Lesson 1: 3.1 Intermolecular Forces

What is the fundamental difference between an intramolecular bond and an intermolecular force?
How do the shape, polarizability, and size of a molecule dictate the magnitude of its London dispersion forces?

Intermolecular Force
Dipole-Dipole
London Dispersion Forces
Polarizability
Hydrogen Bonding
Ion-Dipole
Dipole Moment

Learning Objective 3.1.A; Suggested Skill 4.A (Model Analysis)

This topic establishes the foundational rules for physical properties by evaluating the four primary IMFs: London dispersion forces (LDFs), dipole-dipole interactions, hydrogen bonding, and ion-dipole forces. Students must realize that LDFs are present in all molecules and increase with the size of the electron cloud (polarizability). They must also master the structural requirements for hydrogen bonding (H explicitly covalently bound to N, O, or F).

Support: Provide an IMF flowchart/decision tree that walks students through identifying a molecule's traits (Is it nonpolar? --> LDF only. Does it have H bound to N, O, F? --> Hydrogen bonding).
Extension: Present pairs of isomers (e.g., n-pentane vs. neopentane) and challenge students to explain why the straight-chain structure has a higher boiling point based on surface-area contact and polarizability.

Consider the two substances ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3), which share the identical molecular formula C2H6O.

1. Identify all type(s) of intermolecular forces present in a pure sample of each substance.

2. Predict which substance will have a higher boiling point. Justify your choice based on the types and relative strengths of their intermolecular forces.