S2.2.9 Intermolecular forces and physical properties Notes

Relative Strengths of Intermolecular Forces and Their Impact on Properties

Types of Intermolecular Forces

1. While they are much weaker than covalent or ionic bonds, they strongly influence a substance’s physical properties.
2. The main types of intermolecular forces, ranked by increasing strength, are:
   1. London Dispersion Forces (LDFs): Found in all molecules, these forces arise from temporary dipoles caused by the random movement of electrons.
   2. Dipole-Dipole Forces: Occur between polar molecules with permanent dipoles.
   3. Hydrogen Bonding: A stronger type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F).
3. The relative strengths can be summarized as:
   $$
   \text{London Dispersion Forces}< \text{Dipole-Dipole Forces} < \text{Hydrogen Bonding}
   $$

Why Does Strength Vary?

The strength of intermolecular forces depends on:

1. Molecular Size and Shape (LDFs): Larger molecules with more electrons have stronger LDFs because their electron clouds are more easily polarized.
2. Polarity (Dipole-Dipole): Molecules with a greater difference in electronegativity between bonded atoms exhibit stronger dipole-dipole interactions.
3. Hydrogen Bonding: The strength depends on how electronegative the atom bonded to hydrogen is, as well as how many hydrogen bonds can form.

Properties Explained by Intermolecular Forces

Volatility

1. Volatility describes how easily a substance evaporates.
2. Substances with weaker intermolecular forces are more volatile because less energy is needed to overcome these forces.
   1. High Volatility: Non-polar molecules like methane ($CH_4$) with only weak LDFs evaporate easily.
   2. Low Volatility: Polar molecules like water ($H_2O$) with strong hydrogen bonds require more energy to escape the liquid phase.