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- 1. BT-202 Biochemistry Aug 16, 2011
- 3. <ul><li>Acids and Bases: Definitions </li></ul><ul><li>Ionization of water </li></ul><ul><li>pH Scale </li></ul><ul><li>Dissociation of Weak acids and bases </li></ul><ul><li>Buffers </li></ul>
- 4. Vinegar and Aspirin
- 5. Soaps, toothpaste and cleaning agents.
- 21. HF + H 2 O H 3 O + + F -
- 23. Learning check! <ul><li>Label the acid, base, conjugate acid, and conjugate base in each reaction: </li></ul><ul><li>H 2 SO 4 + H 2 O HSO 4 - + H 3 O + </li></ul><ul><li>HCl + OH - Cl - + H 2 O </li></ul><ul><li>HCl + NH 3 Cl - + NH 4 + </li></ul><ul><li>NH 3 + H 2 O NH 4 + + OH - </li></ul>
- 24. <ul><li>Identify the conjugate base in each of the pairs below? </li></ul><ul><li>RCOOH, RCOO - </li></ul><ul><li>RNH 2 , RNH 3 + </li></ul><ul><li>H 2 PO - ,H 3 PO 4 </li></ul><ul><li>H 2 CO 3 , HCO 3 - </li></ul>
- 28. Proton donor
- 29. <ul><li>Proton Hopping </li></ul><ul><li>Ionization can be measured by its electrical conductivity. </li></ul><ul><li>Movement of hydronium ion is very fast due to “proton hopping”. </li></ul><ul><li>No single proton moves over a long distance but a series of proton hops between water molecules which are hydrogen bonded. </li></ul>
- 31. <ul><li>Most of the acid base reactions in the aqueous solution are exceptionally very fast. </li></ul><ul><li>Proton hopping also plays a role in biological proton transfer reactions. </li></ul>
- 33. <ul><li>Reversible ionization is very crucial to the role of water in cellular function. </li></ul><ul><li>We need to express the extent of ionization in quantitative terms. </li></ul><ul><li>The position of equilirium for any chemical reaction is given by its EQUILIBRIUM CONSTANT, K eq . </li></ul>
- 34. <ul><li>For any generalized reaction: </li></ul><ul><li>Equilibrium constants are dimensionless but biochemists have retained the concentration units (M) or molarity. </li></ul>
- 35. <ul><li>Equilibrium constant for a reversible reaction of water is: </li></ul><ul><li>K eq = [H + ] [OH - ] </li></ul><ul><ul><ul><ul><ul><li> [H 2 O] </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>In pure water at 25°C, the concentration of water is 55.5 M. </li></ul></ul></ul></ul></ul>
- 38. <ul><li>At this pH the concentration of [H + ] and [OH - ] is 1 X 10 -7 . </li></ul><ul><li>Whenever [H + ] is greater than 1 X 10 -7 [OH - ] is less and visa versa. </li></ul>
- 39. Learning check! <ul><li>What is the concentration of H + in a solution of 0.1 M NaOH? </li></ul><ul><li>What is the [H + ] of human saliva if its [OH - ] is 4 x 10 -8 M? Is human saliva acidic, basic, or neutral? </li></ul><ul><li>What is the [H + ] of a sample of lake water with [OH - ] of 4.0 x 10 -9 M? Is the lake acidic, basic, or neutral? </li></ul>
- 40. Learning check! <ul><li>What is the concentration of OH - in a solution with an H + concentration of 1.3 X 10 -4 M? </li></ul>
- 41. <ul><li>If an acidic solution is defined as a solution water where H 3 O + is greater than OH - , what can be said of an acidic solution? </li></ul><ul><li> [OH - ]>[H 3 O + ] </li></ul><ul><li> The concentration of H 3 O + must be greater than 10 -7. </li></ul><ul><li> The concentration of H 3 O + is always 10 -7 if the solution is made in water. </li></ul><ul><li> The solution feels slippery and might taste bitter. </li></ul>
- 45. pH of some aqueous fluids. pH scale ranges from 0-14.
- 47. <ul><li>Measuring pH: </li></ul><ul><li>pH indicator is a substance which changes color around a particular pH Eg. Litmus paper. </li></ul><ul><li>Universal indicator is a mixture of indicators such that it changes color over a range of pH (Thymol blue, phenolpthalein, bromophenol blue etc). </li></ul><ul><li>pH meter is used in laboratory. </li></ul>
- 48. Learning Check! <ul><li>If the [H + ] of a solution is 0.1M, What is the [OH - ]? What is the pH? </li></ul><ul><li>What is the difference in [H + ] between solution at pH 4 and pH 7? </li></ul><ul><ul><li>3M </li></ul></ul><ul><ul><li>1000-fold difference in [H + ] </li></ul></ul><ul><ul><li>3-fold difference in [H + ] </li></ul></ul><ul><ul><li>11M </li></ul></ul>
- 49. <ul><li>pH in biological system: </li></ul><ul><li>-catalytic activity of enzymes. </li></ul><ul><li>-blood pH changes can be life- threatning. pH blood is 7.4. </li></ul><ul><li>-acidosis. </li></ul><ul><li>-alkalosis. </li></ul>
- 51. ACIDS AND BASES-STRENGTH
- 52. <ul><li>Because strong acids completely ionize, the [H + ] of a solution made with a strong acid is easily figured out, since it is equal to the molarity of the solution. </li></ul>ACIDS AND BASES-STRENGTH
- 53. Learning Check! <ul><li>Write out the acid dissociation reaction for hydrochloric acid? </li></ul><ul><li>Calculate the pH of a solution of 5.0 X 10 -4 M HCl? </li></ul><ul><li>Write out the acid dissociation reaction for Sodiaum hydroxide? </li></ul><ul><li>Calculate the pH of a solution of 7.0 X 10 -5 M NaOH? </li></ul>
- 54. ACIDS AND BASES-STRENGTH
- 55. ACIDS AND BASES-STRENGTH
- 56. ACIDS AND BASES-STRENGTH <ul><li>The tendency of any acid to lose proton and form its conjugate base in water is defined by the equilibrium constant ( K eq ) for reversible reaction: </li></ul><ul><li>HA H + + A - </li></ul><ul><li>K eq = [H + ] [A - ] = K a </li></ul><ul><li>[HA] </li></ul>
- 57. ACIDS AND BASES-STRENGTH <ul><li>The degree of dissociation of a weak acid in water is described by the acid dissociation constant, K a . It is also called the ionization constant. </li></ul><ul><li>As seen in the figure. </li></ul>
- 58. ACIDS AND BASES-STRENGTH
- 59. ACIDS AND BASES-STRENGTH <ul><li>Also included in the figure is p K a which is analogous to pH </li></ul><ul><li>p K a = log 1/ K a = -log K a </li></ul><ul><li>eg. </li></ul><ul><li> K a of acetic acid is 1.74 x 10 –5 . </li></ul><ul><li>pK a = –log (1.74 x 10 –5 ) = 4.76. </li></ul>
- 60. <ul><li>Experimental determination of amount of acid in a given solution is done by titration. It also reveals the p K a . </li></ul><ul><li>Concentration of the acid in original solution can be calculated from volume and concentration of NaOH added. </li></ul>
- 61. ACIDS AND BASES-STRENGTH
- 62. ACIDS AND BASES-STRENGTH
- 63. ACIDS AND BASES-STRENGTH <ul><li>The stronger the acid, the lower its p K a ; the stronger the base the higher its p K a . </li></ul><ul><li>p K a can be determined experimentally ; it is the pH at the midpoint of a titration curve for the acid and base. </li></ul>
- 64. Learning Check! <ul><li>Which of the following aqueous solutions has the lowest pH: 0.1 M HCl; 0.1 M acetic acid (pKa = 4.86); 0.1 M formic acid (pKa = 3.75)? </li></ul>
- 65. Buffer
- 66. BUFFER <ul><li>Buffers are aqueous systems that tend to resist changes in pH when small amounts of acid (H + ) or base (OH - ) are added. </li></ul><ul><li>A buffer system consists of weak acid (proton donor) and its conjugate base (proton acceptor). </li></ul>
- 67. Buffer The acetic acid-acetate pair as a buffer system
- 68. Buffers <ul><li>Buffering region </li></ul><ul><li>This is the zone in the titration curve of a weak acid which extends 1 pH unit on either side of the p K a . In this zone additon of small amount of either H + or OH - has much less effect on pH than region outside this zone. </li></ul>
- 69. THE HENDERSON–HASSELBALCH EQUATION <ul><li>The extent of ionization of a weak acid (the pK a ) influences the final concentration of H + ions (the pH) of the solution, there must be a relationship between pH and the pK a of a weak acid. This relationship is given by the Henderson–Hasselbalch equation : </li></ul><ul><li>pH = pK a + log [conjugate base] [acid] </li></ul>
- 70. THE HENDERSON–HASSELBALCH EQUATION <ul><li>Calculate p K a , given pH and the molar ratio of proton donor and acceptor; </li></ul><ul><li>Calculate pH, given p K a and the molar ratio of proton donor and acceptor; and </li></ul><ul><li>Calculate the molar ratio of proton donor and acceptor , given p K a and pH. </li></ul>
- 71. Learning check! <ul><li>Which of the following compounds would be the best buffer at pH 5.0: formic acid (pKa = 3.8), acetic acid (pKa=4.76) or ethylamine (pka=9.0)? </li></ul>
- 72. Learning check! <ul><li>(a) What is the pH of a mixture of 0.042 M NaH 2 PO 4 and 0.058 M Na 2 HPO 4 ? </li></ul><ul><li>(b) If 1.0 ml of 10.0N NaOH is added to a liter of the buffer prepared in (a), how much will the pH change? </li></ul><ul><li>(c) If 1.0 ml of 10.0 N NaOH is added to a liter of pure water at pH 7.0, what is the final pH? Compare with (c)? </li></ul>
- 73. Buffers in cells and tissues Cytoplasm of cells contain many amino acids with functional groups that are weak acids or weak bases
- 74. Buffers in cells and tissues Blood plasma is buffered inpart by the bicarbonate system.

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