H O H. Chapter 3: Outline-2. Chapter 3: Outline-1

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1 Chapter 3: utline-1 Molecular Nature of Water Noncovalent Bonding Ionic interactions van der Waals Forces Thermal Properties of Water Solvent Properties of Water ydrogen Bonds ydrophilic, hydrophobic, and amphipathic molecules smotic pressure 3P1-1 Chapter 3: utline-2 Ionization of Water Acids, bases, and p Buffers Physiological buffers 3P1-2 Water Solvent for all chemical reactions. Transports chemicals from place to place. elps to maintain constant body temperature. Part of digestive fluids. Dissolves excretion products. 3P Molecular Structure of Water The oxygen in water is sp 3 hybridized. ydrogens are bonded to two of the orbitals. Consequently the water molecule is bent. The -- angle is o. 3P1-4 Water Water is a polar molecule. A polar molecule is one in which one end is partially positive and the other partially negative. This polarity results from unequal sharing of electrons in the bonds and the specific geometry of the molecule. Water Water molecule with bond ( ( ) dipoles. ) and net 3P1-5 3P1-6 1

2 Water δ δ Water has an abnormally high boiling point due to intermolecular hydrogen bonding. bonding is a weak attraction between an δ electronegative atom in one molecule and an (on an or N) in another. 3P Noncovalent Bonding Ionic interactions ydrogen bonding Van der Waals forces Dipole-dipole Dipole-induced dipole Induced dipole-induced dipole 3P1-8 Typical Bond Strengths Type Covalent Noncovalent Ionic interactions ydrogen bonds van der Waals ydrophobic interactions kj/mol > P1-9 Ionic Interactions Ionic interactions occur between charged atoms or groups. In proteins, side chains sometimes form ionic salt bridges, particularly in the absence of water which normally hydrates ions. C 2 C 2 C N C C2 2 Salt bridge 3P1-10 ydrogen Bonding Water molecules hydrogen bond with one another. Four hydrogen bonding attractions are possible per molecule: two through the hydrogens and two through the nonbonding electron pairs. 3P1-11 Van der Waals Attractions a. Dipole-dipole b. Dipole-induced dipole c. Induced dipole-induced dipole C C C 3P1-12 2

3 ydrophobic interactions Nonpolar molecules tend to coalesce into droplets in water. The repulsions between the water molecules and the nonpolar molecules cause this phenomenon. The water molecules form a cage around the small hydrophobic droplets. 3P Thermal Properties ydrogen bonding keeps water in the liquid phase between 0 o C and 100 o C. Liquid water has a high: eat of vaporization-energy to vaporize one mole of liquid at 1 atm eat capacity-energy to change the temperataure by 1 o C Water plays an important role in thermal regulation in living organisms. 3P Water-solvent Properties Water dissolves chemicals that have an affinity for it, ie. hydrophilic (water loving) materials. -many ionic compounds -polar organic compounds These compounds are soluble in water due to three kinds of noncovalent interactions: 1. ion-dipole 2. dipole-dipole 3. hydrogen bonding 3P1-15 Ion-dipole Interactions Ions are hydrated by water molecules. The water molecules orient so the opposite charge end points to the ion to partially neutralize charge. The shell of water molecules is a solvation sphere. 3P1-16 Dipole-dipole Interactions The polar water molecule interacts with an or N or an on an or N on an organic molecule. Dipole-dipole interactions 3P1-17 ydrogen Bonding A hydrogen attached to an or N becomes very polarized and highly partial plus. This partial positive charge interacts with the nonbonding electrons on another or N giving rise to the very powerful hydrogen bond. hydrogen bond shown in yellow 3P1-18 3

4 Nonpolar Molecules Nonpolar molecules have no polar bonds or the bond dipoles cancel due to molecular geometry. These molecules do not form good attractions with the water molecule. They are insoluble and are said to be hydrophobic (water hating). eg.: C 3 C 2 C 2 C 2 C 2 C 3, hexane Nonpolar Molecules-2 Water forms hydrogen-bonded cagelike structures around hydrophobic molecules, forcing them out of solution. 3P1-19 3P1-20 Amphipathic Molecules Amphipathic molecules contain both polar and nonpolar groups. Ionized fatty acids are amphipathic. The carboxylate group is water soluble and the long carbon chain is not. Amphipathic molecules tend to form micelles, colloidal aggregates with the charged head facing outward to the water and the nonpolar tail part inside. 3P1-21 A Micelle 3P1-22 smotic Pressure-2 smosis is a spontaneous process in which solvent molecules pass through a semipermeable membrane from a solution of lower solute concentration to a solution of higher solute concentration. smotic pressure is the pressure required to stop osmosis. smotic Pressure-3 smotic pressure (π) is measured in an osmometer. 3P1-23 3P1-24 4

5 smotic Pressure-4 π = imrt i = van t off factor (% as ions) M = molarity (mol/l for dilute solns) R = L atm/ mol K T = Kelvin temperature 1 M NaCl is 90% ionized and 10% ion pairs. i = = 1.9 smolarity (osm/l) = im 3P1-25 smotic Pressure-5 Because cells have a higher ion concentration than the surrounding fluids, they tend to pick up water through the semipermeable cell membrane. The cell is said to be hypertonic relative to the surrounding fluid and will burst (hemolyze) if osomotic control is not effected. 3P1-26 smotic Pressure-6 Cells placed in a hypotonic solution will lose water and shrink (crenate). If cells are placed in an isotonic solution (conc same on both sides of membrane) there is no net passage of water. 3.5 Ionization of Water Water dissociates. (self-ionizes) = K a = [ 3 + ][ - ] [ 2 ] 2 K w = K a [ 2 ] 2 = [ 3 + ][ - ] 3P1-27 3P1-28 Water Ionization-2 The conditions for the water dissociation equilibrium must hold under all situations at 25 o. K w = [ 3 + ][ - ]=1 x In neutral water, [ 3 + ] = [ - ] = 1 x 10-7 M Water: A/B Properties When external acids or bases are added to water, the ion product ([ 3 + ][ - ] ) must equal K w. The effect of added acids or bases is best understood using the Lowry-Bronsted theory of acids and bases. 3P1-29 3P1-30 5

6 Water: A/B Properties-2 Lowry-Bronsted acid = proton donor A + 2 = A - A B CA CB C: conjugate (product) A/B Water: A/B Properties-3 Lowry-Bronsted base = proton acceptor RN = - + RN 3 + B A CB CA 3P1-31 3P1-32 Measuring Acidity Added acids increase the concentration of hydronium ion and bases the concentration of hydroxide ion. In acid solutions [ 3 + ] > 1 x 10-7 M [ - ] < 1 x 10-7 M In basic solutions [ - ] > 1 x 10-7 M [ 3 + ] < 1 x 10-7 M p scale measures acidity without using exponential numbers. 3P1-33 p Scale Define: p = -log (10) [ 3 + ] acidic basic [ 3 + ]=1 x 10-7 M, p =? 7.0 3P1-34 p Scale-2 [ 3 + ]=1 x 10-5 M, p =? 5 (acidic) [ 3 + ]=1 x M, p =? 10 (basic) What if preexponential number is not 1? 3P1-35 p Scale-3 [ 3 + ]=2.6 x 10-5 M, p =? 4.59 (acidic) [ 3 + ]=6.3 x 10-9 M, p =? 8.20 (basic) [ 3 + ]=7.8 x 10-3 M, p =? 2.11 (acidic) 3P1-36 6

7 p Scale-4 p to [ 3 + ]? inverse log of negative p orange juice, p 3.5. [ 3 + ]=? [ 3 + ] = 3.2 x 10-4 M urine, p 6.2. [ 3 + ]=? [ 3 + ] = 6.3 x 10-7 M 3P1-37 Strength of Acids Strength of an acid is measured by the percent which reacts with water to form hydronium ions. Strong acids (and bases) ionize close to 100%. eg. Cl, Br, N 3, 2 S 4 3P1-38 Strength of Acids-2 Weak acids (or bases) ionize typically in the 1-5% range. eg. C 3 CC, pyruvic acid C 3 CC, lactic acid C 3 C, acetic acid 3P1-39 Strength of Acids-3 Strength of an acid is also measured by its K a or pk a values. A + 2 = A - K a = [ 3 + ][A - ] [A] Larger K a and smaller pk a values indicate stronger acids. 3P1-40 Strength of Acids-4 K a pk a C 3 CC 3.2x C 3 CC 1.4x C 3 C 1.8x Larger K a and smaller pk a values indicate stronger acids. 3P1-41 7