Bonding
Intermolecular Forces
Forces between molecules — much weaker than bonds but determine boiling points
Intermolecular forces (IMFs) are attractive forces between molecules — much weaker than chemical bonds (which hold atoms within molecules). Three main types: (1) London dispersion (induced dipoles, all molecules) — weakest. (2) Dipole-dipole (permanent dipoles, polar molecules). (3) Hydrogen bonds (H bonded to N, O, F interacting with another N, O, F) — strongest. Determine: boiling/melting points, viscosity, surface tension, solubility, evaporation rate. Why ice floats, water has high specific heat, DNA holds together.
- London dispersion~0.05-40 kJ/mol; in all molecules
- Dipole-dipole~5-25 kJ/mol; polar molecules
- Hydrogen bond~10-40 kJ/mol; strongest IMF
- Ion-dipole~40-600 kJ/mol; ions in polar solvents
- Compared to bonds~10-100× weaker than covalent bonds
- DeterminesBoiling point, viscosity, solubility
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Why IMFs matter
- Boiling/melting points. Determine phase behavior.
- Solubility. Like dissolves like.
- Biology. DNA, proteins held by H-bonds.
- Surface tension. H-bonds explain water's high γ.
- Viscosity. Strong IMFs → viscous liquids.
- Vapor pressure. Strong IMFs → low VP.
- Materials. Polymer cohesion.
Common misconceptions
- IMFs and bonds are same. 10-100× weaker; different concept.
- All molecules H-bond. Need H attached to N, O, F.
- London forces only in nonpolar. In all molecules; dominant in nonpolar.
- Bigger molecule → more H-bonds. More dispersion; H-bonds depend on H/N/O/F.
- IMFs explain bond strength. Different concept; bonds are stronger.
- Polar always dissolves polar. Same IMF type compatibility, not exact match.
Frequently asked questions
What types of intermolecular forces exist?
Three main. (1) London dispersion (van der Waals): induced dipoles from temporary electron asymmetry. Present in all molecules; only force in nonpolar (e.g., He, CH₄). (2) Dipole-dipole: permanent dipoles attract opposite poles. Polar molecules (e.g., HCl, acetone). (3) Hydrogen bond: special dipole-dipole; H bonded to highly electronegative atom (N, O, F) attracts lone pair on another. Strongest IMF.
What are hydrogen bonds?
Special strong dipole-dipole. Hydrogen attached to highly electronegative atom (N, O, F) acquires significant partial positive charge. This + acts like a tiny proton attracted to lone pairs on other electronegative atoms. Energy ~10-40 kJ/mol — strong for IMF, weak for bond. Critical in: water, ice, DNA base pairing, protein structure.
Why does water have such high boiling point?
Hydrogen bonding. H₂O molecules form extensive H-bond network. Predicted BP of water (no H-bonds): ~-90°C. Actual: 100°C. Difference is hydrogen bond network. Same reason: ice floats (H-bonds hold molecules in open lattice when frozen → less dense than liquid); water has high specific heat (energy goes into breaking H-bonds).
How do IMFs affect boiling point?
Stronger IMFs → higher BP (more energy to separate molecules). HF (BP 19.5°C) higher than HCl (-85°C) due to H-bonds. Larger molecules have more dispersion forces → higher BP. CH₄ (-161°C) < C₂H₆ (-89°C) < C₈H₁₈ (126°C) — increasing dispersion. Polar molecules higher than nonpolar of similar mass.
What's the "like dissolves like" rule?
Polar solvents dissolve polar/ionic solutes; nonpolar dissolves nonpolar. Reason: similar IMFs allow mixing. Water (polar) dissolves NaCl (ions) — ion-dipole interactions. Hexane (nonpolar) dissolves oil (nonpolar) — both London forces. Water + oil don't mix because water's H-bonds + oil's London forces don't compatibly substitute.
How do IMFs compare to chemical bonds?
Much weaker. Covalent bond: ~150-1000 kJ/mol. IMF: ~5-40 kJ/mol. So IMFs break first when heating — molecule melts/boils. Bonds break only at much higher T (decomposition). Difference: bonds within molecule; IMFs between molecules.
What's the role in biology?
Critical. (1) DNA base pairing: H-bonds hold double helix (specific A-T 2 bonds, G-C 3). (2) Protein folding: H-bonds, hydrophobic interactions create 3D structure. (3) Cell membranes: hydrophobic effect organizes lipid bilayer. (4) Drug-receptor binding: combination of IMFs. Biology requires both H-bonds and hydrophobic effects.