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Bt 202 aug 12 2011 ppt1997-2004


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Bt 202 aug 12 2011 ppt1997-2004

  1. 1. BT-202 Biochemistry Aug 16, 2011
  2. 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>
  3. 4. Vinegar and Aspirin
  4. 5. Soaps, toothpaste and cleaning agents.
  5. 21. HF + H 2 O H 3 O + + F -
  6. 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>
  7. 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>
  8. 28. Proton donor
  9. 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>
  10. 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>
  11. 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>
  12. 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>
  13. 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>
  14. 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>
  15. 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>
  16. 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>
  17. 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>
  18. 45. pH of some aqueous fluids. pH scale ranges from 0-14.
  19. 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>
  20. 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>
  21. 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>
  23. 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
  24. 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>
  27. 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>
  28. 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>
  30. 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>
  31. 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>
  34. 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>
  35. 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>
  36. 65. Buffer
  37. 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>
  38. 67. Buffer The acetic acid-acetate pair as a buffer system
  39. 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>
  40. 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>
  41. 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>
  42. 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>
  43. 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>
  44. 73. Buffers in cells and tissues Cytoplasm of cells contain many amino acids with functional groups that are weak acids or weak bases
  45. 74. Buffers in cells and tissues Blood plasma is buffered inpart by the bicarbonate system.