# ch02 - Dimensional analysis How many seconds are there in...

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1/25/2011 1 Dimensional analysis year day day hour hour N 365 24 min 60 min sec 60 * * * = year sec 31536000 = How many seconds are there in one year? Water and Buffers Chapter 2 1. Unusual properties of water 2. Acids and Bases 3. Buffers

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1/25/2011 2 Atoms have an effective interaction distance, the van der Waals radius . It is the sum of attractive forces, typically dispersion, and repulsive forces as outer orbital overlap. This is the “6-12” potential field. Atom/ group vdw radius ( Å ) H 1.2 C 1.7 O 1.4 N 1.5 -CH3 2.0 ½ Ar 1.7 Atoms Bond length ( Å ) C-H 1.09 C-C 1.54 C=C 1.34 C-O 1.43 C-N 1.47
1/25/2011 3 Water is an unusual solvent Relative bond strengths O-H = 366 kJ/mol C-C = 348 kJ/mol H-bond ~20 kJ/mol Dispersion ~ 1 kJ/mol 2-1 Water Clathrate Note: O-O distance, about 2.8 Å, is same as van der Waals contact distance, but there is a shared hydrogen atom in there.

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1/25/2011 4 2-3,4 Note orientation of lone pairs Dissolution of salts (2-6) F = Q 1 Q 2 ε r 2 Q = magnitude of charges r = distance between charges ε = dielectric constant ε Water 78.5 EtOH 24 Benzene 4.6
1/25/2011 5 l 2-7 amphipathic Hydrophobic interaction Lipid bilayer

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1/25/2011 6 Proteins are solvated in an aqueous environment.
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