Intermolecular Forces and Their Impact

Introduction to Intermolecular Forces

• Intermolecular forces are interactions between molecules.
• They influence physical properties: boiling point, volatility, solubility, conductivity, and viscosity.
• Different from intramolecular forces which are the covalent bonds within a molecule.
• Types of intermolecular forces: London dispersion forces, dipole-dipole forces, and hydrogen bonding.

Covalent Molecules and Polarity

• Covalent molecules can be non-polar (e.g., diatomic elements) or polar (e.g., water).
• Polarity affects intermolecular forces and thus physical properties.

Types of Intermolecular Forces

1. London Dispersion Forces

• Present in all atoms and molecules.
• Caused by temporary dipoles due to electron movement within electron clouds.
• Larger atoms/molecules are more polarizable and exhibit stronger London dispersion forces.
• Example: Bromine and iodine have higher boiling points due to more electrons and stronger dispersion forces compared to chlorine and fluorine.

2. Dipole-Dipole Interactions

• Occur in polar molecules with permanent dipoles.
• Stronger than London dispersion forces.
• Example: Hexanol has a higher boiling point than hexane due to increased molecular polarity and stronger dipole-dipole interactions.

3. Hydrogen Bonding

• Not a true bond but a strong dipole interaction.
• Occurs when hydrogen is bonded to highly electronegative elements like nitrogen, oxygen, or fluorine.
• Example: Water's unique properties (e.g., high boiling point, surface tension, solvent ability) are due to hydrogen bonding.

Examination of Boiling Points

• Boiling point data highlights differences in intermolecular forces:
  • H2O, HF, and NH3 exhibit hydrogen bonding, leading to higher boiling points.
  • CH4 does not have hydrogen bonding and has a low boiling point due to only having London dispersion forces.
  • In Group 16, boiling points increase from H2S to H2Te due to increased London dispersion forces as molecule size increases.
  • Across Period 3: H2S, HCl, PH3, and SiH4 show differences in boiling points; SiH4 lacks dipole interactions and has the lowest boiling point.

Summary

• Intermolecular forces (IMFs) are weaker than covalent bonds but crucial for determining physical properties.
• London dispersion forces are the weakest but significant in large atoms/molecules.
• Dipole-dipole interactions are stronger and present in polar molecules.
• Hydrogen bonding is the strongest IMF, critical for properties like boiling points and biological functions.