Understanding Intermolecular Forces (IMF) in Chemistry

Introduction to Intermolecular Forces

• Definition: Intermolecular forces (IMF) are attractions between molecules.
• Contrast with Intramolecular Forces:
  • Intermolecular forces differ from intramolecular forces, which are the bonds within a molecule (e.g., covalent bonds between O and H in water).

Types of Intermolecular Forces

1. London Dispersion Forces
   • Present in all molecules.
2. Dipole-Dipole Forces
   • Occur in polar molecules.
3. Hydrogen Bonds
   • Present when H is directly attached to F, O, or N.

Determining Intermolecular Forces

• Nonpolar Compounds: Only have London dispersion forces.
• Polar Compounds: Have both London dispersion and dipole-dipole forces.
• Compounds with H attached to F, O, or N: Have all three forces (London dispersion, dipole-dipole, and hydrogen bonding).

Example Problems

Carbon Dioxide (CO₂)

• Lewis Structure: C with double bonds to O.
• Polarity: Nonpolar (central atom has no lone pairs and surrounding atoms are the same).
• IMF Present: Only London dispersion forces.

Sulfur Tetrafluoride (SF₄)

• Lewis Structure: S with four F atoms and a lone pair on S.
• Polarity: Polar (central atom has a lone pair).
• IMF Present: London dispersion and dipole-dipole forces.

Methylamine (CH₃NH₂)

• Observation: H attached to N.
• IMF Present: All three forces (London dispersion, dipole-dipole, and hydrogen bonding).

Pentane (C₅H₁₂)

• Composition: Only carbon and hydrogen.
• Polarity: Nonpolar.
• IMF Present: Only London dispersion forces.

Neon (Ne)

• Type: Single element.
• Polarity: Nonpolar.
• IMF Present: Only London dispersion forces.

Hydrogen Chloride (HCl)

• Lewis Structure: H-Cl.
• Polarity: Polar (dipole moment from H to Cl).
• IMF Present: London dispersion and dipole-dipole forces.

Conclusion

• Steps to Determine IMF:
  1. Draw the Lewis Structure.
  2. Determine if the compound is polar or nonpolar.
  3. Check for the presence of hydrogen bonds if applicable.
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