Intermolecular Forces
Edexcel International A-Level Chemistry· 7.1-7.6· 25 min read
1. Types of Intermolecular Forces★★☆☆☆⏱ 7 min
Intermolecular Forces
Weak electrostatic attractive forces between molecules, significantly weaker than covalent or ionic intramolecular bonds.
Three types of intermolecular force are assessed in this unit, ordered by typical strength for molecules of similar size: London (instantaneous dipole-induced dipole) forces, permanent dipole-permanent dipole interactions, and hydrogen bonds.
London forces are present in all molecules, polar and non-polar. They arise from random fluctuations in electron density that create temporary instantaneous dipoles, which induce opposite dipoles in adjacent molecules. Strength increases with increasing electron count (molecular size) and surface area of the molecule.
Permanent dipole-permanent dipole (pd-pd) interactions occur between polar molecules, where the permanent separation of charge (due to differences in electronegativity between bonded atoms) causes electrostatic attraction between the δ+ end of one molecule and the δ- end of a neighbouring molecule.
Hydrogen Bond
A strong intermolecular electrostatic attraction between a hydrogen atom covalently bonded to a highly electronegative N, O or F atom (δ+ H) and a lone pair of electrons on a separate N, O or F atom (δ- acceptor).
Example:
Hydrogen bonds form between water molecules: each δ+ H on one H₂O molecule is attracted to a lone pair on the δ- O of an adjacent H₂O molecule, represented as O–H···O.
Identify all intermolecular forces present in a pure sample of liquid ethanol (C₂H₅OH).
- 1
- London forces are present in all molecules, so they exist between ethanol molecules.
- 2
- Ethanol contains polar C-O and O-H bonds, so it is a polar molecule, meaning permanent dipole-permanent dipole interactions are present.
- 3
- Ethanol has a hydrogen atom covalently bonded to oxygen, and the oxygen atom has lone pairs, so hydrogen bonds form between ethanol molecules.
- 4
Final answer: London forces, permanent dipole-dipole interactions, hydrogen bonds.
Exam tip:
Always list London forces first for any molecule, as they are universal. Only state hydrogen bonds if both the H-bond donor (H bonded to N/O/F) and acceptor (lone pair on N/O/F) are present.
2. Anomalous Properties of Water★★☆☆☆⏱ 5 min
Hydrogen bonding in water causes properties that are unusual for a small, low molecular mass molecule. These are regularly tested in exam questions.
High melting and boiling temperature: Hydrogen bonds are strong relative to other intermolecular forces, so large amounts of energy are required to overcome them to melt or boil water.
Ice is less dense than liquid water: In ice, hydrogen bonds hold water molecules in a rigid open tetrahedral lattice, with empty space between molecules. When ice melts, the lattice breaks down, and molecules pack more closely together, increasing density.
Explain why water has a much higher boiling temperature than hydrogen sulfide (H₂S), even though H₂S has a higher molecular mass.
- 1
- Water molecules contain hydrogen atoms bonded to oxygen (highly electronegative), so hydrogen bonds form between H₂O molecules.
- 2
- Hydrogen sulfide has no hydrogen bonding, as sulfur is not sufficiently electronegative to form H-bonds. Only London forces and weak pd-pd interactions exist between H₂S molecules.
- 3
- The strong hydrogen bonds in water require much more energy to overcome than the weak intermolecular forces in H₂S, so water has a higher boiling temperature.
3. Intermolecular Forces and Boiling Temperature Trends★★★☆☆⏱ 6 min
Boiling temperature is directly related to the strength of intermolecular forces: stronger forces require more energy to overcome, leading to higher boiling temperatures. Edexcel expects you to apply this to specific groups of compounds.
Alkanes: Boiling temperature increases with carbon chain length, as longer chains have more electrons, so stronger London forces. Branched alkanes have lower boiling temperatures than straight chain isomers, as branching reduces surface area, weakening London forces.
Alcohols vs alkanes of similar electron count: Alcohols have hydrogen bonding, so much higher boiling temperatures and lower volatility than equivalent alkanes.
Hydrogen halides (HF to HI): HF has the highest boiling temperature due to hydrogen bonding. From HCl to HI, boiling temperature increases, as increasing molecular mass leads to stronger London forces, which outweigh the decreasing strength of pd-pd interactions.
Arrange the following compounds in order of increasing boiling temperature: butane (C₄H₁₀), propan-1-ol (C₃H₇OH), 2-methylpropane (C₄H₁₀). Justify your answer.
- 1
- First identify intermolecular forces for each compound: Propan-1-ol has hydrogen bonding, so it has the strongest intermolecular forces and highest boiling temperature.
- 2
- Butane and 2-methylpropane are isomers, so they have the same electron count. Butane is straight chain, so it has a larger surface area than branched 2-methylpropane, leading to stronger London forces.
- 3
- Order from lowest to highest boiling temperature: 2-methylpropane < butane < propan-1-ol.
Exam tip:
When comparing boiling temperatures of molecules, always first check for hydrogen bonding, then compare electron count for London force strength, then surface area for isomers.
4. Solubility and Solvent Choice★★★☆☆⏱ 5 min
The principle 'like dissolves like' applies: solutes dissolve in solvents with similar intermolecular force types. This is tested qualitatively in this unit, with no enthalpy calculations required.
Water (polar, H-bonding solvent): Dissolves ionic compounds via hydration of ions (attraction between δ+/- water molecules and ions), and small alcohols (which can form H-bonds with water). It is a poor solvent for halogenoalkanes and non-polar compounds, which cannot form strong interactions with water.
Non-aqueous solvents: Non-polar solvents (e.g. hexane) dissolve non-polar solutes (e.g. alkanes, iodine) via London forces. Polar non-aqueous solvents can dissolve polar solutes that cannot form H-bonds.
Explain why ethanol is miscible with water in all proportions, but 1-chlorobutane is insoluble in water.
- 1
- Ethanol has an -OH group that can form hydrogen bonds with water molecules, so strong interactions form between ethanol and water, replacing the H-bonds between pure water molecules.
- 2
- 1-chlorobutane is a polar molecule but cannot form hydrogen bonds with water, as it has no H bonded to N/O/F, and no N/O/F atom with lone pairs available to act as an H-bond acceptor. The interactions between 1-chlorobutane and water are much weaker than the H-bonds in pure water, so it does not dissolve.
5. Common Pitfalls
Wrong move:
Stating hydrogen bonds exist between any molecule containing H and O/N/F
Why:
Hydrogen bonds require both a donor (H covalently bonded to O/N/F) and an acceptor (lone pair on a separate O/N/F atom). For example, methanal (HCHO) has O with lone pairs but no H bonded to O, so no H-bonding between pure methanal molecules.
Correct move:
Always check both H-bond donor and acceptor are present before stating hydrogen bonding occurs.
Wrong move:
Ranking permanent dipole-dipole forces as always stronger than London forces
Why:
London force strength increases with electron count. For large non-polar molecules (e.g. octadecane, C₁₈H₃₈), London forces are far stronger than pd-pd interactions in small polar molecules like HCl.
Correct move:
Only apply the rank London < pd-pd < H-bond for molecules of comparable electron count/molecular mass.
Wrong move:
Referring to boiling 'point' instead of boiling 'temperature'
Why:
Edexcel uses the specific term 'boiling temperature' in mark schemes for this unit, and using 'point' may lead to lost marks in extended response questions.
Correct move:
Always use the exact term 'boiling temperature' when answering exam questions.
Wrong move:
Claiming branching reduces boiling temperature of alkanes by weakening covalent bonds
Why:
Branching affects intermolecular London forces, not the intramolecular covalent bonds, which remain unchanged regardless of structure.
Correct move:
State that branching reduces molecular surface area, weakening London forces between molecules, leading to lower boiling temperature.
Wrong move:
Stating ionic compounds dissolve in water because ionic bonds are 'weak'
Why:
Ionic bonds are very strong, but the energy released from hydration of ions (attraction between ions and polar water molecules) is sufficient to overcome the ionic lattice forces.
Correct move:
Explain solubility of ionic compounds in water via hydration of ions, not weak ionic bonds.
6. Quick Reference Cheatsheet
Force Type | Required Conditions | Relative Strength (similar size) | Key Property Effects |
|---|---|---|---|
London (instantaneous dipole-induced dipole) | All molecules, strength increases with electron count and surface area | Weakest | Higher electron count = higher boiling temperature; more branching = lower boiling temperature for alkanes |
Permanent dipole-permanent dipole | Polar molecules with permanent charge separation | Medium | Higher boiling temperature than non-polar molecules of similar size |
Hydrogen Bond | H covalently bonded to N/O/F, plus lone pair on separate N/O/F acceptor | Strongest | Very high boiling temperature; allows H-bonding molecules to dissolve in water; causes ice to be less dense than liquid water |
7. Frequently Asked
What is the order of strength of intermolecular forces for molecules of similar size?
For molecules with comparable molecular mass/electron count, intermolecular forces rank in order of increasing strength: London dispersion forces < permanent dipole-dipole interactions < hydrogen bonds. For very large non-polar molecules, London forces can be stronger than hydrogen bonds in small molecules.
Why does ice float on liquid water?
In ice, hydrogen bonds hold water molecules in an open tetrahedral lattice structure. This creates empty spaces in the solid, making ice less dense than liquid water, where hydrogen bonds are constantly breaking and reforming, packing molecules closer together.
Going deeper
What's Next
Now you have mastered intermolecular forces for Edexcel IAL Chemistry Unit 2, you can apply this knowledge to understand the reactivity and physical properties of the organic families covered in later topics. Intermolecular forces are a foundational concept that will reappear when you study rates of reaction, organic synthesis and analytical techniques in both Unit 2 and A2 Unit 4. You will also use your understanding of solubility to explain the outcomes of organic separation and purification practicals, which are assessed in both written papers and practical endorsement tasks. Make sure you can correctly identify intermolecular forces in any given molecule, as this is a common prerequisite for extended response questions on organic properties and reaction mechanisms.