Intermolecular forces
Intermolecular forces
Bonds within molecules and forces between them
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Keep the location of an attraction clear. Covalent bonds hold atoms together within a molecule. Intermolecular forces attract one molecule to another.
Start with induced dipole-dipole attractions, which act between all atoms and molecules. Polar molecules also have permanent dipole-dipole attractions. Suitable molecules can form hydrogen bonds as well.
List all the forces present before comparing substances. Naming hydrogen bonding does not mean the other applicable forces disappear.
London dispersion is another name for induced dipole-dipole interactions. Van der Waals is also used as a broader term for induced and permanent dipole-dipole interactions; the specific name makes your meaning clearer.
- Intramolecular bond
- A bond within a particle, such as an O–H covalent bond within water.
- Intermolecular force
- An electrostatic attraction between separate molecules.
- Volatility
- The tendency to vaporise; higher volatility usually accompanies a lower boiling point.
| Interaction | Where it acts | Cause |
|---|---|---|
| Induced dipole-dipole (London dispersion) | All atoms and molecules | Instantaneous dipoles induce neighbouring dipoles. |
| Permanent dipole-dipole | In addition, between polar molecules | Opposite permanent partial charges attract. |
| Hydrogen bonding | H directly bonded to N/O/F and a neighbouring N/O/F lone pair | A lone pair attracts a strongly δ+ hydrogen on another molecule. |
What changes when iodine boils?
When iodine boils, identify the particles that separate and the attractions overcome. State whether the I–I covalent bonds break.
Worked answer
- Iodine contains discrete I2 molecules.
- Induced dipole-dipole attractions act between the molecules.
- Boiling overcomes those intermolecular attractions.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
(a) Define an induced dipole-dipole interaction. You may also use the term London dispersion force. [2 marks]
(b) Define a permanent dipole-dipole interaction. [2 marks]
(c) Define hydrogen bonding, including the atoms involved and the role of a lone pair. [2 marks]
Total: 6 marks
Show answer & marking points
(a) A fluctuation in electron density creates an instantaneous dipole [1], which induces an opposite dipole in a neighbouring particle and causes attraction [1]. [2]
(b) Opposite partial charges on neighbouring polar molecules attract [2]. [2]
(c) Hydrogen bonding is an intermolecular attraction involving H bonded to N, O or F [1] and a lone pair on N, O or F in another molecule [1]. [2]
(a) State the strongest intermolecular force present between molecules or atoms of each substance. Use the most specific OCR A term. [5]
| Substance | Strongest force |
|---|---|
| Ne | … |
| HCl | … |
| NH3 | … |
| CH3OH | … |
| CO2 | … |
(b) Explain why HCl molecules also experience induced dipole-dipole interactions. [2]
Total: 7 marks
Show answer & marking points
(a) Ne: induced dipole-dipole [1]; HCl: permanent dipole-dipole [1]; NH3: hydrogen bonding [1]; CH3OH: hydrogen bonding [1]; CO2: induced dipole-dipole [1]. [5]
(b) Every electron cloud fluctuates [1], so HCl also forms instantaneous dipoles that induce dipoles in neighbouring molecules [1]. [2]
Induced dipole-dipole interactions
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Electrons are always moving. For an instant, they can be distributed unevenly, giving one side of a particle a small negative charge and the other a small positive charge.
- This instantaneous dipole distorts the electron cloud of a neighbouring particle.
- That distortion creates an induced dipole in the neighbour.
- Opposite partial charges attract. New dipoles continually form as the electrons move.
A larger electron cloud is usually more easily distorted, or more polarisable. This tends to give stronger van der Waals attractions and a higher boiling point.
For isomers with the same electron count, more branching usually reduces surface contact between molecules. The overall attractions are then weaker, so less energy is needed to separate them.
- Induced dipole-dipole interaction
- Attraction between an instantaneous dipole in one particle and the dipole it induces in a neighbour.
| Factor | Effect |
|---|---|
| More electrons / larger cloud | Usually more polarisable, so larger temporary dipoles and stronger attractions. |
| Greater molecular contact | More electron-cloud contact can strengthen the overall attraction. |
I2 compared with Br2
Why does I2 have the higher boiling point?
Explain your answer in terms of intermolecular attractions and state which substance is more volatile.
Worked answer
I2 has more electrons and a larger, more polarisable cloud. Stronger induced dipole-dipole attractions require more energy to overcome.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
The boiling points of three halogens are shown.
| Substance | Boiling point / °C |
|---|---|
| Cl2 | −34.0 |
| Br2 | 58.8 |
| I2 | 184.3 |
- State the strongest intermolecular force between molecules of each halogen. [1]
- Explain the boiling-point trend from Cl2 to I2. [4]
- Predict whether F2 boils above or below −34.0 °C. Justify your answer. [2]
Total: 7 marks
Show answer & marking points
(a) Induced dipole-dipole / London dispersion forces [1]. [1]
(b) From Cl2 to I2 the molecules contain more electrons and have larger, more polarisable electron clouds [1]; instantaneous dipoles are larger [1], so induced dipole-dipole attractions are stronger [1] and more energy is needed to separate the molecules [1]. [4]
(c) F2 should boil below -34.0 °C [1] because it has fewer electrons / a less polarisable cloud and weaker induced dipole-dipole attractions than Cl2 [1]. [2]
Permanent dipole-dipole interactions
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A polar molecule has a permanent uneven distribution of charge. The δ+ end of one molecule attracts the δ− end of a neighbouring molecule.
These permanent dipole-dipole attractions act as well as induced dipole-dipole attractions. For otherwise comparable molecules, the extra attractions can increase the energy needed to separate them.
HCl has an overall dipole because it has one polar bond. SO₂ is bent, so its bond dipoles do not cancel. Use the shape as well as bond polarity when deciding whether this force is present.
Do not rank boiling points from polarity alone. A large non-polar molecule can have stronger overall attractions than a small polar molecule.
- Permanent dipole-dipole interaction
- Electrostatic attraction between the permanent δ+ end of one polar molecule and the permanent δ− end of a neighbour.
| Molecule | Interactions |
|---|---|
| Cl2 or linear CO2 | Induced dipole-dipole |
| HCl or bent SO2 | Induced plus permanent dipole-dipole |
HCl compared with F2
HCl and F₂ have similar numbers of electrons. Explain why HCl has the higher boiling point.
Worked answer
- Their electron counts provide a useful starting comparison for induced interactions.
- HCl is polar; F2 is non-polar.
- HCl also has permanent dipole-dipole attractions.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
(a) Draw two HCl molecules to show a permanent dipole-dipole attraction. Use solid lines for H-Cl bonds, a dotted line for the intermolecular attraction and correct partial charges. [4 marks]
(b) Name the dotted attraction. [1 mark]
(c) A student calls the dotted attraction 'another covalent bond'. Explain why this is incorrect. [2 marks]
Total: 7 marks
Show answer & marking points
(a) Two solid H-Cl covalent bonds [1]; H δ+ and Cl δ- on both molecules [1]; a dotted line between Cl δ- and H δ+ [1]; molecules aligned so opposite partial charges attract [1]. [4]
(b) Permanent dipole-dipole attraction [1]. [1]
(c) No electron pair is shared between the molecules [1]; the dotted line is an electrostatic intermolecular attraction and is much weaker than the intramolecular H-Cl covalent bond [1]. [2]