Intermolecular Forces Lecture Notes

Overview

• Intermolecular Forces: Responsible for holding solids and liquids together; present in real gases but not in ideal gas models.
• Comparison to Chemical Bonds: Intermolecular forces are weaker, usually 10s-20s kJ/mol compared to hundreds of kJ/mol for chemical bonds.

Types of Intermolecular Forces

• Ion-Dipole Interactions: Example is saltwater.
• Dipole-Dipole Interactions: Involves polar molecules with asymmetric symmetry.
• Hydrogen Bonding: A special kind of dipole-dipole interaction:
  • Involves hydrogen bonded to nitrogen, oxygen, or fluorine.
  • Significant electronegativity difference leads to strong attraction.

Dipole-Induced Dipole Interactions

• Occurs between polar and nonpolar substances.
• Example: Oxygen and iodine dissolving in water, despite nonpolar nature.
  • Nonpolar substances can dissolve in polar solvents due to induced dipoles.
  • Water induces a dipole in the electron cloud of nonpolar molecules like oxygen.

Polarization

• Process: Interaction with a polar molecule distorts the electron cloud of a nonpolar molecule.
• Result: Causes temporary polarity due to asymmetric electron distribution.
• Example: Oxygen's electron cloud is distorted, leading to polarization, crucial for dissolved oxygen in water (e.g., in fish habitats).

Factors Influencing Polarization

• Molecule Size: Larger molecules with bigger electron clouds are more easily polarized.
  • Example: Iodine (I2) is more polarizable than Chlorine due to larger atomic radius.
• Electrons' Distance from Nucleus: Electrons further from the nucleus are less tightly held and more easily polarized.

Polarizability

• Defined as the ability of an electron cloud to be distorted.
• Larger molecules or atoms tend to have higher polarizability due to loosely held outer electrons.
• Examples:
  • Nonpolar diatomic molecules like hydrogen, nitrogen, oxygen.
  • Small chain organic molecules and compounds like carbon dioxide.

Additional Notes

• Polarizability increases with molar mass.
• Not limited to diatomic molecules; applies to various small nonpolar molecules.

Question Prompt

• Which of the following is more polarizable or the most polarized? (This is left as an exercise or study question for the reader.)