BCH461_Chapter2

BCH461_Chapter2 - BCH 461 Chapter 2 Water Outline Weak...

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BCH 461 Chapter 2 Water
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Outline • Weak interactions in Water • Ionization of water • Buffers • Water as a reactant • Water supports life
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Properties of Water Compared to Some Other Low-Molecular-Weight Compounds Compound Molecular Weight Melting Point (°C) Boiling Point (°C) Heat of Vaporization (kJ/mol) CH 4 16.04 -182 -164 8.16 NH 3 17.03 -78 -33 23.26 H 2 O 18.02 0 +100 40.71 H 2 S 34.08 -86 -61 18.66
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Structure of the Water Molecule
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(23 kJ/mol) (420 kJ/mol) Two Water Molecules Joined by H-bond
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Hydrogen bonds between water molecules H-bond donor and acceptor Near-tetrahedral bond angles Potential to form 4 H-bonds per molecule with neighboring molecules Extensive network of H-bonded water molecules makes liquid water: High boiling point, high heat of vaporization and surface tension.
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Hydrogen Bonding in Ice 4 H-bonds/water molecule High melting point and heat of melting.
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Ice versus Water Liquid Water (0 to 100 °C) Ice (-273 to 0 °C) This easily explains why the density of ice is slightly less than that of liquid water.
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Comparison of Ice and Water • Ice: 4 H-bonds/water molecule • Water: 2.3 H-bonds/water molecule • Ice: H-bond lifetime - about 10 μ s • Water: H-bond lifetime - about 10 ps
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Water as Solvent Solvent properties of water derive from its polar nature Ions are always hydrated in water and carry around a “hydration shell” Water forms H-bonds with polar solutes Hydrophobic interactions - a “secret of life”
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Hydration of Ions in Solution Hydrated Na + ion Hydrate d Cl - ion NaCl Crystal Water screens electrostatic interactions between dissolved ions, correlated with the high dielectric constant of water. Entropy increases. Ion-dipole interactions
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Alcohol Ketone Polar Compounds Dissolve in Water dipole-dipole interactions and H-bonding
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Common H-bonds in biological system (not unique to water) Water forms hydrogen bonds with polar solutes, making them readily soluble. • C-H does not form H-bond with water or with itself.
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Some important H-bonds Base-pairing α -helix and β -sheets
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Directionality of the H-bond Strongest when three atoms lie in a straight line. Precise 3D structure for protein and DNA: many inter and intra- molecular H-bonds.
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Nonpolar compounds are poorly soluble in water:
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Nonpolar compounds force energetically unfavorable changes in the structure of water • A nonpolar solute "organizes" water • The H-bond network of water reorganizes to accommodate the nonpolar solute • This is an increase in "order" of water • This is a decrease in the entropy of water molecules. Δ G = Δ H ± T Δ S. Δ H>0, Δ S<0, Δ G>0 (energetically unfavorable).
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Formation of a Cage Structure by Water Molecules Surrounding a nonpolar Solute Hydrophobic interaction
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Amphiphilic (Amphipathic) Molecules Refers to molecules that contain both polar and nonpolar groups Are attracted to both polar and nonpolar environments Good examples: fatty acids, lipids
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Sodium Palmitate
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