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WHY?

WHY?. Why is water usually a liquid and not a gas? Why does liquid water boil at such a high temperature for such a small molecule? Why does ice float on water? Why do snowflakes have 6 sides? Why is I 2 a solid whereas Cl 2 is a gas? Why are NaCl crystals little cubes?.

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WHY?

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  1. WHY? • Why is water usually a liquid and not a gas? • Why does liquid water boil at such a high temperature for such a small molecule? • Why does ice float on water? • Why do snowflakes have 6 sides? • Why is I2 a solid whereas Cl2 is a gas? • Why are NaCl crystals little cubes?

  2. Liquids, Solids & Intermolecular ForcesChap. 13

  3. Inter-molecular Forces Have studied INTRAmolecular forces—the forces holding atoms together to form molecules. Now turn to forces between molecules —INTERmolecular forces. Forces between molecules, between ions, or between molecules and ions.

  4. Ion-Ion Forcesfor comparison of magnitude Na+—Cl- in salt These are the strongest forces. Lead to solids with high melting temperatures. NaCl, mp = 800 oC MgO, mp = 2800 oC

  5. Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydratedions in water.

  6. Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydratedions in water.

  7. Attraction Between Ions and Permanent Dipoles • Many metal ions are hydrated. This is the reason metal salts dissolve in water.

  8. Attraction Between Ions and Permanent Dipoles Attraction between ions and dipole depends on ion charge and ion-dipole distance. Measured by ∆H for Mn+ + H2O --> [M(H2O)x]n+ -1922 kJ/mol -405 kJ/mol -263 kJ/mol

  9. Dipole-Dipole Forces Such forces bind molecules having permanent dipoles to one another.

  10. Dipole-Dipole Forces Influence of dipole-dipole forces is seen in the boiling points of simple molecules. Compd Mol. Wt. Boil Point N2 28 -196 oC CO 28 -192 oC Br2 160 59 oC ICl 162 97 oC

  11. Hydrogen Bonding A special form of dipole-dipole attraction, which enhances dipole-dipole attractions. H-bonding is strongest when X and Y are N, O, or F

  12. H-bond H-Bonding Between Methanol and Water - + -

  13. H-Bonding Between Two Methanol Molecules - + - H-bond

  14. H-Bonding Between Ammonia and Water - + - H-bond This H-bond leads to the formation of NH4+ and OH-

  15. Hydrogen Bonding in H2O H-bonding is especially strong in water because • the O—H bond is very polar • there are 2 lone pairs on the O atom Accounts for many of water’s unique properties.

  16. Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid. And so solid floats on water. Snow flake: http://www.its.caltech.edu/~atomic/snowcrystals/snow3x.jpg

  17. Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid and so solid floats on water.

  18. Hydrogen Bonding in H2O H bonds ---> abnormally high specific heat capacity of water (4.184 g/K•mol). This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms release huge energy.

  19. Hydrogen Bonding H bonds leads to abnormally high boiling point of water. See Screen 13.7

  20. Boiling Points of Simple Hydrocarbon Compounds

  21. Methane Clathrate

  22. Hydrogen Bonding in Biology H-bonding is especially strong in biological systems — such as DNA. DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O. Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases. —adenine with thymine —guanine with cytosine

  23. Double helix of DNA Portion of a DNA chain

  24. Base-Pairing through H-Bonds

  25. Double Helix of DNA

  26. Hydrogen Bonding in Biology Hydrogen bonding and base pairing in DNA. See Screen 13.6

  27. FORCES INVOLVING INDUCED DIPOLES How can non-polar molecules such as O2 and I2 dissolve in water? The water dipole INDUCES a dipole in the O2 electric cloud. Dipole-induced dipole

  28. FORCES INVOLVING INDUCED DIPOLES Solubility increases with mass the gas

  29. d - I-I I-I d + d - d O - O R H R H d + d + FORCES INVOLVING INDUCED DIPOLES • Consider I2 dissolving in alcohol, CH3CH2OH. The alcohol temporarily creates or INDUCES a dipole in I2.

  30. FORCES INVOLVING INDUCED DIPOLES Formation of a dipole in two nonpolar I2 molecules. Induced dipole-induced dipole

  31. FORCES INVOLVING INDUCED DIPOLES The induced forces between I2 molecules are very weak, so solid I2 sublimes (goes from a solid to gaseous molecules).

  32. FORCES INVOLVING INDUCED DIPOLES The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight ---> larger induced dipoles. Molecule Boiling Point (oC) CH4 (methane) - 161.5 C2H6 (ethane) - 88.6 C3H8 (propane) - 42.1 C4H10 (butane) - 0.5

  33. C4H10 C3H8 C2H6 CH4 Boiling Points of Hydrocarbons Note linear relation between bp and molar mass.

  34. Intermolecular Forces Summary

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