2.2.9Additional lessons
Intermolecular Forces — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 30 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6The Three Types
Intermolecular forces act between molecules, not within them.
London dispersion
- London dispersion Temporary dipoles from moving electrons induce dipoles in neighbours, giving weak attractions present between all molecules.
Dipole-dipole
- Dipole-dipole Permanent dipoles in polar molecules attract the opposite poles of nearby molecules.
Hydrogen bonding
- Hydrogen bonding A hydrogen atom bonded to N, O or F is attracted to a lone pair on another N, O or F atom,
- which is stronger than other dipole-dipole forces.
Strength order
- Strength order Generally dispersion forces are weakest for small molecules,
- then dipole-dipole,
- then hydrogen bonds,
- but all are much weaker than covalent bonds.
Section 2 of 6London Forces and Molecular Size
London Forces and Molecular Size
More electrons
- More electrons Molecules with more electrons have electron clouds that are more easily distorted, so dispersion forces are stronger.
Larger molar mass
- Larger molar mass Within a similar family, the greater the molar mass the stronger the dispersion forces and the higher the boiling point.
Surface contact
- Surface contact Long straight molecules touch over a larger area than branched ones, so they attract more strongly.
Halogen trend
- Halogen trend Boiling points rise from F₂ (−188 °C) to Cl₂ (−34 °C), Br₂ (59 °C) and I₂ (184 °C) as dispersion forces increase.
Section 3 of 6Hydrogen Bonding
1
In a molecule such as water, hydrogen is bonded to highly electronegative oxygen, which makes the hydrogen carry a strong δ+ charge.
2
This hydrogen is attracted to a lone pair on an oxygen atom of a neighbouring molecule.
3
Each water molecule can form up to four hydrogen bonds, two through its hydrogens and two through its oxygen lone pairs.
4
A great deal of energy is needed to break these bonds, so water has a high boiling point for its small size (100 °C).
5
Similar effects occur in NH₃, HF and alcohols, giving them higher boiling points than molecules of similar mass without hydrogen bonding.
| Substance | Main force | Boiling point / °C |
|---|---|---|
| CH₄ | dispersion | −162 |
| HCl | dipole-dipole + dispersion | −85 |
| NH₃ | hydrogen bonds | −33 |
| H₂O | hydrogen bonds | 100 |
Section 4 of 6Applying the Ideas
Ethanol and methoxymethane
- Ethanol and methoxymethane Both are C₂H₆O, but ethanol (78 °C) forms hydrogen bonds, whereas methoxymethane (−24 °C) cannot.
Ice is less dense
- Ice is less dense Hydrogen bonding holds water in an open lattice in ice, so ice floats on liquid water.
State at room temperature
- State at room temperature Weak forces for small non-polar molecules give gases;
- stronger forces give liquids and solids.
Comparing answers
- Comparing answers Always compare the types and strengths of forces in each substance, not just the covalent bonds.
Section 5 of 6Worked Example
Question Explain why the boiling point of iodine (184 °C) is much higher than that of chlorine (−34 °C).
- Both are non-polar diatomic molecules, so the only intermolecular forces are London dispersion forces.
- Iodine has many more electrons (106 compared with 34), so its electron cloud is larger and more easily distorted.
- This gives stronger temporary dipoles and stronger attractions, which need more energy to overcome.
Answer Iodine has stronger London dispersion forces because it has more electrons, so more energy is needed to separate its molecules.
Section 6 of 6Exam Checklist & Tips
2.2.9 – Key Points for the Exam
- London dispersion force A weak attraction between temporary dipoles in neighbouring molecules.
- Dipole-dipole force An attraction between permanent dipoles of polar molecules.
- Hydrogen bond A strong dipole-dipole attraction involving H bonded to N, O or F and a lone pair on N, O or F.
- Intermolecular force An attraction between separate molecules.
Common Mistakes
- Saying covalent bonds break when a molecular substance boils; only intermolecular forces are overcome.
- Claiming hydrogen bonds are bonds within a molecule.
- Forgetting that dispersion forces exist in all molecules, including polar ones.
- Describing hydrogen bonding in any molecule that contains hydrogen.
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