Intermolecular Forces Lecture Notes

Key Topics Covered

• Ion-Ion Interactions
• Ion-Dipole Interactions
• Dipole-Dipole Interactions
• Hydrogen Bonds
• London Dispersion Forces (LDF) / Van der Waals Forces

Ion-Ion Interactions

• Opposite charges attract (e.g., sodium ion (Na⁺) and chloride ion (Cl⁻)).
• This attraction is called an electrostatic force.
• Electrostatic force is proportional to the product of charges and inversely related to the distance between them.
• Higher charges result in stronger ionic interactions.
• Smaller ion sizes result in stronger electrostatic forces.
• Lattice energy: Proportional to the product of charges and inversely related to the distance, affects melting points.
• Examples: Aluminum nitride (AlN) vs. magnesium oxide (MgO) - AlN has a higher melting point due to higher lattice energy.

Ion-Dipole Interactions

• Involve an ion interacting with a polar molecule.
• Dipole: Substance with two charges, one positive and one negative (e.g., carbon monoxide (CO)).
• Example: Sodium ion (cation) and water, chloride ion (anion) and water.
• These interactions help explain why water can dissolve salts like NaCl.

Dipole-Dipole Interactions

• Occur between two polar molecules.
• Example: Between two carbon monoxide molecules (CO).
• Bromine in HBr can also form dipole-dipole interactions due to different electronegativity values.
• Hydrogen Bonds: A strong type of dipole-dipole interaction, found in molecules like water.
  • Hydrogen bonds form between H and N, O, or F atoms from different molecules.

Hydrogen Bonds

• Strong dipole interactions between hydrogen and nitrogen, oxygen, or fluorine in different molecules.
• Important in water, leads to unique properties.
• **Intermolecular vs. Intramolecular Forces: **
  • Intermolecular: between two molecules (e.g., hydrogen bonds).
  • Intramolecular: within a molecule (e.g., covalent bonds).

London Dispersion Forces (LDF) / Van der Waals Forces

• Found in all molecules and atoms, but most significant in nonpolar molecules.
• Temporary dipoles: Form when electrons in an atom or molecule become unevenly distributed.
• Induced dipoles: Temporary dipoles that cause neighboring molecules to become dipoles.
• Weaker than other dipole interactions but present in all molecules.
• Significantly affect nonpolar molecules.

Strength of Intermolecular Forces

• Order of Strength:
  1. Ion-Ion Interaction
  2. Ion-Dipole Interaction
  3. Hydrogen Bond
  4. Dipole-Dipole Interaction
  5. London Dispersion Forces (LDF)

Practice Examples

• Magnesium oxide (MgO): Ion-ion interaction.
• Potassium chloride (KCl) and water: Ion-dipole interaction.
• Methane (CH₄): Only London dispersion forces since it is nonpolar.
• Carbon Dioxide (CO₂): London dispersion forces as it is nonpolar despite having polar bonds.
• Sulfur dioxide (SO₂): Dipole-dipole interaction as it is polar.
• Hydrofluoric Acid (HF): Hydrogen bonding.
• Methanol (CH₃OH) and Lithium Chloride (LiCl): Ion-dipole interaction.
• Formaldehyde (CH₂O) and Carbon Monoxide (CO): Dipole-dipole interactions.

Boiling Points and Molecular Size

• **Factors Affecting Boiling Point: **
  • Type of intermolecular force.
  • Size of molecules.
  • More electrons and larger size increases boiling point due to more London dispersion forces.
  • Branched alkanes have lower boiling points than straight-chain alkanes.

Solubility in Water

• Polar molecules: Generally more soluble in water (e.g., methanol).
• Nonpolar regions: Reduce solubility (e.g., longer hydrocarbon chains in alcohols).

Ranking Compounds by Boiling Point

• Example: Comparing HF, HBR, HI, and HCL
  • HF has the highest boiling point due to hydrogen bonds.
  • Larger molecules generally have higher boiling points due to more London dispersion forces.