Lesson 5.1: Intermolecular Forces

Key Concepts

• Intramolecular forces: Forces within a molecule, such as covalent bonds.
• Intermolecular forces: Forces between different molecules.
• Intermolecular forces are generally attractive forces based on electrostatic attraction (positive attracts negative).
• Ion-Ion Forces: Forces within ionic compounds where anions and cations attract each other.

Factors Influencing Ion-Ion Forces

• Larger charges result in stronger attraction.
• Closer proximity of ions leads to stronger attraction.

Example Comparisons

• Magnesium Oxide vs Magnesium Fluoride
  • MgO has a higher melting point due to stronger ion-ion forces from larger charges (O²⁻ vs F⁻).
• Magnesium Fluoride vs Magnesium Chloride
  • MgF₂ has a higher melting point than MgCl₂ because F⁻ is smaller than Cl⁻, allowing ions to be closer.

Intermolecular Forces in Covalent Compounds

• Dipole-Dipole Forces: Attraction between two polar molecules.
  • Example: Hydrogen chloride (HCl) has partial charges attracting each other.
• Ion-Ion Forces vs Dipole-Dipole Forces
  • Ion-ion forces are stronger due to larger charges.

Comparing Boiling Points

• Li₂O vs H₂O
  • Lithium oxide has a much higher boiling point than water due to strong ion-ion forces.
• H₂O vs H₂S
  • Water has a higher boiling point due to a stronger dipole (1.85 vs 0.97).
  • Water's strong dipole-dipole forces are called hydrogen bonds.

Hydrogen Bonds

• Not actual bonds but extra strong dipole-dipole attractions.
• Occur in molecules with N-H, O-H, or F-H bonds due to the small size of hydrogen.
• Significant in biological systems (e.g., DNA pairing).

London Dispersion Forces

• Present in nonpolar molecules due to instantaneous and induced dipoles.
• Weak but significant in large molecules due to extensive surface area.

Example Analysis

• Hydrocarbons: Larger hydrocarbons have higher boiling points due to increased London dispersion forces.
• Molecular Shape: Elongated molecules have greater surface area, resulting in stronger London dispersion forces.

Force Comparison Summary

• Covalent Bonds > Ionic Bonds > Hydrogen Bonds > Dipole-Dipole Forces > London Dispersion Forces

Practice Problem Analysis

• CH₂Cl₂: Has dipole-dipole forces and London dispersion forces but no hydrogen bonds.
• CO₂: Nonpolar, only has London dispersion forces.
• NH₃ (Ammonia): Has London dispersion, dipole-dipole forces, and hydrogen bonding (due to N-H bond).

Conclusion

• Intermolecular forces play a significant role in determining the physical properties of substances, such as boiling and melting points.
• Understanding these forces is crucial for predicting molecular behavior in different states and environments.