify intermolecular force types
2021 · 2
Justify boiling point trend
2023 · 1
Intra vs intermolecular comparison

What's Next

Understanding intermolecular forces is foundational for many upcoming topics in IB Chemistry SL, including how and why substances dissolve, phase behavior, and the physical properties of organic compounds. Intermolecular forces explain many everyday observations, from why water has an unusually high boiling point for its size to why vegetable oil is liquid at room temperature while saturated fats are solid. This concept is heavily assessed in both Paper 1 multiple choice and Paper 2 extended response questions, so mastering the ability to identify force types and predict physical property trends is critical for earning high marks on your exam.