Lecture Notes: Intermolecular Forces and Boiling Points

Overview

• Objective: Identify intermolecular forces in given substances and predict the highest boiling point.
• Substances Discussed: Alkane, Methanol, Ethanol

Intermolecular Forces in Substances

Molecule A: Alkane

• Structure: CH₃-CH₃
• Forces:
  • Dispersion Forces: Present
  • Dipole-Dipole: Not present (Nonpolar molecule)
  • Hydrogen Bonding: Not applicable
  • Ionic Interaction: None

Molecule B: Methanol

• Structure: CH₃OH
• Forces:
  • Dispersion Forces: Present
  • Dipole-Dipole: Present (Polar molecule)
  • Hydrogen Bonding: Present (OH group can form hydrogen bonds)
  • Ionic Interaction: None

Molecule C: Ethanol

• Structure: C₂H₅OH
• Forces:
  • Dispersion Forces: Present
  • Dipole-Dipole: Present (Polar molecule)
  • Hydrogen Bonding: Present (OH group can form hydrogen bonds)
  • Ionic Interaction: None

Predicting Boiling Points

Considerations

1. Intermolecular Forces:
   • List all possible forces for better interaction understanding.
   • More forces generally indicate higher boiling points.
2. Molecular Size:
   • Larger molecules have higher boiling points due to increased dispersion forces.
   • Larger molecules have more electrons and can exhibit more fluctuation in dispersion forces.

Conclusion

• Alkane: Lowest boiling point due to only dispersion forces and being nonpolar.
• Comparison of Methanol and Ethanol:
  • Both have similar intermolecular forces.
  • Ethanol is larger than Methanol.
  • Ethanol: Expected to have a higher boiling point due to larger size and stronger dispersion forces.

Summary

• Evaluate intermolecular forces and molecular size to predict boiling points.
• Larger molecules with more intermolecular forces tend to have higher boiling points.