© 2006 Brooks/Cole - Thomson Inter- molecular Forces Have studied INTRA molecular forces— the forces holding atoms together to form molecules. Now turn to forces between molecules — INTER molecular forces. Forces between molecules, between ions, or between molecules and ions.

© 2006 Brooks/Cole - Thomson Ion-Ion Forces for comparison of magnitude Na + —Cl - in salt These are the strongest forces. Lead to solids with high melting temperatures. NaCl, mp = 800 o C MgO, mp = 2800 o C

© 2006 Brooks/Cole - Thomson Covalent Bonding Forces for comparison of magnitude C–H, 413 kJ/mol C=C, 610 kJ/mol C–C, 346 kJ/mol CN, 887 kJ/mol

© 2006 Brooks/Cole - Thomson Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydrated ions in water.

© 2006 Brooks/Cole - Thomson Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydrated ions in water.

© 2006 Brooks/Cole - Thomson Attraction Between Ions and Permanent Dipoles Many metal ions are hydrated. This is the reason metal salts dissolve in water.

© 2006 Brooks/Cole - Thomson Attraction between ions and dipole depends on ion charge and ion-dipole distance. Measured by ∆H for M n+ + H 2 O --> [M(H 2 O) x ] n kJ/mol -405 kJ/mol -263 kJ/mol Attraction Between Ions and Permanent Dipoles

© 2006 Brooks/Cole - Thomson Dipole-Dipole Forces Such forces bind molecules having permanent dipoles to one another.

© 2006 Brooks/Cole - Thomson Dipole-Dipole Forces Influence of dipole-dipole forces is seen in the boiling points of simple molecules. CompdMol. Wt.Boil Point N o C CO o C Br o C ICl16297 o C

© 2006 Brooks/Cole - Thomson 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

© 2006 Brooks/Cole - Thomson Hydrogen Bonding in H 2 O H-bonding is especially strong in water because the O—H bond is very polarthe O—H bond is very polar there are 2 lone pairs on the O atomthere are 2 lone pairs on the O atom Accounts for many of water’s unique properties.

© 2006 Brooks/Cole - Thomson Hydrogen Bonding in H 2 O Ice has open lattice-like structure. Ice density is < liquid. And so solid floats on water. Snow flake:

© 2006 Brooks/Cole - Thomson Hydrogen Bonding in H 2 O 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.

© 2006 Brooks/Cole - Thomson Hydrogen Bonding in H 2 O H bonds ---> abnormally high specific heat capacity of water (4.184 J/gK) 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.

© 2006 Brooks/Cole - Thomson Boiling Points of Simple Hydrogen-Containing Compounds Active Figure 13.8

© 2006 Brooks/Cole - Thomson Portion of a DNA chain Double helix of DNA 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 —adenine with thymine —guanine with cytosine —guanine with cytosine

© 2006 Brooks/Cole - Thomson Base-Pairing through H-Bonds

© 2006 Brooks/Cole - Thomson Base-Pairing through H-Bonds

© 2006 Brooks/Cole - Thomson FORCES INVOLVING INDUCED DIPOLES How can non-polar molecules such as O 2 and I 2 dissolve in water? The water dipole INDUCES a dipole in the O 2 electric cloud. Dipole-induced dipole

© 2006 Brooks/Cole - Thomson FORCES INVOLVING INDUCED DIPOLES Solubility increases with mass the gas

© 2006 Brooks/Cole - Thomson FORCES INVOLVING INDUCED DIPOLES Process of inducing a dipole is polarizationProcess of inducing a dipole is polarization Degree to which electron cloud of an atom or molecule can be distorted in its polarizability.Degree to which electron cloud of an atom or molecule can be distorted in its polarizability.

© 2006 Brooks/Cole - Thomson IM FORCES — INDUCED DIPOLES Consider I 2 dissolving in ethanol, CH 3 CH 2 OH. O H -  +  I-I R -  +  O H +  -  R The alcohol temporarily creates or INDUCES a dipole in I 2.

© 2006 Brooks/Cole - Thomson FORCES INVOLVING INDUCED DIPOLES Formation of a dipole in two nonpolar I 2 molecules. Induced dipole-induced dipole The induced forces between I 2 molecules are very weak, so solid I 2 sublimes (goes from a solid to gaseous molecules).

© 2006 Brooks/Cole - Thomson FORCES INVOLVING INDUCED DIPOLES The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight ---> larger induced dipoles. MoleculeBoiling Point ( o C) MoleculeBoiling Point ( o C) CH 4 (methane) CH 4 (methane) C 2 H 6 (ethane) C 2 H 6 (ethane) C 3 H 8 (propane) C 3 H 8 (propane) C 4 H 10 (butane) C 4 H 10 (butane) - 0.5

© 2006 Brooks/Cole - Thomson Boiling Points of Hydrocarbons Note linear relation between bp and molar mass. CH 4 C2H6C2H6C2H6C2H6 C3H8C3H8C3H8C3H8 C 4 H 10

© 2006 Brooks/Cole - Thomson Intermolecular Forces Summary

© 2006 Brooks/Cole - Thomson Equilibrium Vapor Pressure Active Figure 13.18

© 2006 Brooks/Cole - Thomson 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. 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 C 2 H 5 H 5 C 2 H H 5 C 2 H Hwateralcoholether increasing strength of IM interactions extensive H-bondsH-bonds dipole- dipole O O O

© 2006 Brooks/Cole - Thomson Liquids HEAT OF VAPORIZATION is the heat req’d (at constant P) to vaporize the liquid. LIQ + heat ---> VAP Compd.∆H vap (kJ/mol) IM Force H 2 O40.7 (100 o C)H-bonds SO (-47 o C)dipole Xe12.6 (-107 o C)induced dipole

© 2006 Brooks/Cole - Thomson 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.)

© 2006 Brooks/Cole - Thomson Phase Equilibria — Water Solid-liquid Gas-Liquid Gas-Solid

© 2006 Brooks/Cole - Thomson Triple Point — Water At the TRIPLE POINT all three phases are in equilibrium.

© 2006 Brooks/Cole - Thomson Phases Diagrams— Important Points for Water T(˚C)P(mmHg) Normal boil point Normal freeze point0 760 Triple point

© 2006 Brooks/Cole - Thomson Critical T and P Above critical T no liquid exists no matter how high the pressure. As P and T increase, you finally reach the CRITICAL T and P At P < 4.58 mmHg and T < ˚C solid H 2 O can go directly to vapor. This process is called SUBLIMATION This is how a frost-free refrigerator works.

© 2006 Brooks/Cole - Thomson Critical T and P COMPDT c ( o C)P c (atm) H 2 O CO CH Freon (CCl 2 F 2 ) Notice that T c and P c depend on intermolecular forces.

© 2006 Brooks/Cole - Thomson Solid-Liquid Equilibria Raising the pressure at constant T causes water to melt. The NEGATIVE SLOPE of the S/L line is unique to H 2 O. Almost everything else has positive slope. Solid H 2 O Liquid H 2 O P T 760 mmHg 0 °C Normal freezing point 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).