12_Forces2011

12_Forces2011 - Physical properties of the phases of matter...

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164 Types of Bonding Physical properties of the phases of matter Liquids significant interaction between molecules higher density fixed volume variable shape moderate viscosity (resistance to flow) Solids strong interaction between molecules highest density fixed volume fixed shape high viscosity Gases little or no interaction between molecules low density variable volume (compressible) variable shape low viscosity
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165 Types of solids Ionic electrons localized on ions ions held in a rigid, brittle lattice by electrostatic forces Metallic delocalized valence electrons metal atom cores held in a deformable (ductile) lattice by sea of delocalized valence electrons Network covalent ( crystalline or amorphous ) electrons localized in covalent bonds extended (infinite) network of (polar or non-polar) covalent (directional) bonds , eg. graphite, diamond, sand (SiO 2 ) , glass Molecular e.g. H 2 O, I 2 , C 6 H 12 O 6 , CO 2 , C 60 , etc. low melting points discrete molecular units held together by intermolecular forces ...
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166 Intermolecular forces What forces hold molecules together to form molecular solids , or molecular liquids ? These forces are also responsible for non-ideal behavior in gases - non-ideal gas behavior is observed at sufficiently low temperature (approaching the boiling point of the liquid) or high pressure (approaching the vapor pressure of the liquid). Dipole moment interactions Recall that molecules with polar bonds and non- symmetric shapes have permanent dipole moments The negative end, or region, of one molecule will be attracted to the positive end, or region, of another This leads to ordered structure in solids and some ordering in liquids Larger dipole moments stronger forces
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167 Hydrogen bonding An extreme form of dipole-dipole interaction is called hydrogen-bonding, or H-bonding. When H is bonded to a very electronegative atom (F, O or N) there is a very large dipole associated with the X-H bond (egs. H 2 O, HF, NH 3 , CH 3 CH 2 OH, CH 3 NH 2 ) The positively charged H atom will be strongly attracted to negatively charged atoms on other molecules. This attraction is "directional" like a covalent bond H-bonding is responsible for the double helix of DNA & the ability of the cell to "read" DNA & produce the proteins encoded in the DNA H-bonding is responsible for the α -helix of proteins DNA double helix
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168 Dipoles may interact with ions stronger than dipole-dipole interactions eg. ions dissolved in water: hydration energy increases with decreasing size and increasing charge of ions - relates to solubility Hydration energy - eg. Li + (g) Li + (aq) H hydration Another eg. . .. soap dissolves grease (oil) in water because soap molecules are ionic at one end ~~~~~~~~ COO the non-polar part of the soap molecule interacts well the the grease - How do non-polar molecules interact?
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12_Forces2011 - Physical properties of the phases of matter...

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