Intermolecular Forces and Liquids Chap. 12

Inter- molecular Forces Have studied INTRAmolecular forces—the forces holding atoms together to form molecules. These are the forces that hold COVALENT bonds together. Now turn to forces between molecules — INTERmolecular forces. Forces between molecules, between ions, or between molecules and ions. Electrons are NOT shared in these interactions! Inter- molecular Forces

Ionic Bonds are an extreme case of intERmolecular forces Na+—Cl- in salt These are the strongest forces. Lead to solids with high melting temperatures. NaCl, mp = 800 oC Strength = 787kJ/mol MgO, mp = 2800 oC

Covalent Bonding intRAmolecular Forces for comparison of magnitude C=C, 610 kJ/mol C–C, 346 kJ/mol C–H, 413 kJ/mol CN, 887 kJ/mol

Dipole-Dipole Forces Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. Strengths that range from 5 kJ to 20 kJ per mole http://www.wisc-online.com/Objects/ViewObject.aspx?ID=GCH6804

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 IBr 206 116 oC

London Forces London forces are exhibited by nonpolar molecules because of the random, correlated movements of the electrons in interacting molecules.

H-bonding is strongest when X and Y are N, O, or F 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

H-Bonding Between Methanol and Water H-Bonding Between Ammonia and Water H-bond H-bond - + - - - +

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

Base-Pairing through H-Bonds

Liquids—Evaporation To evaporate, molecules must have sufficient energy to break IM forces. Breaking IM forces requires energy. The process of evaporation is endothermic.

Boiling Points of Simple Hydrogen-Containing Compounds See Active Figure 12.8

When molecules of liquid are in the vapor state, they exert a VAPOR PRESSURE EQUILIBRIUM VAPOR PRESSURE is the pressure exerted by a vapor over a liquid in a closed container when the rate of evaporation = the rate of condensation. Liquids PLAY MOVIE

Equilibrium Vapor Pressure See Active Figure 12.17

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