Dispersion Forces fluctuations in the electron distribution in atoms and molecules result in a temporary dipole region with excess electron density has partial () charge region with depleted electron density has partial (+) charge the attractive forces caused by these temporary dipoles are called dispersion forces aka London Forces all molecules and atoms will have them as a temporary dipole is established in one molecule, it induces a dipole in all the surrounding molecules Tro, Chemistry: A Molecular Approach 1 Dispersion Force Tro, Chemistry: A Molecular Approach 2 Size of the Induced Dipole the magnitude of the induced dipole depends on several factors polarizability of the electrons volume of the electron cloud larger molar mass = more electrons = larger electron cloud = increased polarizability = stronger attractions shape of the molecule more surface-to-surface contact = larger induced dipole = stronger attraction Tro, Chemistry: A Molecular Approach 3 Effect of Molecular Size on Size of Dispersion Force Noble Gases are all increases, As the molar mass nonpolar atomic elements. the number of electrons increase. Therefore the strength of the dispersion forces increases. The stronger the attractive forces between the molecules, the higher the boiling point will be. Tro, Chemistry: A Molecular Approach 4 Tro, Chemistry: A Molecular Approach 5 Properties of Straight Chain Alkanes Non-Polar Molecules Name Molar Mass BP, C MP, C Density, g/mL Methane 16 -162 -183 0.47 Ethane 30 -89 -183 0.57 Propane 44 -42 -188 0.5 Butane 58 0 -138 0.58 Pentane 72 36 -130 0.56 Hexane 86 69 -95 0.66 Heptane 100 98 -91 0.68 Octane 114 126 -57 0.7 Nonane 128 151 -54 0.72 Decane 142 174 -30 0.74 Undecane 156 196 -26 0.75 Dodecane 170 216 -10 0.76 Tridecane 184 235 -5 0.76 Tetradecane 198 254 6 0.77 Pentadecane 212 271 10 0.79 Hexadecane 226 287 18 0.77 Tro, Chemistry: A Molecular Approach 6 Boiling Points of n-Alkanes Tro, Chemistry: A Molecular Approach 7 Effect of Molecular Shape on Size of Dispersion Force Tro, Chemistry: A Molecular Approach 8 Dipole Dipole Forces A relatively strong IMF takes place between polar molecules due to mutual interactions of their permanent dipole moments The molecules interact to align the charges to minimize the energy of the dipole moments + + - - - + + + + Three chloromethane molecules situated randomly in solution How would these molecules line up in a crystal of chloromethane? They would line up to minimized the energy of the dipole moments! Head to tail to head to tail to head to tail to head to head to tail to head Effect of Dipole-Dipole Attraction on Boiling and Melting Points Tro, Chemistry: A Molecular Approach 12 CH3CH2CH3 CH3-O-CH3 CH3 - CH=O CH3-CN Molar Mass 44.09 46.07 44.05 41.05 Boiling Dipole Point Size -42 C 0.08 D -24 C 1.30 D 20.2 C 2.69 D 81.6 C 3.92 D 13 Solubility depends on the attractive forces of solute and solvent molecules Like dissolves Like miscible liquids will always dissolve in each other Attractive Forces and Solubility polar substance dissolve in polar solvents hydrophilic groups = OH, CHO, C=O, COOH, NH2, Cl nonpolar molecules dissolve in nonpolar solvents hydrophobic groups = C-H, C-C Many molecules have both hydrophilic and hydrophobic parts - solubility becomes competition between parts Tro, Chemistry: A Molecular Approach 14 Immiscible Liquids Tro, Chemistry: A Molecular Approach 15 Polar Solvents Dichloromethane (methylene chloride) Ethanol Water (ethyl alcohol) Tro, Chemistry: A Molecular Approach 16 Nonpolar Solvents H3C H2 C C H2 H2 C C H2 CH3 HC HC CH3 C CH CH n-hexane C H toluene Cl Cl Cl C Cl carbon tetrachloride Tro, Chemistry: A Molecular Approach 17 When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it O-H, N-H, or F-H Hydrogen Bonding Since hydrogen has no other electrons, when it loses the electrons, the nucleus becomes deshielded exposing the H proton The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules Tro, Chemistry: A Molecular Approach 18 H-Bonding HF Tro, Chemistry: A Molecular Approach 19 H-Bonding in Water Tro, Chemistry: A Molecular Approach 20 Summary Dispersion forces are the weakest of the intermolecular attractions. Dispersion forces are present in all molecules and atoms. The magnitude of the dispersion forces increases with molar mass Polar molecules also have dipole-dipole attractive forces 21 Tro, Chemistry: A Molecular Approach Summary (cont'd) Hydrogen bonds are the strongest of the intermolecular attractive forces Hydrogen bonds will be present when a molecule has H directly bonded to either O , N, or F atoms a pure substance can have Ion-dipole attractions are present in mixtures of ionic compounds with polar molecules. Ion-dipole attractions are the strongest intermolecular attraction Ion-dipole attractions are especially important in aqueous solutions of ionic compounds Tro, Chemistry: A Molecular Approach 22 only example of H bonded to F is HF