12/6 IMF lecture

Dipole-Dipole Interactions

  • Definition: Attraction of polar molecules (those with dipoles).

  • Boiling Point Influence: Strong dipole-dipole interactions in the liquid phase make it difficult to form a gas, resulting in higher boiling points (BP).

    • Relates to enthalpy of vaporization (DHvap):

      • Liquid to gas: positive value (breaking intermolecular forces).

      • Gas to liquid: negative value (forming intermolecular forces).

  • Molar Mass & Polarity: While molar mass generally indicates boiling point, polarity affects this significantly. Two compounds with the same molecular weight but different polarities may have drastically different boiling points.

    • Example:

      • Br2 = 59°C (molecular weight ~160 g/mol)

      • ICl = 97°C (molecular weight ~160 g/mol)

Nature of Dipole-Dipole Forces

  • Strength: Dipole-dipole forces are relatively strong compared to other intermolecular forces and preferentially occur.

  • Solubility Principle: Reflects the "like dissolves like" principle in chemistry, where polar compounds dissolve in polar solvents.

    • Example: Water (polar) and ethanol (CH3CH2OH) mix due to polar -OH groups, whereas water and oil (non-polar) do not mix due to differing intermolecular forces.

Hydrogen Bonding

  • Definition & Strength: Hydrogen bonding is stronger than typical dipole-dipole interactions (5-30 kJ/mol) but weaker than covalent bonds (about 1/10 of covalent strength).

  • Self-Interaction: Molecules like H2O, NH3, and CH3CH2OH can both donate and accept hydrogen bonds.

  • Boiling Point Examples:

    • Example BP:

      • H2O = 100°C

      • H2Se = -40°C

Effects of Hydrogen Bonding

  • Water's Unique Properties: Water is essential for life largely due to hydrogen bonding.

    • Ice floats because hydrogen bonds create an open lattice structure, trapping empty space.

    • Lakes and rivers freeze from the top down, allowing aquatic life to survive underneath.

  • Molecular Comparison:

    • H2O: 18 g/mol BP +100°C

    • H2Te: 130 g/mol BP -2°C

    • H2S: 34 g/mol BP -58°C

Induced Dipole-Dipole Interactions

  • Definition: Occurs when a non-polar molecule becomes polarized in the presence of a polar molecule.

  • Polarizability: Larger orbitals or π-systems often have higher polarizability: F2 < Cl2 < Br2 < I2.

Induced Dipole - Induced Dipole Interactions

  • Definition: Fleeting interactions from induced dipoles among non-polar solids, referred to as "London Dispersion Forces."

  • General Characteristics:

    • Common to all molecules but weak compared to stronger intermolecular forces.

    • Influenced by molecular weight and polarizability.

Molecular Weight and Boiling Points

  • Halogen Examples:

    • F2: MW 38, BP -188°C

    • Cl2: MW 71, BP -34°C

    • Br2: MW 160, BP 59°C

    • I2: MW 254, BP 184°C

  • Trends: As molecular weight and polarizability increase, induced dipole-induced dipole interactions strengthen, leading to higher boiling points.

Non-Polar Hydrocarbons: Molecular Weight Influence

  • For linear hydrocarbons of similar polarizability, molecular weight is the key factor for boiling point.

    • Octane: 126°C, 114.23 g/mol

    • Heptane: 98°C, 100.20 g/mol

    • Hexane: 69°C, 86.18 g/mol

    • Pentane: 36°C, 72.15 g/mol

    • Butane: -0.5°C, 58.12 g/mol

Non-Polar Hydrocarbons: Shape Influence

  • Branching Impact: Structure and degree of branching significantly affect boiling points.

    • Examples:

      • Pentane (BP 36°C)

      • 2-Methylbutane (BP 28°C)

      • 2,2-Dimethylpropane (BP 9°C)

Summary of Intermolecular Forces (IMFs)

  • To determine the most significant IMF in a pure substance, visualize how two of those molecules interact:

    • Ions = ion interactions

    • Polar molecules = dipole interactions

    • Non-polar molecules = induced dipole interactions

  • Example: Between hexane and iodine (both non-polar), the significant forces are induced dipole-induced dipole.

Solubility Principles

  • Principle: A solute is soluble in a solvent when the solvent can form intermolecular forces with the solute.

  • Miscibility: Involves the same concept for two liquids.

    • Example combinations:

      • Water and CH3OH (soluble)

      • Water and C6H14 (not miscible)

  • I2 Solubility: I2 is soluble in both water and C6H14 but preferentially dissolves in hexane.

Applications in Diving

  • Health Risks: SCUBA divers can experience health issues due to nitrogen (N2) solubility at depth; it can "boil" out if ascent is too rapid.

  • Gas Alternatives: Deep-sea divers use Helium/Oxygen instead of Nitrogen/Oxygen since He has lower solubility.

Gases Used in SCUBA Diving

  • Composition:

    • Diving air: 21% oxygen, 78% nitrogen.

    • Nitrox: Reduced N2 content to prevent decompression sickness.

    • Heliox: Replacement of N2 with He for deeper dives to avoid nitrogen narcosis.

Lipids and Structure

  • Formation: Complex lipids form structures through intermolecular forces to lower energy states.

  • Regions of Lipid Molecules:

    • Non-polar region (tail)

    • Polar region (head)

  • Examples:

    • Liposome, Micelle, Triglyceride, Phospholipid.