Notes on Intermolecular Forces

Learning Outcomes

• Describe the types of Intermolecular Forces
• Explain the origin of Dipole-Dipole Interactions and London Dispersion Forces
• Explain the origin of Hydrogen Bonding
• Understand the importance of these forces in explaining physical and biological properties

Intermolecular Forces Overview

• Intermolecular forces influence physical properties like melting and boiling points.
• Melting Point: Temperature at which solid converts to liquid.
• Boiling Point: Temperature at which average kinetic energy of molecules equals the attractive energy of intermolecular interactions.
  • Normal boiling point occurs at a pressure of $1.013 imes 10^5 ext{ Pa}$.
• Vaporization: Liquid to gas conversion.
• Condensation: Gas to liquid conversion.
• Freezing Point: Temperature below which substances solidify, also at $1.013 imes 10^5 ext{ Pa}$.
• Both melting and boiling points depend on the strength of intermolecular forces.

Types of Intermolecular Forces

1. Dispersion Forces

• Attraction between negatively charged electron clouds and positively charged nuclei of neighboring molecules.
• Present in all substances, but is the weakest of the intermolecular forces.

2. Dipole-Induced Dipole Forces

• Occur when a polar molecule induces a dipole in a neighboring molecule.

3. Dipole-Dipole Forces

• Attraction between the negative end of one polar molecule and the positive end of another.
• A special case is the Hydrogen Bond, which is a strong type of dipole-dipole interaction.

London Dispersion Forces

• Polarisability: Larger electron clouds are more easily distorted, leading to stronger dispersion forces.
• Examples:
  • With an increase in the number of electrons, the boiling points of halogens increase, demonstrating strength from dispersion forces.
  • The boiling point of alkanes increases with carbon chain length indicating stronger dispersion due to increased polarisability.

Dipole-Dipole Forces

• Different polar molecules have different boiling points based on their dipole moments.
  • Example: Acetone has a higher boiling point (56 ºC) than Butane (0.5 ºC) despite similar molecular weights due to its significant dipole moment resulting from its polar C=O bond.

Hydrogen Bonds

• A special type of dipole-dipole interaction:
  • Occurs when the positive side of a dipole (H) is attached to highly electronegative atoms like N, O, or F, and interacts with a negative dipole on a neighboring molecule.
• Examples: Hydrogen bonds in water (H₂O) contribute to its unusual boiling point (373 K), which is higher than many similar compounds.

Importance of Intermolecular Forces

• The strength of intermolecular forces directly correlates with melting and boiling points.
• All molecules and nonpolar atoms have dispersion forces.
• Polar molecules have both London Dispersion Forces and Dipole-Dipole Interactions; those with hydrogen bonding conditions exhibit all three forces.

Applications in Biology

• Intermolecular forces play crucial roles in biological structures like DNA, where hydrogen bonds ensure base-pairing fidelity.
• Understanding the properties of organic bases in nucleic acids involves recognizing hydrogen bonding patterns within the molecules, contributing to DNA's overall structure and function.