02Water - Bio 20A-M. Dalbey Assigned Reading: WATER...

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Unformatted text preview: Bio 20A-M. Dalbey Assigned Reading: WATER (Chapter 2) Chapter 2 Section 2.4 1/7/09 p. 31 The structure of the water molecule 2-D R esen ion o the W r M cul epr tat s f ate ole e H HO ~105o [theoretical = ~110o] X X δ+0.33 H H Oδ δ+0.33 -0.33 δ-0.33 Therepr sen on o th le d notshowthe2 e tron inthe1sorbialof e tati n e ft oes lec s t Oxyg n b usetheyare no " enceEectr ns"(ie. t eydo notpar patein e eca t Val lo h tici bonding ). Theserep resen onsobscu t e rue 3 te tati re h t D trahe str ctur ofwat r. dral u e e HYDROGEN BONDING Linus Pauling developed concept of H-bonding to explain the unusual properties of liquid H2O. H bonds are 10X stronger than VDW interactions in ordinary liquids, 10X weaker than covalent bonds. Note that in a H2O dimer, both molecules are free to rotate about the axis of the H bond. Association of 2 Water Molecules by Intermolecular Hydrogen Bo ding n ~0.10 nm 460 kJ / m e ol ~0 8 nm .1 20 kJ / mo le H OH H O 0.2 nm 8 H In an iso ed, H-bon lat dedpa of wat r mol cu , bot ir e e les arefreeto rot abo theH b ate ut ondaxi . s ICE A CPK model of 5 water molecules H-bonded in the typical tetrahedral lattice structure of ice. 1 of 4 Bio 20A-M. Dalbey WATER (Chapter 2) 1/7/09 Important properties of water • Cohesion, Adhesion ("Surface Tension"; "Capillary Action") and Elasticity • Specific Heat • Heat of Vaporization • Expansion on Freezing • *Solvent Properties • *Ionization pH Buffering • *Reactivity (hydrolysis / dehydration; oxidation / reduction) • Supercooling (not mentioned in the text) * Denotes those properties most important for Bio 20A. Other properties are more germane to ecology and physiology. The properties listed above should be understood as manifestations of the difference in electronegativities between Hydrogen and Oxygen, leading to polarity of the water molecule and to intermolecular Hydrogen Bonding. The structure of liquid water is sometimes said to contain "flickering microclusters" of ice. Liquid water is somewhere between an ideal liquid and ideal crystal in that it has about 3040% of the H bonds of ice at 25° C. WATER AS A SOLVENT ions and polar molecules = hydrophilic non-polar molecules = hydrophobic If a non-polar molecule is surrounded by water molecules, the ability of the water molecules to form hydrogen bonds to each other is diminished, and not compensated by their ability to interact with the solute molecule (as would be the case if the solute were polar or ionic). This amounts to forcing the water molecules into a more highly structured arrangement; i.e. causes a decrease in entropy for the system. To minimize the entropy decrease non-polar molecules tend to associate with each other rather than with the water molecules. This is the physical basis of the so-called hydrophobic effect. A nonpolar molecule in an aqueous environment decreases entropy. DISSOCIATION OF WATER MOLECULES Ionization of the water molecule can be viewed as an extreme manifestation of its polarity. This is the basis for defining pH values. 2 of 4 Bio 20A-M. Dalbey WATER (Chapter 2) pH = log 1 ⎡H+ ⎤ ⎣⎦ 1/7/09 = − log ⎡ H + ⎤ ⎣⎦ " Neutral " pH = log 1 ⎡10−7 ⎤ ⎣ ⎦ =7 ACID/BASE CONCEPTS Acid H+ Donor H+ Donor 2 e- Acceptor Arrhenius 1880's Brønstead-Lowry 1923 Lewis Base OH- Donor H+ Acceptor 2 e- Donor The Arrhenius concept is too limited because it fails to account for the properties of some important functional groups , notably the amino group. NH2 + H+ Base ↔ NH3 Acid The Lewis Acid/Base concept is useful in evaluating non-aqueous systems. This is not often necessary in biochemistry. For any acid/base dissociation: HA ↔ H+ + A- Buffers TheCar ate - Bic ona Buf er Sy bon arb te f stem CO 2 Air pKa= CO + H2O 2 H2CO 3 Carbon Dioxide Carbonic Ac d i 3.8 HCO3+ H+ Bicarbonate 10. 2 Wa r te CO -2 3 + H+ Carbonate Can you use this to explain why laboratory di water usually has a pH of between 5 and 6? Blood leaving the capillary circulation in the human lungs normally has a pH near value 7.6, while blood leaving capillary circulation in tissues has a pH value near 7.2. Can you explain the basis for this difference in pH? 3 of 4 Bio 20A-M. Dalbey WATER (Chapter 2) 1/7/09 Suggested Problems from the TEXT Chapter 2 SELF-QUIZ (p.37) #6, 7, 8, 9 FOR DISCUSSION #4 4 of 4 ...
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