Summary
Intermolecular forces are weaker than covalent and metallic bonds and include hydrogen bonding, van der Waals forces, and dipole-dipole interactions.
- Bond Polarity and Dipole Moments — The electronegativity of atoms in a molecule determines bond polarity and dipole moment. Example: In HCl, the dipole moment points from Cl to H due to Cl's higher electronegativity.
- Instantaneous Dipole-Induced Dipole Forces — Temporary dipoles in molecules cause adjacent molecules to form temporary dipoles, resulting in attraction. Example: London dispersion forces in noble gases like argon.
- Permanent Dipole-Permanent Dipole Forces — Attraction between two permanent dipoles in adjacent molecules. Example: The interaction between HCl molecules.
- Hydrogen Bonding — A strong type of dipole-dipole interaction between molecules containing N-H or O-H bonds. Example: Hydrogen bonds in water (H2O) and ammonia (NH3).
Exam Tips
Key Definitions to Remember
- Bond Polarity: Difference in electronegativity between atoms in a molecule.
- Dipole Moment: A measure of the separation of positive and negative charges in a molecule.
- Hydrogen Bonding: A strong intermolecular force between molecules containing N-H or O-H bonds.
Common Confusions
- Confusing hydrogen bonds with covalent bonds.
- Misunderstanding the difference between temporary and permanent dipoles.
Typical Exam Questions
- What is a dipole moment? A measure of the separation of positive and negative charges in a molecule.
- How do hydrogen bonds affect water's properties? They increase water's boiling and melting points and contribute to its high surface tension.
- What is the effect of branching on boiling points? Branching reduces surface area, decreasing boiling points due to weaker London dispersion forces.
What Examiners Usually Test
- Understanding of different types of intermolecular forces.
- Ability to explain how intermolecular forces affect physical properties.
- Comparison of boiling points based on molecular structure and intermolecular forces.
