Inter- molecular Forces

1. 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.

2. 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

3. C=C, 610 kJ/mol C–C, 346 kJ/mol C–H, 413 kJ/mol CN, 887 kJ/mol Covalent Bonding Forcesfor comparison of magnitude

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

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

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

7. 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

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

9. 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

10. 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

11. 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.

12. Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid. And so solid floats on water. Snow flake: www.snowcrystals.com

13. Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid and so solid floats on water. One of the VERY few substances where solid is LESS DENSE than the liquid.

14. Hydrogen Bonding in H2O H bonds ---> abnormally high specific heat capacity of water (4.184 J/g•K) 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.

15. Boiling Points of Simple Hydrogen-Containing Compounds Active Figure 13.8

16. Double helix of DNA Portion of a DNA chain 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

17. Base-Pairing through H-Bonds

18. Base-Pairing through H-Bonds

19. 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

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

21. FORCES INVOLVING INDUCED DIPOLES • Process of inducing a dipole is polarization • Degree to which electron cloud of an atom or molecule can be distorted in its polarizability.

22. d - I-I I-I d The alcohol temporarily creates or INDUCES a dipole in I2. + d - d O - O R H R H d + d + IM FORCES — INDUCED DIPOLES Consider I2 dissolving in ethanol, CH3CH2OH.

23. FORCES INVOLVING INDUCED DIPOLES The induced forces between I2 molecules are very weak, so solid I2 sublimes (goes from a solid to gaseous molecules). Formation of a dipole in two nonpolar I2 molecules. Induced dipole-induced dipole

24. 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

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

26. Intermolecular Forces Summary

27. Equilibrium Vapor PressureActive Figure 13.18

28. ether alcohol water O O O C H H C H C H H 2 5 H 5 2 5 2 dipole- extensive dipole H-bonds H-bonds increasing strength of IM interactions Liquids Equilibrium Vapor Pressure FIGURE 13.18:VP as a function of T. 1. The curves show all conditions of P and T where LIQ and VAP are in EQUILIBRIUM 2. The VP rises with T. 3. When VP = external P, the liquid boils. This means that BP’s of liquids change with altitude. 4. If external P = 760 mm Hg, T of boiling is the NORMAL BOILING POINT 5. VP of a given molecule at a given T depends on IM forces. Here the VP’s are in the order

29. Liquids HEAT OF VAPORIZATION is the heat req’d (at constant P) to vaporize the liquid. LIQ + heat ---> VAP Compd. ∆Hvap (kJ/mol) IM Force H2O 40.7 (100 oC) H-bonds SO2 26.8 (-47 oC) dipole Xe 12.6 (-107 oC) induced dipole

30. Phase Diagrams Lines connect all conditions of T and P where EQUILIBRIUM exists between the phases on either side of the line. (At equilibrium particles move from liquid to gas as fast as they move from gas to liquid, for example.)

31. Phase Equilibria — Water Solid-liquid Gas-Liquid Gas-Solid

32. Triple Point — Water At the TRIPLE POINTall three phases are in equilibrium.

33. Phases Diagrams—Important Points for Water T(˚C) P(mmHg) Normal boil point 100 760 Normal freeze point 0 760 Triple point 0.0098 4.58

34. Critical T and P As P and T increase, you finally reach the CRITICAL T and P Above critical T no liquid exists no matter how high the pressure. At P < 4.58 mmHg and T < 0.0098 ˚C solid H2O can go directly to vapor. This process is called SUBLIMATION This is how a frost-free refrigerator works.

35. Critical T and P COMPD Tc(oC) Pc(atm) H2O 374 218 CO2 31 73 CH4 -82 46 Freon-12 112 41(CCl2F2) Notice that Tc and Pc depend on intermolecular forces.

36. P Liquid H O 2 Solid Normal H O 2 freezing point 760 mmHg 0 °C T Solid-Liquid Equilibria Raising the pressure at constant T causes water to melt. The NEGATIVE SLOPE of the S/L line is unique to H2O. Almost everything else has positive slope. The behavior of water under pressure is an example of LE CHATELIER’S PRINCIPLE At Solid/Liquid equilibrium, raising P squeezes the solid. It responds by going to phase with greater density, i.e., the liquid phase. In any system, if you increase P the DENSITY will go up. Therefore — as P goes up, equilibrium favors phase with the larger density (or SMALLER volume/gram).