Chapter 11 Kinetic Molecular Description of Liquids and Solids Noncovalent Forces Intermolecular interactions Electrostatic Forces Dispersion Forces H bonding DNA RNA Mary J Bojan Chem 110 1 Kinetic Molecular Description of Liquids and Solids gas Kinetic energy intermolecular forces Liquid Kinetic energy intermolecular forces Solid Kinetic energy intermolecular forces Kinetic Energy T solid Mary J Bojan Heating T KE liquid Chem 110 gas 2 States of Matter 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 110 3 Non covalent Interactions Strength of chemical bonds intermolecular IM forces 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 Mary J Bojan Chem 110 4 Intermolecular IM Forces Strength of IM forces depends on Q charge on ion dipole moment Polarizability polarizability ease with which electron clouds become distorted increases as number of electrons increases increases as size MW increases Mary J Bojan Chem 110 5 TYPES OF INTERMOLECULAR FORCES Depends on Depends on Ion Ion ionic bond Q Ion Dipole Ions in aqueous solutions of electrolytes Q Dipole Dipole Ion Induced Dipole ions in nonpolar solvents Mary J Bojan Q Dipole Induced Dipole polar molecules in nonpolar solvents Dispersion induced dipole induced dipole Hydrogen Bonding must have H bonded to N O F Chem 110 directional dipole dipole 6 ELECTROSTATIC FORCES Ion Ion depends on ion charge Q and distance r Q1Q2 E r Q charge r distance between charges Opposite charges STRONG interaction Called Mary J Bojan Chem 110 7 ELECTROSTATIC FORCES Ion Dipole Depends on ion charge Q dipole moment and r Q E 2 r Example salt dissolved in water Ions Na and Cl Polar solvent water Mary J Bojan Chem 110 8 ELECTROSTATIC FORCES Dipole Dipole depends on and r attraction 1 2 E 3 r repulsion Net effect averaged over time Example Mary J Bojan Chem 110 9 DISPERSION FORCES Induced Dipoles depends on and r Ease of electronic distortion is polarizability Ion neutral molecule 1 2 E 6 r Induce dipole London Dispersion forces Induced dipoles induced dipoles forces Mary J Bojan Chem 110 10 London Dispersion Forces 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 110 11 Dispersion Interactions H Cl Ar Ar Cl LDF in addition to dipole dipole H dispersion forces are related to SIZE Ar Xe size shape BP 185 7 C BP 107 1 C Size number of electrons is related to polarizability Mary J Bojan Chem 110 12 HYDROCARBONS H ALKANES H C H H H H H ALKENES H C H H H H C C C H H H C H CH 3 CH3 C C H H H C C C C H H H H C H C C H H Mary J Bojan H C H AROMATIC C H H H ALKYNES C H H C C H H Chem 110 H H C C C C H C C H H 13 Dispersion Interactions hydrocarbons non polar LDF only 200 150 100 Boiling Point Alkanes Boiling Points of Alkanes 50 0 50 100 150 200 0 2 4 6 8 10 Number of Carbons As size of hydrocarbon increases boiling point and melting point increase Mary J Bojan Chem 110 14 Dispersion Interactions SHAPE N pentane BP 36 2 C C isopentane BP 28 C C C C C C C C C C C Neopentane BP 9 5 C C C C C Mary J Bojan Chem 110 15 Figure 11 07 Mary J Bojan Chem 110 16 Hydrogen Bonding H bonding IM force BUT directional like covalent bonds 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 Mary J Bojan Chem 110 17 Hydrogen Bonding H bonds are strong as intermolecular forces go 4 25kJ mole weak compared to covalent bonds but are directional like covalent bonds Water is especially interesting Mary J Bojan Chem 110 18 How do you know which has the strongest Intermolecular IM forces 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 CH3Br CH3I 24 2 C 3 6 C 42 4 C increases size increases polarizability BP follows trend in size London Dispersion forces dominate here Mary J Bojan Chem 110 19 Biological polymers Polymers high molecular weight materials formed from many small molecules called monomers Monomer repeating unit Proteins Monomers amino acids Primary Structure of protein sequence of amino acids Mary J Bojan Chem 110 20 H bonding in Proteins Secondary structure is influenced by H bonding Mary J Bojan Chem 110 21 DNA RNA Polymer DNA or RNA chain of nucleotides Monomer nucleotides Mary J Bojan Chem 110 22 H bonding in DNA 4 bases for DNA come in complementary pairs based on H bonding H bonding holds alphahelix together Mary J Bojan Chem 110 23 H bonding in DNA And plays key role in replication Mary J Bojan Chem 110 24
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