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Intermolecular Forces, Liquids, and Solids

Intermolecular Forces, Liquids, and Solids. Chapter 11 BLB 11 th. Questions?. Why does water have such anomalous properties? Why do you add salt to the water before cooking pasta? How can ice melt below 0 °C? How does antifreeze work?.

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Intermolecular Forces, Liquids, and Solids

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  1. Intermolecular Forces, Liquids, and Solids Chapter 11 BLB 11th

  2. Questions? • Why does water have such anomalous properties? • Why do you add salt to the water before cooking pasta? • How can ice melt below 0°C? • How does antifreeze work? Answer: Solutions have different properties than pure solvents.

  3. 11.2 Intermolecular Forces • The attractive forces between molecules or between ions and molecules • Most properties of liquids and solids (and solutions) are due to the strength of the intermolecular forces present. • Boiling point • Melting (freezing) point • ΔHvap°

  4. Types of Intermolecular Forces: In order from strongest to weakest: • Ion-dipole – attraction of an ion and the partial charges of a polar molecule (hydration of ionic salts) • Dipole-dipole – attraction between two polar molecules (water, HF, alcohols) • London Dispersion (induced dipole) – instantaneous attraction between two nonpolar molecules due the polarizability of the electron cloud.

  5. Ion-dipole

  6. Dipole-dipole

  7. London dispersion

  8. Hydrogen “bonding” • Special type of dipole-dipole interaction • Occurs between a H atom attached to an electronegative atom (N, O, F) on one molecule and a lone pair on an electronegative element on another molecule. (whew!) • Common in water(!) and alcohols (C–OH) • Very strong collectively

  9. Hydrogen-bonding examples Between same molecule Between different molecules

  10. Protein structure(α-helix)

  11. DNA(double helix)

  12. Hydrogen-bonding between base pairs in DNA

  13. Why is water so weird? • High specific heat (only NH3 higher) • High ΔHfus° (only NH3 higher) • Highest ΔHvap° • Highest surface tension • Higher boiling point than expected • Ice floats on water. Answer: Strong network of hydrogen bonding

  14. Hydrogen bonding in ice

  15. Boiling Points (See Table 11.3)

  16. 11.3 Some Properties of Liquids • Viscosity – resistance to flow • Surface tension – a measure of the inward forces that must be overcome in order to expand the surface area of a liquid • Cohesive – between molecules • Adhesive – attraction between substance and a surface (glass ⇒ water) • Capillary action

  17. 11.5 Vapor Pressure • In an open container: liquid → gas (evaporation) • In a closed container: liquid ⇌ gas (dynamic equilibrium) • Vapor pressure – pressure exerted by the vapor in equilibrium with the liquid or solid in a closed container at a given temperature

  18. Vapor Pressure

  19. Vapor Pressure, cont. • As T ↑, vapor pressure ↑. • When vapor pressure = atmospheric pressure, boiling occurs. • When pressure = 1 atm: normal boiling point • Volatile substances have a higher vapor pressure.

  20. Vapor Pressure

  21. 11.6 Phase Diagrams • Show graphically the relationship between the different phases of a substance. • Solid, liquid, gas regions • Normal melting and boiling points • Triple point • Critical point • Slope of solid-liquid line

  22. Generic Phase Diagram

  23. H2O vs. CO2

  24. H2O

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