9. Intermolecular Forces (3.4) (2025)

Intermolecular Forces Overview

• Focus on understanding intermolecular forces in covalent molecules.

Key Ideas Recap

• Polar Covalent Bond: Unequal sharing of electrons.

• Non-polar Covalent Bond: Equal sharing of electrons.

• Molecules can be identified as polar or non-polar based on charge symmetry.

Learning Intention

• Understand three main types of intermolecular forces: dispersion forces, dipole-dipole attractions, and hydrogen bonding.

• Success Criteria: Ability to define intramolecular and intermolecular forces, describe dispersion forces, compare strengths, and predict intermolecular forces in different molecules.

Intramolecular vs Intermolecular Forces

• Intramolecular Forces: Forces within molecules (e.g., covalent bonds).

• Intermolecular Forces: Forces between molecules (e.g., electrostatic attraction).

Electron Movement in Covalent Bonds

• Electrons are in constant motion, creating instantaneous dipoles at various times.

Dispersion Forces

• Result from instantaneous dipoles causing electrostatic attraction between molecules.

• Present in all molecules, regardless of polarity, but are weak in nature.

• Larger molecules have stronger dispersion forces due to more electrons.

Dipoles

• Dipole: Presence of two poles in a polar molecule formed by atoms with different electronegativities.

Dipole-Dipole Attraction

• Intermolecular force acting between polar molecules.

• Stronger than dispersion forces as they involve permanent dipoles.

Hydrogen Bonding

• Stronger dipole-dipole attraction between polar molecules involving hydrogen and F, O, or N.

• Hydrogen bonding is the strongest intermolecular force due to high electronegativity differences.

Identifying the Strongest Intermolecular Force

• Steps to determine the strongest intermolecular force present in molecules:

  • Hydrogen Bond: Requires hydrogen and F, O, or N.

  • Dipole-Dipole Bond: Asymmetrical charge (polar) without H bonds to F, O, N.

  • Dispersion Force: Present in all molecules (polar and non-polar).

Review Questions Overview

• Identify types of intermolecular forces in polar vs non-polar substances.

• Diagram the arrangement of water molecules in ice and identify strongest intermolecular force.

• Predict boiling points based on intermolecular forces.