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ERT106 BIOCHEMISTRY WATER Pn Syazni Zainul Kamal.

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1 ERT106 BIOCHEMISTRY WATER Pn Syazni Zainul Kamal

2 Lecture contents 1. Why water important to biochemistry 2. Uses of water 3. Physics & chemistry of water 4. Unique physical properties of water 5. Molecular structure of water 6. Noncovalent Bonding in water 1.Ionic interactions 2.Hydrogen Bonds 3.van der Waals Forces 7. Thermal Properties of Water 8. Solvent Properties of Water 1.Hydrophilic, hydrophobic, and amphipathic molecules 2.Osmotic pressure

3 9.Ionization of Water 1.Acids, bases, and pH 2.Buffers 3.titration

4 Why water is important to biochemistry More than 70% earth’s surface covered with water More than 70% earth’s surface covered with water The substance that make possible life on earth The substance that make possible life on earth Solvent & substrate for many cellular reaction Solvent & substrate for many cellular reaction Transports chemicals from place to place Transports chemicals from place to place Helps to maintain constant body temperature Helps to maintain constant body temperature Cell components and molecules (protein, polysaccharides, nucleic acid, membranes) assume their shape in response to water Cell components and molecules (protein, polysaccharides, nucleic acid, membranes) assume their shape in response to water

5 USES OF WATER?

6 Introduction Physic and chemistry of water Water is the chemical substance with chemical formula H 2 O: one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom. Water is the chemical substance with chemical formula H 2 O: one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom.chemical substance chemical formulaH Omoleculehydrogenatomscovalently bondedoxygenchemical substance chemical formulaH Omoleculehydrogenatomscovalently bondedoxygen Water is a tasteless, odorless liquid at ambient temperature and pressure, and appears colorless in small quantities, although it has its own intrinsic very light blue hue. Water is a tasteless, odorless liquid at ambient temperature and pressure, and appears colorless in small quantities, although it has its own intrinsic very light blue hue. ambient temperature and pressure ambient temperature and pressure

7 Oxygen attracts electrons much more strongly than hydrogen, resulting in a net positive charge on the hydrogen atoms, and a net negative charge on the oxygen atom. Oxygen attracts electrons much more strongly than hydrogen, resulting in a net positive charge on the hydrogen atoms, and a net negative charge on the oxygen atom. The presence of a charge on each of these atoms gives each water molecule a net dipole moment. The presence of a charge on each of these atoms gives each water molecule a net dipole moment.dipole momentdipole moment

8 Introduction Unique physical properties of water Exist in all three physical states of matter: solid, liquid, and gas. Exist in all three physical states of matter: solid, liquid, and gas. Has high specific heat Has high specific heat Water conducts more easily than any liquid except mercury Water conducts more easily than any liquid except mercury Water has a high surface tension Water has a high surface tension Water is a universal solvent Water is a universal solvent Water in a pure state has a neutral pH Water in a pure state has a neutral pH

9 Molecular Structure of Water Molecular Structure of Water Tetrahedral geometry Tetrahedral geometry The oxygen in water is sp 3 hybridized. Hydrogens are bonded to two of the orbitals. Consequently the water molecule is bent. The H-O-H angle is o. The oxygen in water is sp 3 hybridized. Hydrogens are bonded to two of the orbitals. Consequently the water molecule is bent. The H-O-H angle is o.

10 The bent structure indicate water is polar coz linear structure is nonpolar. The bent structure indicate water is polar coz linear structure is nonpolar. Phenomenon where charge is separated to partial –ve charge and partial +ve charge is called dipoles. Phenomenon where charge is separated to partial –ve charge and partial +ve charge is called dipoles.

11 Water is a polar molecule. Water is a polar molecule. A polar molecule is one in which one end is partially positive and the other partially negative.A polar molecule is one in which one end is partially positive and the other partially negative. Oxygen is more electronegative than hydrogen, so oxygen atom bears a partial –ve charge, hydrogen atoms are partial +ve chargeOxygen is more electronegative than hydrogen, so oxygen atom bears a partial –ve charge, hydrogen atoms are partial +ve charge

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13 Molecules eg water, in which charge is separated are called dipoles. Molecules eg water, in which charge is separated are called dipoles. Molecular dipoles will orient themselves in the direction opposite to that of the field when subjected to an electric field. Molecular dipoles will orient themselves in the direction opposite to that of the field when subjected to an electric field.

14 Noncovalent Bonding Usually electrostatic Usually electrostatic They occur between the positive nucleus of one atom and the negative electron clouds of another nearby atom They occur between the positive nucleus of one atom and the negative electron clouds of another nearby atom Relatively weak, easily disrupted Relatively weak, easily disrupted Large no. of noncovalent interactions stabilize macromolecules Large no. of noncovalent interactions stabilize macromolecules

15 Types of noncovalent bonding : Types of noncovalent bonding : 1)Ionic interactions 1)Ionic interactions 2)Hydrogen bonding 2)Hydrogen bonding 3)Van der Waals forces 3)Van der Waals forces -Dipole-dipole -Dipole-dipole -Dipole-induced dipole -Dipole-induced dipole -Induced dipole-induced dipole -Induced dipole-induced dipole

16 Typical “Bond” Strengths TypekJ/mol Covalent>210 Noncovalent Ionic interactions Ionic interactions4-80 Hydrogen bonds Hydrogen bonds12-30 van der Waals van der Waals0.3-9 Hydrophobic interactions Hydrophobic interactions3-12

17 1) Ionic Interactions Interaction occur between charged atoms or group. Interaction occur between charged atoms or group. Oppositely charged ions are attracted to each other. (eg. NaCl) Oppositely charged ions are attracted to each other. (eg. NaCl) ions with similar charges eg K + and Na + will repel each other ions with similar charges eg K + and Na + will repel each other

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19 In proteins, certain amino acid side chains contain ionizable groups. In proteins, certain amino acid side chains contain ionizable groups. Glutamic acid ionized as –CH 2 CH 2 COO - Glutamic acid ionized as –CH 2 CH 2 COO - Lysine ionized as -CH 2 CH 2 CH 2 CH 2 NH 3 + Lysine ionized as -CH 2 CH 2 CH 2 CH 2 NH 3 + Attraction between +ve and –ve charged amino acid side chains forms a salt bridge (-COO -+ H 3 N-) Attraction between +ve and –ve charged amino acid side chains forms a salt bridge (-COO -+ H 3 N-) Salt bridge

20 Hydrogen bonding is a weak attraction between an electronegative atom (O,N,F) in one molecule and a hydrogen atom in another molecule. 2) Hydrogen bonding *Has both electrostatic (ionic) and covalent character.

21 Water molecule form hydrogen bond with one another Water molecule form hydrogen bond with one another Four hydrogen bonding attraction are possible for each molecule: Four hydrogen bonding attraction are possible for each molecule: *2 through the hydrogen *2 through the nonbonding electron pairs

22 The resulting intermolecular hydrogen bond acts as bridge between water molecules. The resulting intermolecular hydrogen bond acts as bridge between water molecules. Large no. of intermolecular bond (in liquid/solid states of water),the molecules become large, dynamic. Large no. of intermolecular bond (in liquid/solid states of water),the molecules become large, dynamic. This explain why water have high boiling & melting point. This explain why water have high boiling & melting point.

23 3)Van Der Waals Forces Force between molecules Force between molecules Occur between permanent and/or induced dipoles Occur between permanent and/or induced dipoles 3 types of van der waals forces : 3 types of van der waals forces : - Dipole-dipole interactions - Dipole-induced dipole interactions - Induced dipole-induced dipole interactions interactions

24 a) Dipole-dipole interaction Occur between molecules containing electronegative atoms, cause positive end of one molecule is directed toward negative end of another Occur between molecules containing electronegative atoms, cause positive end of one molecule is directed toward negative end of another eg. Hydrogen bonds are strong type of dipole-dipole interaction eg. Hydrogen bonds are strong type of dipole-dipole interaction

25 b)Dipole-induced dipole interaction A permanent dipole induces a transient dipole in a nearby molecule by distorting its electron distribution A permanent dipole induces a transient dipole in a nearby molecule by distorting its electron distribution eg. Carbonyl-containing molecule is weakly attracted to hydrocarbon eg. Carbonyl-containing molecule is weakly attracted to hydrocarbon Weaker than dipole-dipole interaction Weaker than dipole-dipole interaction

26 c)Induced dipole-induced dipole interactions Forces between nonpolar molecules Forces between nonpolar molecules Because of the constant motion of electron, an atom/molecule can develop a temporary dipole (induced dipole) when the electron are distributed unevenly around nucleus Because of the constant motion of electron, an atom/molecule can develop a temporary dipole (induced dipole) when the electron are distributed unevenly around nucleus Neighboring atom can be distorted by the appearance of the temporary dipole which lead to an electrostatic interaction between them Neighboring atom can be distorted by the appearance of the temporary dipole which lead to an electrostatic interaction between them Also known as London dispersion forces Also known as London dispersion forces eg. Stacking of base ring in DNA molecule eg. Stacking of base ring in DNA molecule

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28 Thermal Properties of Water Hydrogen bonding keeps water in the liquid phase between 0 o C and 100 o C. Hydrogen bonding keeps water in the liquid phase between 0 o C and 100 o C. Liquid water has a high: Liquid water has a high: Heat of vaporization - energy to vaporize one mole of liquid at 1 atm Heat capacity - energy to change the temperature by 1 o C Water plays an important role in thermal regulation in living organisms. Water plays an important role in thermal regulation in living organisms.

29 Max number of hydrogen bonds form when water has frozen into ice. Max number of hydrogen bonds form when water has frozen into ice. Hydrogen bonds is approximately 15% break when ice is warmed. Hydrogen bonds is approximately 15% break when ice is warmed. Liquid water consists of continuously breaking and forming hydrogen bonds. Liquid water consists of continuously breaking and forming hydrogen bonds. As the tempt rise, the broken of hydrogen bonds are accelerating. As the tempt rise, the broken of hydrogen bonds are accelerating. When boiling point is reached, the water molecules break free from one another and vaporize. When boiling point is reached, the water molecules break free from one another and vaporize. Relationship between temperature and hydrogen bond

30 Solvent properties of water Water is an ideal biological solvent Water is an ideal biological solvent Water easily dissolves a wide variety of the constituents of living organisms. Water easily dissolves a wide variety of the constituents of living organisms. Water also unable to dissolve some substances Water also unable to dissolve some substances This behavior is called hydrophilic and hydrophobic properties of water. This behavior is called hydrophilic and hydrophobic properties of water.

31 Hydrophilic molecules Ionic or polar substances that has an affinity for water Ionic or polar substances that has an affinity for water In Greek= Hydro, “water” philios, “loving” In Greek= Hydro, “water” philios, “loving” Water dipole structure and its capacity to form hydrogen bond with electronegative atoms enable water to dissolve ionic and polar substance Water dipole structure and its capacity to form hydrogen bond with electronegative atoms enable water to dissolve ionic and polar substance These substances soluble in water due to 3 kinds of noncovalent bonding : These substances soluble in water due to 3 kinds of noncovalent bonding : a) ion-dipole a) ion-dipole b) dipole-dipole b) dipole-dipole c) hydrogen bonding c) hydrogen bonding

32 Salts (KCl,NaCl) held together by ionic interactions Salts (KCl,NaCl) held together by ionic interactions When ionic compound eg. KCl,NaCl dissolved in water, its ions separate because the polar water molecules attract ions more than the ions attract each other. (ion-dipole interaction) When ionic compound eg. KCl,NaCl dissolved in water, its ions separate because the polar water molecules attract ions more than the ions attract each other. (ion-dipole interaction) Shells of water mol. cluster around the ions = solvation spheres Shells of water mol. cluster around the ions = solvation spheres

33 Dipole-dipole Interactions Organic molecules with ionize group Organic molecules with ionize group The polar water molecule interacts with carboxyl group of aldehyd & ketones (carbohyd) and hydroxyl group of alcohol The polar water molecule interacts with carboxyl group of aldehyd & ketones (carbohyd) and hydroxyl group of alcohol Dipole-dipole interactions

34 Hydrogen Bonding A hydrogen attached to an O or N becomes very polarized and highly partial plus. This partial positive charge interacts with the nonbonding electrons on another O or N giving rise to the very powerful hydrogen bond. A hydrogen attached to an O or N becomes very polarized and highly partial plus. This partial positive charge interacts with the nonbonding electrons on another O or N giving rise to the very powerful hydrogen bond. hydrogen bond shown in yellow

35 Hydrophobic molecules Non ionic or nonpolar substance Non ionic or nonpolar substance These molecules do not form good attractions with the water molecule. They are insoluble and are said to be hydrophobic (water hating). These molecules do not form good attractions with the water molecule. They are insoluble and are said to be hydrophobic (water hating). eg. Hydrocarbon : CH 3 CH 2 CH 2 CH 2 CH 2 CH 3, hexane eg. Hydrocarbon : CH 3 CH 2 CH 2 CH 2 CH 2 CH 3, hexane

36 Water forms hydrogen-bonded cagelike structures around hydrophobic molecules, forcing them out of solution. (droplet/into a separate layer) Water forms hydrogen-bonded cagelike structures around hydrophobic molecules, forcing them out of solution. (droplet/into a separate layer)

37 Amphipathic Molecules Amphipathic molecules contain both polar and nonpolar groups. Amphipathic molecules contain both polar and nonpolar groups. Ionized fatty acids are amphipathic. The carboxylate group is water soluble (hydrophilic) and the long carbon chain is not (hydrophobic). Ionized fatty acids are amphipathic. The carboxylate group is water soluble (hydrophilic) and the long carbon chain is not (hydrophobic). Amphipathic molecules tend to form micelles when mixed with water. Amphipathic molecules tend to form micelles when mixed with water.

38 polar head – orient themselves in contact with water molecules polar head – orient themselves in contact with water molecules Nonpolar tails – aggregate in the center, away from water Nonpolar tails – aggregate in the center, away from water

39 Osmotic Pressure Osmosis is a spontaneous process in which solvent (eg water) molecules pass through a semi permeable membrane from a solution of lower solute concentration (dilute) to a solution of higher solute concentration (concentrated). Osmosis is a spontaneous process in which solvent (eg water) molecules pass through a semi permeable membrane from a solution of lower solute concentration (dilute) to a solution of higher solute concentration (concentrated). Osmotic pressure is the pressure required to stop osmosis (22.4 atm for 1M solution) Osmotic pressure is the pressure required to stop osmosis (22.4 atm for 1M solution)

40 B A B A Over time, water diffuses from side B (more dilute) to side A (concentrated)

41 Osmotic Pressure Osmotic pressure () is measured using an osmometer. Osmotic pressure () is measured using an osmometer.

42 Osmotic Pressure iMRT i = van’t Hoff factor (degree of ionization of solute) M = molarity (concentration of solute in mole/L) R = gas constant (0.082 L.atm/K.mole) T = absolute temp (in Kelvin) Osmolarity = iM (osmol/Liter)

43 i is the van't Hoff coefficient. i is the van't Hoff coefficient. For non-electrolytes (non ionizable solute) i=1 For non-electrolytes (non ionizable solute) i=1 For strong electrolytes i= the number of ions that are produced by the dissociation according to the molecular formula For strong electrolytes i= the number of ions that are produced by the dissociation according to the molecular formula e.g for NaCl you have 2 ions (1 Na + and 1 Cl - ) so i=2 for CaCl 2, 3 ions (1 Ca +2 and 2 Cl - ) so i=3 for CaCl 2, 3 ions (1 Ca +2 and 2 Cl - ) so i=3 For weak electrolytes i=(1-a)+na For weak electrolytes i=(1-a)+na n = the number of ions coming from the 100% dissociation according to the molecular formula n = the number of ions coming from the 100% dissociation according to the molecular formula a = the degree of dissociation a = the degree of dissociation e.g the degree of ionization of 1M CH 3 COOH solution is 80% e.g the degree of ionization of 1M CH 3 COOH solution is 80% a=80%/0.8, n=2 a=80%/0.8, n=2 so, so, i=(1-0.8) + 2(0.8) =1.8 i=(1-0.8) + 2(0.8) =1.8

44 Question 1 1)Estimate the osmotic pressure of a solution 1M NaCl at 25°C. Assume 100% ionization of solute. iMRT i= 2 (1 Na + and 1 Cl - ) i= 2 (1 Na + and 1 Cl - ) M= 1 mol/L R= L.atm/K.mol T= 298K T= 298K

45 Question 2 Question 2 Estimate the osmotic pressure of a solution 0.2M Magnesium chloride at 25°C. Assume 70% ionization of solute.

46 Osmotic pressure is an important factor affecting cells Osmotic pressure is an important factor affecting cells Cells contain high concentration of solutes – small organic mol., ionic salts, macromolecule Cells contain high concentration of solutes – small organic mol., ionic salts, macromolecule Cells may gain or lose water depend on concentration of solute in their environment. Cells may gain or lose water depend on concentration of solute in their environment.

47 Isotonic – solutions of equal concentration on either side of the membrane Isotonic – solutions of equal concentration on either side of the membrane Cells placed in isotonic solution no net movement of water across the membrane Cells placed in isotonic solution no net movement of water across the membrane Volume of cells are unchanged bcoz water entering & leaving the cell at the same rate. Volume of cells are unchanged bcoz water entering & leaving the cell at the same rate. Definitions of solutions

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49 Hypotonic – solution with a lower solute concentration than the solution on the other side of the membrane Hypotonic – solution with a lower solute concentration than the solution on the other side of the membrane Cells placed in hypotonic solution water moves into the cells Cells placed in hypotonic solution water moves into the cells Cause cells rupture Cause cells rupture eg. Red blood cells swell & rupture when immersed in pure water (hemolysis) eg. Red blood cells swell & rupture when immersed in pure water (hemolysis)

50 Hypertonic – solution with higher concentration of solutes than the solution on the other side of the membrane Hypertonic – solution with higher concentration of solutes than the solution on the other side of the membrane Cells placed in hypertonic solution water moves out the cells Cells placed in hypertonic solution water moves out the cells Cause cells to shrink Cause cells to shrink eg. Red blood cells shrink when immersed in 3% NaCl solution. (crenation) eg. Red blood cells shrink when immersed in 3% NaCl solution. (crenation)

51 Water ionization, pH, titration, buffer The self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium (H 3 O + ) and a hydroxide (OH − ) ion. The self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium (H 3 O + ) and a hydroxide (OH − ) ion.hydroniumhydroxidehydroniumhydroxide Water ionization occurs endothermically due to electric field fluctuations between molecules caused by nearby dipole librations resulting from thermal effects, and favorable localized hydrogen bonding. Water ionization occurs endothermically due to electric field fluctuations between molecules caused by nearby dipole librations resulting from thermal effects, and favorable localized hydrogen bonding.

52 Water dissociates. (self- ionizes) Water dissociates. (self- ionizes) H 2 O + H 2 O = H 3 O + + OH - H 2 O + H 2 O = H 3 O + + OH -

53 Ions may separate but normally recombine within a few min. to seconds. Ions may separate but normally recombine within a few min. to seconds. Rarely (about once every eleven hours per molecule at 25°C, or less than once a week at 0°C) the localized hydrogen bonding arrangement breaks before allowing the separated ions to return, and the pair of ions (H+, OH-) hydrate independently and continue their separate existence. Rarely (about once every eleven hours per molecule at 25°C, or less than once a week at 0°C) the localized hydrogen bonding arrangement breaks before allowing the separated ions to return, and the pair of ions (H+, OH-) hydrate independently and continue their separate existence.

54 may be expressed as may be expressed as K eq = [H 3 O + ][OH - ] K eq = [H 3 O + ][OH - ] [H 2 O] 2 [H 2 O] 2 The conditions for the water dissociation equilibrium must hold under all situations at 25°C. The conditions for the water dissociation equilibrium must hold under all situations at 25°C. K w = [H 3 O + ][OH - ] = 1 x M K w = [H 3 O + ][OH - ] = 1 x M Pure water ionize into equal amount of Pure water ionize into equal amount of [H 3 O + ] = [OH - ] = 1 x M [H 3 O + ] = [OH - ] = 1 x M Ionization of water

55 Acids, Bases and pH When external acids or bases are added to water, the ion product ([H 3 O + ][OH - ] ) must equal. When external acids or bases are added to water, the ion product ([H 3 O + ][OH - ] ) must equal. K w = [H 3 O + ][OH - ] = 1 x The effect of added acids or bases is best understood using the Bronsted- Lowry- theory of acids and bases. The effect of added acids or bases is best understood using the Bronsted- Lowry- theory of acids and bases.

56 Bronsted-Lowry theory is an acid-base theory Bronsted-Lowry theory is an acid-base theory Acid is a substance that can donate proton (ion H + donor) Acid is a substance that can donate proton (ion H + donor) acid + base = conjugate base + conjugate acid acid + base = conjugate base + conjugate acid HCl + H 2 O = H 3 O + + Cl - HCl + H 2 O = H 3 O + + Cl - Asid Base CA CB Asid Base CA CB C: conjugate (product) A/B Bronstead-Lowry theory

57 base is a substance that can accept proton base is a substance that can accept proton RNH 2 + H 2 O = OH - + RNH 3 + RNH 2 + H 2 O = OH - + RNH 3 + B A CB CA B A CB CA C: conjugate (product) A/B

58 Measuring Acidity Added acids, increase concentration of hydronium ion Added acids, increase concentration of hydronium ion In acid solutions [H 3 O + ] > 1 x M In acid solutions [H 3 O + ] > 1 x M [OH - ] < 1 x M [OH - ] < 1 x M Added bases, increase concentration of hydroxide ion. Added bases, increase concentration of hydroxide ion. In basic solutions [OH - ] > 1 x M In basic solutions [OH - ] > 1 x M [H 3 O + ] < 1 x M [H 3 O + ] < 1 x M pH scale measures acidity without using exponential numbers. pH scale measures acidity without using exponential numbers.

59 pH Scale Define: pH = - log (10) [H 3 O + ] Define: pH = - log (10) [H 3 O + ] acidic basic acidic basic [H 3 O + ]=1 x M, pH = ?

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61 pH Scale Questions 1) [H 3 O + ]=1 x M, pH = ? 2) [H 3 O + ]=1 x M, pH = ? 3) [H 3 O + ]=1 x M, pH = ?

62 pH Scale Questions 1) [H 3 O + ]=2.6 x M, pH = ? 2)[H 3 O + ]=6.3 x M, pH = ? 3)[H 3 O + ]=7.8 x M, pH = ?

63 pH Scale pH to [H 3 O + ]? pH to [H 3 O + ]? inverse log of negative pH inverse log of negative pH orange juice, pH 3.5. [H 3 O + ]=? orange juice, pH 3.5. [H 3 O + ]=? urine, pH 6.2. [H 3 O + ]=? urine, pH 6.2. [H 3 O + ]=?

64 Strength of Acids Strength of an acid is measured by the percent which reacts with water to form hydronium ions. 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%. Strong acids (and bases) ionize close to 100%. eg. HCl, HBr, HNO 3, H 2 SO 4 eg. HCl, HBr, HNO 3, H 2 SO 4 eg. NaOH, KOH, CaOH eg. NaOH, KOH, CaOH

65 Strength of Acids Weak acids (or bases) ionize typically in the 1-5% range Weak acids (or bases) ionize typically in the 1-5% range eg. Organic acid (contain carboxyl groups) eg. Organic acid (contain carboxyl groups) CH 3 COCOOH, pyruvic acid CH 3 COCOOH, pyruvic acid CH 3 CHOHCOOH, lactic acid CH 3 COOH, acetic acid CH 3 CHOHCOOH, lactic acid CH 3 COOH, acetic acid

66 Strength of Acids Strength of an acid is also measured by its K a or pK a values Strength of an acid is also measured by its K a or pK a values Dissociation of weak acid : Dissociation of weak acid : HA + H 2 O = H 3 O + + A - HA + H 2 O = H 3 O + + A - Weak acid conjugate base of HA Strength of weak acid may be determined : Strength of weak acid may be determined : pK a = -log K a

67 Strength of Acids K a pK a K a pK a CH 3 COCOOH 3.2x CH 3 CHOHCOOH 1.4x CH 3 COOH 1.8x Larger K a and smaller pK a values indicate stronger acids. Larger K a and smaller pK a values indicate stronger acids.

68 Monitoring acidity The Henderson-Hasselbalch (HH) equation is derived from the equilibrium expression for a weak acid. The Henderson-Hasselbalch (HH) equation is derived from the equilibrium expression for a weak acid.

69 HH equation The HH equation enables us to calculate the pH during a titration and to make predictions regarding buffer solutions. The HH equation enables us to calculate the pH during a titration and to make predictions regarding buffer solutions. What is a titration? What is a titration? It is a process in which carefully measured volumes of a base are added to a solution of an acid in order to determine the acid concentration.

70 When chemically equal (equivalent) amounts of acid and base are present during a titration, the equivalence point is reached. When chemically equal (equivalent) amounts of acid and base are present during a titration, the equivalence point is reached. The equivalence point is detected by using an indicator chemical that changes color or by following the pH of the reaction versus added base, ie. a titration curve. The equivalence point is detected by using an indicator chemical that changes color or by following the pH of the reaction versus added base, ie. a titration curve.

71 Titration Curve (HOAc with NaOH) Equivalence point End point NaOH (equivalents added) pH

72 Titration Curve (HOAc with NaOH) At the end point, only the salt (NaOAc) is present in solution. At the end point, only the salt (NaOAc) is present in solution. At the equivalence point, equal moles of salt and acid are present in solution. At the equivalence point, equal moles of salt and acid are present in solution. [HOAc] = [NaOAc] [HOAc] = [NaOAc] pH = pK a pH = pK a

73 Questions 1) By using HH equation, calculate the pH of a mixture of 0.25M acetic acid and 0.1M sodium acetate. The pK a of acetic acid is 4.76 pH = pK a + log [A - ] pH = pK a + log [A - ] [HA] [HA] pH = log [acetate] [acetic acid] [acetic acid] pH = log 0.1 =

74 2) Calculate the ratio of lactic acid to lactate in a buffer at pH The pK a for lactic acid is 3.86 = 13.8

75 3) During the fermentation of wine, a buffer system consisting of tartaric acid and potassium hydrogen tartrate is produced by a chemical reaction. Assuming that at some time the concentration of potassium hydrogen tartrate is twice that of tartaric acid, calculate the pH of the wine. The pK a of tartaric acid is 2.96

76 pH = pKa + log [A-] [HA] [HA] = log [hydrogen tertrate] = log [hydrogen tertrate] [tartaric acid] [tartaric acid] = log 2 = log 2 = 3.26 = 3.26

77 Buffer solution Buffer : a solution that resists change in pH when small amounts of strong acid or base are added. Buffer : a solution that resists change in pH when small amounts of strong acid or base are added. A buffer consists of: A buffer consists of: a weak acid and its conjugate base ora weak acid and its conjugate base or a weak base and its conjugate acida weak base and its conjugate acid

78 How does buffer work? Accepting hydrogen ions from the solution when they are in excess Accepting hydrogen ions from the solution when they are in excess Donating hydrogen ions from the solution when they have depleted Donating hydrogen ions from the solution when they have depleted

79 Buffer Solutions Maximum buffer effect occurs at the pK a for an acid. Maximum buffer effect occurs at the pK a for an acid. Effective buffer range is at 1 pH unit above and below the pK a value for the acid or base. Effective buffer range is at 1 pH unit above and below the pK a value for the acid or base. eg. H 2 PO 4 - /HPO 4 2-, pK a =7.20 eg. H 2 PO 4 - /HPO 4 2-, pK a =7.20 buffer range pH buffer range pH

80 Buffer Solutions High concentrations of acid and conjugate base give a high buffering capacity. High concentrations of acid and conjugate base give a high buffering capacity. Buffer systems are chosen to match the pH of the physiological situation, usually around pH 7. Buffer systems are chosen to match the pH of the physiological situation, usually around pH 7.

81 Physiological buffer 3 most important buffer in body: 3 most important buffer in body: Within cells the primary buffer is the phosphate buffer: H 2 PO 4 - /HPO 4 2- Within cells the primary buffer is the phosphate buffer: H 2 PO 4 - /HPO 4 2- The primary blood buffer is the bicarbonate buffer: HCO 3 - /H 2 CO 3. The primary blood buffer is the bicarbonate buffer: HCO 3 - /H 2 CO 3. Proteins also provide buffer capacity. Side chains can accept or donate protons. (eg. Hemoglobin, serum albumins) Proteins also provide buffer capacity. Side chains can accept or donate protons. (eg. Hemoglobin, serum albumins)

82 A zwitterion is a compound with both positive and negative charges. A zwitterion is a compound with both positive and negative charges. Zwitterionic buffers have become common because they are less likely to cause complications with biochemical reactions. Zwitterionic buffers have become common because they are less likely to cause complications with biochemical reactions.

83 N-tris(hydroxymethyl)methyl-2- aminoethane sulfonate (TES) is a zwitterion buffer example. N-tris(hydroxymethyl)methyl-2- aminoethane sulfonate (TES) is a zwitterion buffer example. (HOCH 2 ) 3 CN + H 2 CH 2 CH 2 SO 3 - (HOCH 2 ) 3 CN + H 2 CH 2 CH 2 SO 3 -

84 Assignment (water) Date of submission: 30/7/10 1. Explain how the changes in temperature give effect to hydrogen bonds in water molecule. Elaborate the situation with drawing of water molecules at every temperature level. 2. Explain how the acids produced in metabolism are transported to the liver without greatly affecting the pH of the blood.

85 The End Water : The Medium of Life


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