Lecture on Intermolecular Forces

Overview

• Focus on intermolecular forces including ion-ion, ion-dipole, dipole-dipole interactions, hydrogen bonds.
• Difference between inter and intramolecular forces.
• Discuss London dispersion forces and Van der Waals forces.
• Examples of compounds to determine types of interactions.

Ion-Ion Interactions

• Definition: Attraction between ions of opposite charges (electrostatic force).
• Dependence: Proportional to charge magnitude; inversely related to ion distance (lattice energy also considered).
• Example: Calcium ion (Ca²⁺) and oxide ion (O²⁻) have stronger interactions than sodium ion (Na⁺) and chloride ion (Cl⁻) due to higher charges.
• Comparison: Aluminum nitride (Al³⁺N³⁻) has a higher melting point than magnesium oxide (Mg²⁺O²⁻) due to higher charge product.

Ion-Dipole Interactions

• Definition: Interaction between an ion and a polar molecule.
• Example: Sodium ion (Na⁺) interacting with water; oxygen pulls electrons, creating a dipole.
• Anions: Chloride ion (Cl⁻) interacts with the hydrogen part of water.

Dipole-Dipole Interactions

• Definition: Interaction between two polar molecules.
• Example: Carbon monoxide (CO) molecules; oxygen is more electronegative, creating a dipole.
• Hydrogen Bonds: Special type of dipole-dipole interaction involving hydrogen and N, O, or F atoms. Found between separate molecules.

Difference Between Inter- and Intramolecular Forces

• Intermolecular Forces: Forces between separate molecules (e.g., hydrogen bonds).
• Intramolecular Forces: Forces within a single molecule (e.g., covalent bonds).

London Dispersion Forces and Van der Waals Forces

• Nature: Weak temporary dipole interactions, significant in non-polar molecules.
• Example: Neon atom with temporary dipole induces dipole in another atom.
• Strength: Weaker than regular dipole-dipole interactions.

Strength of Intermolecular Forces

1. Ion-Ion Interaction
2. Ion-Dipole Interaction
3. Hydrogen Bonds
4. Dipole-Dipole Interaction
5. London Dispersion Forces

Examples of Compounds and Interactions

• Magnesium Oxide (MgO): Ion-ion interaction.
• Potassium Chloride (KCl) and Water: Ion-dipole interaction.
• Methane (CH₄): Non-polar, London dispersion forces.
• Carbon Dioxide (CO₂): Linear, non-polar, London dispersion forces.
• Sulfur Dioxide (SO₂): Polar, dipole-dipole interaction.
• Hydrochloric Acid (HCl): Hydrogen bonds.

Boiling Point Comparisons

• Factors: Type of intermolecular force, size, polarity.
• Rankings:
  • I₂ > Br₂ due to larger size and more London dispersion forces.
  • Methanol (CH₃OH) > Methane (CH₄) due to hydrogen bonds.
  • Propanol (C₃H₇OH) > Methanol due to larger size and more dispersion forces.

Solubility in Water

• Methanol is more soluble in water than propanol due to smaller non-polar region.

Structural Influence on Boiling Point

• Pentane vs. Neopentane: Straight chain alkanes have higher boiling points due to larger surface area.

Conclusion

Understanding intermolecular forces and their implications on physical properties (melting point, boiling point, solubility) is crucial in chemistry.