Dipole–dipole forces | Intermolecular forces and properties | AP Chemistry | Khan Academy

TL;DR

The boiling point of a molecule is influenced by intermolecular forces. Propane and acetaldehyde have similar molar masses, but acetaldehyde has a higher boiling point due to its stronger permanent dipole.

Transcript

• [Instructor] So I have these two molecules here, propane on the left and acetaldehyde here on the right. And we've already calculated their molar masses for you, and you see that they have very close molar masses. And so based on what you see in front of you, which of these, you think, would have a higher boiling point, a sample of pure propane o... Read More

Key Insights

• 😥 Boiling points are influenced by intermolecular forces.
• 😥 London dispersion forces are related to molar mass and play a role in boiling points.
• 😥 Permanent dipoles contribute to dipole-dipole forces and affect boiling points.
• ✋ Asymmetric molecules have higher dipole moments and stronger dipole-dipole interactions.
• ❓ Dipole-dipole forces are in addition to London dispersion forces.
• ❓ Temporary dipoles can induce dipoles in neighboring molecules.
• ❓ Permanent dipoles can induce dipoles and attract other permanent dipoles.

Questions & Answers

Q: Why do molecules with higher intermolecular forces have higher boiling points?

Molecules with higher intermolecular forces require more energy to break free from each other, leading to higher boiling points.

Q: How do London dispersion forces contribute to boiling points?

London dispersion forces, which increase with molar mass, play a role in determining the boiling points of molecules.

Q: What is the difference between temporary and permanent dipoles?

Temporary dipoles occur due to random fluctuations in electron distribution, while permanent dipoles exist due to differences in electronegativity.

Q: How does asymmetry in a molecule affect its dipole moment?

Asymmetric molecules have a higher dipole moment as the individual dipole moments of bonds do not cancel each other out.

Summary & Key Takeaways

• Propane and acetaldehyde have similar molar masses but different boiling points.

• Boiling point depends on intermolecular forces, specifically dipole-dipole forces.

• Acetaldehyde has a higher boiling point due to its stronger permanent dipole compared to the symmetric propane.