Chapter 11 Kinetic Molecular Description of Liquids and Solids Noncovalent Forces Intermolecular interactions Electrostatic Forces Dispersion Forces H-bonding DNA, RNA 1 Mary J. Bojan Chem 110Kinetic Molecular Description of Liquids and Solids gas: Kinetic energy >> intermolecular forces Liquid: Kinetic energy ≅ intermolecular forces Solid: Kinetic energy << intermolecular forces 2 Kinetic Energy ∝ T Heating: T ↑, KE ↑ solid → liquid → gas Mary J. Bojan Chem 110States of Matter 3 The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: – The kinetic energy of the particles – The strength of the attractions between the particles Mary J. Bojan Chem 110Non covalent Interactions Relative strengths of intermolecular forces can be seen experimentally: higher boiling point (or melting point) ____________ intermolecular interactions IM FORCES: due to electrostatic attraction increase as intermolecular distances ________________ Strength of: chemical bonds intermolecular (IM) forces 4 Mary J. Bojan Chem 110Intermolecular (IM) Forces Strength of IM forces depends on: Q charge on ion µ dipole moment α Polarizability 5 α - polarizability: ease with which electron clouds become distorted α increases as number of electrons increases α increases as size (MW) increases Mary J. Bojan Chem 110TYPES OF INTERMOLECULAR FORCES Depends on Ion-Ion Q ionic bond Ion-Dipole Q, µ Ions in aqueous solutions of electrolytes Dipole-Dipole µ Ion-Induced Dipole Q, α ions in nonpolar solvents 6 Depends on Dipole-Induced Dipole µ, α polar molecules in nonpolar solvents Dispersion α induced-dipole induced-dipole Hydrogen Bonding directional must have H bonded dipole-dipole to N,O,F Mary J. Bojan Chem 110ELECTROSTATIC FORCES Ion-Ion: depends on ion charge Q and distance r Q = charge r = distance between charges Opposite charges ______________ STRONG interaction Called: 7 rQQE21∝Mary J. Bojan Chem 110Ion-Dipole Depends on ion charge Q, dipole moment, µ and r ELECTROSTATIC FORCES Example: salt dissolved in water. Ions: Na+ and Cl− Polar solvent; water 8 2rQEµ∝Mary J. Bojan Chem 110Dipole-Dipole depends on µ and r ELECTROSTATIC FORCES attraction repulsion Net effect averaged over time. Example: 9 δ +δ−δ +δ−δ +δ−δ +δ−321rEµµ∝Mary J. Bojan Chem 110DISPERSION FORCES Induced Dipoles: depends on α and r Ion neutral molecule Induce dipole London Dispersion forces = Induced dipoles - induced dipoles forces Ease of electronic distortion is polarizability: α 10 ++δ−δ+621rEαα∝Mary J. Bojan Chem 110London Dispersion Forces 11 SUMMARY electrons are in constant motion instantaneous dipole moment forms when there are more electrons on one side of the molecule instantaneous dipole moment induces an instantaneous dipole moment in a neighboring molecule THEY MOVE IN SYNC Mary J. Bojan Chem 110Dispersion Interactions 12 LDF in addition to dipole dipole dispersion forces are related to size shape BP =−185.7°C BP = −107.1°C SIZE: Size (number of electrons) is related to polarizability ArArH ClCl HArXeMary J. Bojan Chem 110HYDROCARBONS 13 ALKANES ALKENES ALKYNES H⎯C≡C⎯H CH3⎯C≡C⎯H AROMATIC HC HHHHC CHHHHHHC CHHHHCHHHCHHHHCCHHHCH3CCCCCCCHHHHHH CCCCCCHHHHHHMary J. Bojan Chem 110Dispersion Interactions 14 As size of hydrocarbon increases boiling point (and melting point) increase. Boiling Points of Alkanes-200-150-100-500501001502000 2 4 6 8 10Number of CarbonsBoiling PointAlkanes: hydrocarbons non polar LDF only Mary J. Bojan Chem 110Dispersion Interactions SHAPE 15 N-pentane BP = 36.2°C isopentane BP = 28°C Neopentane BP = 9.5°C C C C C CC C C CCC CCCCMary J. Bojan Chem 110Figure 11.07 16 Mary J. Bojan Chem 110Hydrogen Bonding • The dipole-dipole interactions experienced when H is bonded to N, O, or F (the most electronegative elements) are unusually strong. When hydrogen is bonded to N, O, F the hydrogen nucleus is exposed. H-bonding: IM force BUT “directional” like covalent bonds. 17 Mary J. Bojan Chem 110Hydrogen Bonding 18 H bonds are: • strong as intermolecular forces go: ~ 4-25kJ/mole • weak compared to covalent bonds, but are “directional” like covalent bonds. Mary J. Bojan Chem 110 Water is especially interestingHow do you know which has the strongest Intermolecular (IM) forces? 19 • compare boiling point (BP) and melting (MP) higher BP (MP) means stronger IM forces • If substances are similar, follow trends. Eg. inert gases, shape is the same IM forces increase as MW increases (as polarizability increases) • If trends compete, need BP or MP Eg. CH3Cl −24.2°C µ increases CH3Br 3.6°C size increases CH3I 42.4°C (polarizability) BP follows trend in size: London Dispersion forces dominate here Mary J. Bojan Chem 110Biological polymers • Polymers: high molecular weight materials formed from many small molecules called monomers. • Monomer = repeating unit 20 Proteins: Monomers = amino acids Primary Structure of protein = sequence of amino acids Mary J. Bojan Chem 110H-bonding in Proteins 21 Secondary structure is influenced by H-bonding Mary J. Bojan Chem 110DNA, RNA Monomer = nucleotides 22 Polymer = DNA or RNA = chain of nucleotides Mary J. Bojan Chem 110H-bonding in DNA H-bonding holds alpha-helix together. 23 Mary J. Bojan Chem 110 4 bases for DNA come in complementary pairs, based on: H-bondingH-bonding in DNA And plays key role in replication 24 Mary J. Bojan Chem
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