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Unformatted text preview: Chapter 2 Water: The Medium of Life . . . . . . . . . . . . . . . . . . . . . . . . Chapter Outline v Properties of water: High boiling point, high melting point, high heat of vaporization, high surface tension, high dielectric constant, maximum density as a liquid: All due to ability of water to hydrogen bond v Water structure Electronegative oxygen, two hydrogens: Nonlinear arrangement: Dipole Two lone pairs on oxygen: H-bond acceptors Partially positively charged hydrogens: H-bond donors v Ice Lattice with each water interacting with 4 neighboring waters H-bonds: Directional, straight and stable v Liquid: H-bonds present but less than 4 and transient v Solvent properties of water High dielectric constant decreases strength of ionic interactions between other molecules Force of ionic interaction , F = e 1 e 2 /Dr 2 , inversely dependent on D Salts dissolve in water Interaction with polar solutes through H-bonds Hydrophobic interactions: Entropy-driven process minimizes solvation cage v Colligative properties: Freezing point depression, boiling point elevation, lowering of vapor pressure, osmotic pressure effects: Depend on solute particles per volume v Ionization of water Ions: hydrogen ion H + (protons), hydroxyl ion OH- , hydronium ion H 3 O + (protonated water) Ion product: K w = [H 2 O]xK eq = 55.5xK eq =10-14 = [H + ][OH- ] pH = -log 10 [H + ], pOH = -log 10 [OH- ], pH + pOH = 14 v Strong electrolytes: Completely dissociate: Salts, strong acids, strong bases v Weak electrolytes: Do not fully dissociate: Hydrogen ion buffers v Buffers Henderson-Hasselbalch equation: pH = pK a + log 10 ([A- ]/[HA]) Biological buffers: Phosphoric acid (pK 1 = 2.15, pK 2 = 7.2, pK 3 = 12.4); histidine (pK a = 6.04); bicarbonate (pK overall = 6.1) Good buffers: pK a s in physiological pH range and not influenced by divalent cations Chapter 2 . Water: The Medium of Life 12 Chapter Objectives Water Its properties arise because of the ability of water molecules to form H bonds and to dissociate to H + and OH- . Thus, water is a good solvent, has a high heat capacity and a high dielectric constant. Acid-Base Problems For acid-base problems the key points to remember are: Henderson-Hasselbalch: pH = pK a + log([A- ]/[HA]) Conservation of acid and conjugate base: [A- ] + [HA] = Total concentration of weak electrolyte added. Conservation of charge: [cations] = [anions] i.e., the sum of the cations must equal the sum of the anions. In many cases, simplifications can be made to this equation. For example, [OH- ] or [H + ] may be small relative to other terms and ignored in the equation. For strong acids, it can be assumed that the concentration of the conjugate base is equal to the total concentration of the acid. For example, an x M solution of HCl is x M in Cl- . Likewise for x M NaOH, the [Na + ] is x M....
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This note was uploaded on 01/21/2011 for the course ENG 167 taught by Professor Phillips during the Winter '07 term at UC Davis.
- Winter '07