BCH2333_lecture_3_w2016

BCH2333_lecture_3_w2016 - Lecture 3 pH pKa acids/bases...

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Lecture 3 pH, pKa, acids/bases buffers Amino acids
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Approximate Bond Strength, kJ/mole 12-30 20 <40 0.4 – 4.0 Distance, nm 0.3 0.25 - 0.2
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Maintenance (homeostasis) of internal ionic composition - including pH - is critical for the structure and function of biomolecules. Control of pH is critical in biochemical experiments. Acid-base chemistry
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Brønsted Acids  and  Bases Acid is a substance that can donate a proton . Base is a substance that can accept a proton. HA  +  :B    A -      +     HB +
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5 Water dissociates: H 2 O + H 2 O H 3 O + + OH - K eq [H 3 O + ][OH - ] [H 2 O] 2 K w = [H 3 O + ][OH - ] K w  1  10 -14 M 2 Neutrality: [H 3 O + ] = [OH - ] [H 3 O + ] = [OH - ] = 1 × 10 - 7 M pH = -log [H 3 O + ] This is a equilibrium reaction: Therefore you can define it with an equilibrium constant Keq Which is the concentration of the production / reactions Kw= ionic product of water H2O=55 M D=1.0 g/mland Molecular weight=18.02 g/mol pH=-log[10^-7] = 7
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0 (10 0 ) 1.0 0.00000000000001 (10 -14 ) 1 (10 -1 ) 0.1 0.0000000000001 (10 -13 ) 2 (10 -2 ) 0.01 0.000000000001 (10 -12 ) 3 (10 -3 ) 0.001 0.00000000001 (10 -11 ) 4 (10 -4 ) 0.0001 0.0000000001 (10 -10 ) 5 (10 -5 ) 0.00001 0.000000001 (10 -9 ) 6 (10 -6 ) 0.000001 0.00000001 (10 -8 ) 7 (10 -7 ) 0.0000001 0.0000001 (10 -7 ) 8 (10 -8 ) 0.00000001 0.000001 (10 -6 ) 9 (10 -9 ) 0.000000001 0.00001 (10 -5 ) 10 (10 -10 ) 0.0000000001 0.0001 (10 -4 ) 11 (10 -11 ) 0.00000000001 0.001 (10 -3 ) 12 (10 -12 ) 0.000000000001 0.01 (10 -2 ) 13 (10 -13 ) 0.0000000000001 0.1 (10 -1 ) 14 (10 -14 ) 0.00000000000001 1.0 (10 0 ) pH [H + ] [OH - ] Each pH unit changes h3O 10 folds
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7 Most systems operate at 6-8
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8 strong acids, bases: HCl + H 2 O Cl - + H 3 O + weak acids, bases: CH 3 COOH + H 2 O CH 3 COO - + H 3 O + Strength of an acid: qualitative measure Comlete dissociation of HCL All HCL is completed to H and Cl Partially dissociate will have some CH3COOH and some CH3CH3COO-
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Acid dissociation reaction acid conjugate base Quantitative measure of the strength of acids
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Strength (pKa) of some biologically relevant acids
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11 For any acid: H 2 O + HA H 3 O + + A - log K a = log [H 3 O + ] + log [A - ] - log [HA] K eq [H 3 O + ] [A - ] [H 2 O] [HA] Henderson-Hasselbalch equation: derivation [HA] [H 3 O + ] [A - ] K a  [H 2 O] K eq
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12 by adding – log [H2O] and – log ka to both sides you get pH = pK a + log [A - ] - log [HA] log K a = log [H 3 O + ] + log [A - ] - log [HA] - log [H 3 O + ] = - log K a + log [A - ] - log [HA] pH  pK a  log [A - ] [HA] Henderson-Hasselbalch equation
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13 At the mid-point of the titration : [A - ] = [HA] The pK a of an acid is the pH at which the acid is half-dissociated.  pH  pK a  log pH  pK a  log 1 = 0  pH  pK a [HA] [HA]
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14 OH - H + 0.5 0.3 0.1 0.1 0.3 0.5 4 5 6 7 8 9 Equivalents pK a = 7.0 pH Henderson-Hasselbalch Equation Also called atitration curve HA = A More A – than HA More HA than A - PH change slowly here, Buffer region of an acid
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Polyprotic acids Have more than one acid-base group H 3 PO 4 and H 2 CO 3 The pK a ’s of two closely associated acid-base groups are not independent - the closer they are, the greater the effect.
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