Intermolecular Forces Lecture Notes

Introduction to Intermolecular Forces

• Intermolecular forces are electrostatic interactions between molecules.
• They differ from covalent and ionic bonds within molecules.

Types of Intermolecular Forces

Ion-Ion Interactions

• Strongest intermolecular forces.
• Occur in ionic solids due to networks of ionic bonds involving formal charges.

Ion-Dipole Interactions

• Occur when ions interact with polar molecules.
• Example: Sodium chloride in water: Sodium ions interact with the negative side of water’s dipole, and chloride ions with the positive side.

Dipole-Dipole Interactions

• Happen between polar molecules when dipoles interact.
• Example: Water molecules in liquid form.
• Special case: Hydrogen Bonds: Strong dipole-dipole interactions involving NH, OH, or FH bonds.

Van der Waals (London Dispersion Forces)

• Weakest forces, yet universal as any substance can exhibit them.
• Arise from momentary dipoles which induce dipoles in neighboring atoms.
• More significant in large molecules like hydrocarbons.

Intermolecular Forces and Phase Changes

• Solid: Particles are close together and not moving.
• Liquid: Particles move but still interact closely.
• Gas: Particles move freely and don't interact much.
• Heat energy is required to overcome intermolecular forces and change phases.

Boiling Points and Intermolecular Forces

• The stronger the intermolecular forces, the more energy required to change phases.
  • Helium: Weak van der Waals forces; requires little energy to boil.
  • Water: Strong hydrogen bonds; requires significant energy to boil.
  • Sodium Chloride: Strong ion-ion interactions; requires a lot of energy to boil.

Determining Intermolecular Forces in Compounds

• Nonpolar Covalent Compounds: Only van der Waals forces.
• Polar Covalent Compounds: Geometry dictates overall dipole.
  • Example: Water has a dipole, carbon dioxide does not.
  • Molecular geometry affects overall polarity (e.g., BF3 is nonpolar, NH3 is polar).
• Formally Charged Ions: Engage in ion-ion interactions.

Conclusion

• To predict boiling points, determine the strength and type of intermolecular forces in compounds.
• Molecular geometry and composition influence the kind of interactions.

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