ERT106 BIOCHEMISTRYERT106 BIOCHEMISTRY

WATERWATERPn Syazni Zainul KamalPn Syazni Zainul Kamal

Lecture contentsLecture contents1.1. Why water important to biochemistryWhy water important to biochemistry

2.2. Uses of waterUses of water

3.3. Physics & chemistry of waterPhysics & chemistry of water

4.4. Unique physical properties of waterUnique physical properties of water

5.5. Molecular structure of waterMolecular structure of water

6.6. Noncovalent Bonding in waterNoncovalent Bonding in water1.1. Ionic interactionsIonic interactions

2.2. Hydrogen BondsHydrogen Bonds

3.3. van der Waals Forcesvan der Waals Forces

7.7. Thermal Properties of WaterThermal Properties of Water

8.8. Solvent Properties of WaterSolvent Properties of Water1.1. Hydrophilic, hydrophobic, and amphipathic moleculesHydrophilic, hydrophobic, and amphipathic molecules

2.2. Osmotic pressureOsmotic pressure

9.9. Ionization of WaterIonization of Water1.1. Acids, bases, and pHAcids, bases, and pH

2.2. BuffersBuffers

3.3. titrationtitration

Why water is important to Why water is important to biochemistrybiochemistry

More than 70% earth’s surface covered with More than 70% earth’s surface covered with waterwater

The substance that make possible life on The substance that make possible life on earthearth

Solvent & substrate for many cellular Solvent & substrate for many cellular reactionreaction

Transports chemicals from place to placeTransports chemicals from place to place Helps to maintain constant body Helps to maintain constant body

temperaturetemperature Cell components and molecules (protein, Cell components and molecules (protein,

polysaccharides, nucleic acid, membranes) polysaccharides, nucleic acid, membranes) assume their shape in response to waterassume their shape in response to water

USES OF WATER?USES OF WATER?

IntroductionIntroductionPhysic and chemistry of waterPhysic and chemistry of water

Water is the Water is the chemical substancechemical substance with with chemical formulachemical formula HH22OO: one : one moleculemolecule of of water has two water has two hydrogenhydrogen atomsatoms covalentlycovalently bondedbonded to a single to a single oxygenoxygen atom. atom.

Water is a tasteless, odorless liquid at Water is a tasteless, odorless liquid at ambient temperature and pressureambient temperature and pressure, and , and appears colorless in small quantities, appears colorless in small quantities, although it has its own intrinsic very light although it has its own intrinsic very light blue hue.blue hue.

Oxygen attracts electrons much Oxygen attracts electrons much more strongly than hydrogen, more strongly than hydrogen, resulting in a net positive charge on resulting in a net positive charge on the hydrogen atoms, and a net the hydrogen atoms, and a net negative charge on the oxygen atom.negative charge on the oxygen atom.

The presence of a charge on each of The presence of a charge on each of

these atoms gives each water these atoms gives each water molecule a net molecule a net dipole momentdipole moment. .

IntroductionIntroductionUnique physical properties of waterUnique physical properties of water

Exist in all three Exist in all three physical states of matter: physical states of matter: solid, liquid, solid, liquid, and and gas. gas.

Has high specific heatHas high specific heat Water conducts more easily than any Water conducts more easily than any

liquid except mercuryliquid except mercury Water has a high surface tensionWater has a high surface tension Water is a universal Water is a universal solventsolvent Water in a pure state has a Water in a pure state has a neutral pH neutral pH

Molecular Structure of WaterMolecular Structure of Water

Tetrahedral geometryTetrahedral geometry The oxygen in water is The oxygen in water is

spsp33 hybridized. hybridized. Hydrogens are bonded Hydrogens are bonded to two of the orbitals. to two of the orbitals. Consequently the Consequently the water molecule is bent. water molecule is bent. The H-O-H angle is The H-O-H angle is 104.5104.5oo..

The bent structure indicate water is The bent structure indicate water is polarpolar coz linear structure is nonpolar. coz linear structure is nonpolar.

Phenomenon where charge is Phenomenon where charge is separated to partial –ve charge and separated to partial –ve charge and partial +ve charge is called partial +ve charge is called dipolesdipoles..

Water is a Water is a polar molecule. polar molecule. • A polar molecule is one in which one A polar molecule is one in which one

end is partially positive and the end is partially positive and the other partially negative.other partially negative.

• Oxygen is more electronegative than Oxygen is more electronegative than hydrogen, so oxygen atom bears a hydrogen, so oxygen atom bears a partial –ve charge, hydrogen atoms partial –ve charge, hydrogen atoms are partial +ve chargeare partial +ve charge

Molecules eg water, in which charge Molecules eg water, in which charge is separated are called is separated are called dipolesdipoles..

Molecular dipoles will orient Molecular dipoles will orient themselves in the direction opposite themselves in the direction opposite to that of the field when subjected to to that of the field when subjected to an electric field.an electric field.

Noncovalent BondingNoncovalent Bonding

Usually electrostatic Usually electrostatic They occur between the positive They occur between the positive

nucleus of one atom and the negative nucleus of one atom and the negative electron clouds of another nearby electron clouds of another nearby atomatom

Relatively weak, easily disruptedRelatively weak, easily disrupted Large no. of noncovalent interactions Large no. of noncovalent interactions

stabilize macromoleculesstabilize macromolecules

Types of noncovalent bonding : Types of noncovalent bonding : 1)Ionic interactions1)Ionic interactions 2)Hydrogen bonding2)Hydrogen bonding 3)Van der Waals forces3)Van der Waals forces

-Dipole-dipole-Dipole-dipole -Dipole-induced dipole-Dipole-induced dipole -Induced dipole-induced dipole-Induced dipole-induced dipole

Typical “Bond” StrengthsTypical “Bond” Strengths

TypeType kJ/molkJ/mol

CovalentCovalent >210>210

NoncovalentNoncovalent

Ionic interactionsIonic interactions 4-804-80

Hydrogen bondsHydrogen bonds 12-3012-30

van der Waalsvan der Waals 0.3-90.3-9

Hydrophobic interactionsHydrophobic interactions 3-123-12

1) Ionic Interactions1) Ionic Interactions

Interaction occur between charged atoms or Interaction occur between charged atoms or group.group.

Oppositely charged ions are attracted to each Oppositely charged ions are attracted to each other. (eg. NaCl)other. (eg. NaCl)

ions with similar charges eg Kions with similar charges eg K++ and Na and Na++ will repel will repel each othereach other

In In proteinsproteins, certain amino acid side , certain amino acid side chains contain ionizable groups.chains contain ionizable groups.

Glutamic acid ionized as Glutamic acid ionized as –CH–CH22CHCH22COOCOO--

Lysine ionized as Lysine ionized as -CH-CH22CHCH22CHCH22CHCH22NHNH33++

Attraction between +ve and –ve Attraction between +ve and –ve charged amino acid side chains forms charged amino acid side chains forms a a salt bridgesalt bridge (-COO (-COO-+-+HH33N-)N-)

CH2CH2COO-

CH2CH2NH3

+

Salt bridge

HO

H

HO

H

HO

H

Hydrogen bonding is a

weak attraction between an electronegative atom (O,N,F) in one molecule and a hydrogen atomin another molecule.

2) Hydrogen bonding2) Hydrogen bonding

*Has both electrostatic (ionic) and covalentcharacter.

Water molecule form hydrogen bond with Water molecule form hydrogen bond with one anotherone another

Four hydrogen bonding attraction are Four hydrogen bonding attraction are possible for each molecule:possible for each molecule:

*2 through the hydrogen*2 through the nonbonding electron pairs

HO

H

HO

H

HO

H

HO

HH

OH

The resulting intermolecular hydrogen The resulting intermolecular hydrogen bond acts as bridge between water bond acts as bridge between water molecules.molecules.

Large no. of intermolecular bond (in Large no. of intermolecular bond (in liquid/solid states of water),the liquid/solid states of water),the molecules become large, dynamic.molecules become large, dynamic.

This explain why water have high This explain why water have high boiling & melting point. boiling & melting point.

3)Van Der Waals Forces3)Van Der Waals Forces

Force between moleculesForce between molecules Occur between permanent and/or Occur between permanent and/or

induced dipolesinduced dipoles 3 types of van der waals forces :3 types of van der waals forces :

- Dipole-dipole interactions- Dipole-dipole interactions

- Dipole-induced dipole interactions- Dipole-induced dipole interactions

- Induced dipole-induced dipole - Induced dipole-induced dipole

interactionsinteractions

a) Dipole-dipole interactiona) Dipole-dipole interaction Occur between molecules Occur between molecules

containing electronegative atoms, containing electronegative atoms, cause positive end of one molecule cause positive end of one molecule is directed toward negative end of is directed toward negative end of anotheranother

eg. Hydrogen bonds are strong type eg. Hydrogen bonds are strong type of dipole-dipole interactionof dipole-dipole interaction

C O C O+-

+-

b)Dipole-induced dipole interactionb)Dipole-induced dipole interaction A permanent dipole induces a A permanent dipole induces a

transient dipole in a nearby molecule transient dipole in a nearby molecule by distorting its electron distributionby distorting its electron distribution

eg. Carbonyl-containing molecule is eg. Carbonyl-containing molecule is weakly attracted to hydrocarbon weakly attracted to hydrocarbon

Weaker than dipole-dipole interactionWeaker than dipole-dipole interaction

C O H

H

HH

+- +

-

c)Induced dipole-induced dipole interactionsc)Induced dipole-induced dipole interactions Forces between nonpolar moleculesForces between nonpolar molecules Because of the constant motion of electron, Because of the constant motion of electron,

an atom/molecule can develop a temporary an atom/molecule can develop a temporary dipole (induced dipole) when the electron dipole (induced dipole) when the electron are distributed unevenly around nucleusare distributed unevenly around nucleus

Neighboring atom can be distorted by the Neighboring atom can be distorted by the appearance of the temporary dipole which appearance of the temporary dipole which lead to an electrostatic interaction between lead to an electrostatic interaction between themthem

Also known as Also known as London dispersion forcesLondon dispersion forces eg. Stacking of base ring in DNA moleculeeg. Stacking of base ring in DNA molecule

Thermal Properties of WaterThermal Properties of Water Hydrogen bonding keeps water in the liquid Hydrogen bonding keeps water in the liquid

phase between 0phase between 0ooC and 100C and 100ooC.C.

Liquid water has a high:Liquid water has a high:

Heat of vaporizationHeat of vaporization - energy to - energy to vaporize vaporize one mole of liquid at 1 atmone mole of liquid at 1 atm

Heat capacityHeat capacity - energy to change the - energy to change the temperature by 1temperature by 1ooC C

Water plays an important role in thermal Water plays an important role in thermal regulation in living organisms.regulation in living organisms.

Max number of hydrogen bonds form Max number of hydrogen bonds form when water has frozen into ice.when water has frozen into ice.

Hydrogen bonds is approximately 15% Hydrogen bonds is approximately 15% break when ice is warmed.break when ice is warmed.

Liquid water consists of continuously Liquid water consists of continuously breaking and forming hydrogen bonds.breaking and forming hydrogen bonds.

As the tempt rise, the broken of hydrogen As the tempt rise, the broken of hydrogen bonds are accelerating.bonds are accelerating.

When boiling point is reached, the water When boiling point is reached, the water molecules break free from one another molecules break free from one another and vaporize.and vaporize.

Relationship between temperature and Relationship between temperature and hydrogen bondhydrogen bond

Solvent properties of waterSolvent properties of water Water is an ideal biological solventWater is an ideal biological solvent Water easily dissolves a wide variety Water easily dissolves a wide variety

of the constituents of living of the constituents of living organisms.organisms.

Water also unable to dissolve some Water also unable to dissolve some substances substances

This behavior is called hydrophilic This behavior is called hydrophilic and hydrophobic properties of water.and hydrophobic properties of water.

Hydrophilic moleculesHydrophilic molecules Ionic or polar substances that has an Ionic or polar substances that has an

affinity for wateraffinity for water In Greek= Hydro, “water” philios, In Greek= Hydro, “water” philios,

“loving”“loving” Water dipole structure and its capacity to Water dipole structure and its capacity to

form hydrogen bond with electronegative form hydrogen bond with electronegative atoms enable water to dissolve ionic and atoms enable water to dissolve ionic and polar substancepolar substance

These substances soluble in water due to These substances soluble in water due to 3 kinds of noncovalent bonding :3 kinds of noncovalent bonding :

a) ion-dipolea) ion-dipole b) dipole-dipoleb) dipole-dipole c) hydrogen bondingc) hydrogen bonding

Salts (KCl,NaCl) held together by ionic Salts (KCl,NaCl) held together by ionic interactionsinteractions

When ionic compound eg. KCl,NaCl When ionic compound eg. KCl,NaCl dissolved in water, its ions separate dissolved in water, its ions separate because the polar water molecules attract because the polar water molecules attract ions more than the ions attract each other. ions more than the ions attract each other. ((ion-dipole interactionion-dipole interaction))

Shells of water mol. cluster around the ions Shells of water mol. cluster around the ions = solvation spheres= solvation spheres

K + Cl-

HO

H

HO

H

HO

H

H

OH

HO

HHO

H

HO

H H

OH

Dipole-dipole InteractionsDipole-dipole Interactions Organic molecules with ionize groupOrganic molecules with ionize group The polar water molecule interacts with carboxyl The polar water molecule interacts with carboxyl

group of aldehyd & ketones (carbohyd) and hydroxyl group of aldehyd & ketones (carbohyd) and hydroxyl group of alcoholgroup of alcohol

HO

H

HO H

CH3

CCH3

OHOH

+

-

Dipole-dipoleinteractions

Hydrogen BondingHydrogen Bonding A hydrogen attached to an O A hydrogen attached to an O

or N becomes very polarized or N becomes very polarized and highly partial plus. This and highly partial plus. This partial positive charge partial positive charge interacts with the nonbonding interacts with the nonbonding electrons on another O or N electrons on another O or N giving rise to the very giving rise to the very powerful hydrogen bond.powerful hydrogen bond.

R1 O H

HO

H

HOH

hydrogen bondshown in yellow

Hydrophobic moleculesHydrophobic molecules

Non ionic or nonpolar substance Non ionic or nonpolar substance These molecules do not form good These molecules do not form good

attractions with the water molecule. attractions with the water molecule. They are insoluble and are said to be They are insoluble and are said to be hydrophobichydrophobic (water hating). (water hating).

eg. Hydrocarbon : eg. Hydrocarbon : CHCH33CHCH22CHCH22CHCH22CHCH22CHCH33, hexane, hexane

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

Amphipathic MoleculesAmphipathic Molecules Amphipathic molecules contain both Amphipathic molecules contain both

polar and nonpolar groups.polar and nonpolar groups. Ionized fatty acids are amphipathic. Ionized fatty acids are amphipathic.

The carboxylate group is water soluble The carboxylate group is water soluble (hydrophilic) and the long carbon (hydrophilic) and the long carbon chain is not (hydrophobic).chain is not (hydrophobic).

Amphipathic molecules tend to form Amphipathic molecules tend to form micelles micelles when mixed with water. when mixed with water.

polar head – orient themselves in contact polar head – orient themselves in contact with water moleculeswith water molecules

Nonpolar tails – aggregate in the center, Nonpolar tails – aggregate in the center, away from wateraway from water

Osmotic PressureOsmotic Pressure OsmosisOsmosis is a spontaneous process in is a spontaneous process in

which solvent (eg water) molecules pass which solvent (eg water) molecules pass through a semi permeable membrane through a semi permeable membrane from a solution of from a solution of lower solute lower solute concentrationconcentration (dilute) to a solution of (dilute) to a solution of higher solute concentrationhigher solute concentration (concentrated).(concentrated).

Osmotic pressureOsmotic pressure is the pressure required is the pressure required to stop osmosis (22.4 atm for 1M solution)to stop osmosis (22.4 atm for 1M solution)

BA BA

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

Osmotic PressureOsmotic Pressure Osmotic pressure (Osmotic pressure () is measured using an ) is measured using an

osmometerosmometer..

Osmotic PressureOsmotic Pressure iMRTiMRT

ii = van’t Hoff factor (degree of ionization of = van’t Hoff factor (degree of ionization of solute) solute)

M M = molarity (concentration of solute in mole/L)= molarity (concentration of solute in mole/L)

RR = gas constant (0.082 L.atm/K.mole) = gas constant (0.082 L.atm/K.mole)

T T = absolute temp (in Kelvin)= absolute temp (in Kelvin)

Osmolarity = iM (osmol/Liter)Osmolarity = iM (osmol/Liter)

ii is the is the van't Hoff coefficientvan't Hoff coefficient. .

For non-electrolytes (non ionizable solute) For non-electrolytes (non ionizable solute) ii=1=1

For strong electrolytes For strong electrolytes ii= the number of ions that are = the number of ions that are produced by the dissociation according to the molecular formula produced by the dissociation according to the molecular formula

e.g for NaCl you have 2 ions (1 Nae.g for NaCl you have 2 ions (1 Na++ and 1 Cl and 1 Cl--) so i=2) so i=2 for CaClfor CaCl22, 3 ions (1 Ca, 3 ions (1 Ca+2+2 and 2 Cl and 2 Cl--) so i=3) so i=3

For weak electrolytes For weak electrolytes ii=(1-=(1-aa)+)+nana nn = the number of ions coming from the 100% = the number of ions coming from the 100%

dissociation according to the molecular formula dissociation according to the molecular formula a = a = the degree of dissociationthe degree of dissociation e.g the degree of ionization of 1M CHe.g the degree of ionization of 1M CH33COOH solution is 80% COOH solution is 80% a=80%/0.8 , n=2a=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

Question 1Question 1

1)Estimate the osmotic pressure of a 1)Estimate the osmotic pressure of a solution 1M NaCl at 25°C. Assume solution 1M NaCl at 25°C. Assume 100% ionization of solute.100% ionization of solute.

iMRTiMRT

i= 2 (1 Nai= 2 (1 Na++ and 1 Cl and 1 Cl--))

M= 1 mol/LM= 1 mol/L

R= 0.0821 L.atm/K.molR= 0.0821 L.atm/K.mol

T= 298KT= 298K

Question 2Question 2

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

Osmotic pressure is an important Osmotic pressure is an important factor affecting cellsfactor affecting cells

Cells contain high concentration of Cells contain high concentration of solutes – small organic mol., ionic solutes – small organic mol., ionic salts, macromoleculesalts, macromolecule

Cells may gain or lose water depend Cells may gain or lose water depend on concentration of solute in their on concentration of solute in their environment.environment.

IsotonicIsotonic – solutions of equal – solutions of equal concentration on either side of the concentration on either side of the membrane membrane

Cells placed in isotonic solution no Cells placed in isotonic solution no net movement of water across the net movement of water across the membranemembrane

Volume of cells are unchanged bcoz Volume of cells are unchanged bcoz water entering & leaving the cell at water entering & leaving the cell at the same rate.the same rate.

Definitions of solutionsDefinitions of solutions

HypotonicHypotonic – solution with a lower – solution with a lower solute concentration than the solution solute concentration than the solution on the other side of the membrane on the other side of the membrane

Cells placed in hypotonic solution Cells placed in hypotonic solution water moves into the cellswater moves into the cells

Cause cells ruptureCause cells rupture eg. Red blood cells swell & rupture eg. Red blood cells swell & rupture

when immersed in pure water when immersed in pure water ((hemolysishemolysis))

HypertonicHypertonic – solution with higher – solution with higher concentration of solutes than the concentration of solutes than the solution on the other side of the solution on the other side of the membrane membrane

Cells placed in hypertonic solution Cells placed in hypertonic solution water moves out the cellswater moves out the cells

Cause cells to Cause cells to shrinkshrink eg. Red blood cells shrink when eg. Red blood cells shrink when

immersed in 3% NaCl solution. immersed in 3% NaCl solution. ((crenationcrenation))

Water ionization, pH, titration, bufferWater ionization, pH, titration, buffer

The The self-ionization of waterself-ionization of water is the is the chemical reaction in which two water chemical reaction in which two water molecules react to produce a molecules react to produce a hydroniumhydronium (H(H33OO++)) and a and a hydroxidehydroxide (OH (OH−−) ) ion.ion.

Water ionization occurs endothermically Water ionization occurs endothermically due to electric field fluctuations between due to electric field fluctuations between molecules caused by nearby molecules caused by nearby dipole dipole librations librations resulting from thermal effects, resulting from thermal effects, and favorable localized hydrogen bonding. and favorable localized hydrogen bonding.

Water dissociates. (self-ionizes)Water dissociates. (self-ionizes) HH22O + HO + H22O = HO = H33OO++ + OH + OH--

Ions may separate but normally Ions may separate but normally recombine within a few min. to seconds. recombine within a few min. to seconds.

Rarely Rarely (about once every eleven hours per molecule at (about once every eleven hours per molecule at

25°C, or less than once a week at 0°C)25°C, or less than once a week at 0°C) the localized the localized hydrogen bonding arrangement breaks hydrogen bonding arrangement breaks before allowing the separated ions to before allowing the separated ions to return, and the pair of ions (H+, OH-) return, and the pair of ions (H+, OH-) hydrate independently and continue their hydrate independently and continue their separate existence. separate existence.

may be expressed asmay be expressed as

KKeqeq = [H = [H33OO++][OH][OH--] ]

[H[H22O]O]22

The conditions for the water dissociation The conditions for the water dissociation equilibrium must hold under all situations at equilibrium must hold under all situations at 25°C. 25°C.

KKww= [H= [H33OO++][OH][OH--] = 1 x 10] = 1 x 10-14-14MM

Pure water ionize into equal amount of Pure water ionize into equal amount of

[H[H33OO++ ] = [OH ] = [OH--] = 1 x 10] = 1 x 10-7-7 M M

Ionization of waterIonization of water

Acids, Bases and pHAcids, Bases and pH When external acids or bases are When external acids or bases are

added to water, the ion product added to water, the ion product ([H([H33OO++ ][OH ][OH--] ) must equal.] ) must equal.

KKww= [H= [H33OO++][OH][OH--] = 1 x 10] = 1 x 10-14-14

The effect of The effect of addedadded acids or bases is acids or bases is best understood using the best understood using the Bronsted-Bronsted-Lowry-Lowry- theory of acids and bases. theory of acids and bases.

Bronsted-Lowry theoryBronsted-Lowry theory is an acid-base is an acid-base theorytheory

Acid is a substance that can donate Acid is a substance that can donate protonproton (ion H (ion H++ donor) donor)

acid + base = conjugate base + acid + base = conjugate base + conjugate acid conjugate acid

HCl + HHCl + H22O = HO = H33OO++ + Cl + Cl--

Asid Base CA CBAsid Base CA CB

C: conjugate (product) A/BC: conjugate (product) A/B

Bronstead-Lowry theoryBronstead-Lowry theory

base is a substance that can accept base is a substance that can accept proton proton

RNHRNH22 + H + H22O = OHO = OH-- + RNH + RNH33++

B A CB CAB A CB CA

C: conjugate (product) A/BC: conjugate (product) A/B

Measuring AcidityMeasuring Acidity Added acids, increase concentration of Added acids, increase concentration of

hydronium ion hydronium ion In acid solutions [HIn acid solutions [H33OO++]] > > 1 x 10 1 x 10-7 -7 MM

[OH[OH--] ] << 1 x 10 1 x 10-7 -7 MM Added bases, increase concentration of Added bases, increase concentration of

hydroxide ion.hydroxide ion. In basic solutions [OHIn basic solutions [OH--] ] >> 1 x 10 1 x 10-7 -7 MM [H[H33OO++] ] << 1 x 10 1 x 10-7 -7 MM

pH scale measures acidity without using pH scale measures acidity without using exponential numbers.exponential numbers.

pH ScalepH Scale

Define: Define: pH = - logpH = - log(10)(10)[H[H33OO++]]

0---------------7---------------140---------------7---------------14

acidic basicacidic basic

[H[H33OO++]=1 x 10]=1 x 10-7 -7 M, pH = ?M, pH = ?

pH Scale pH Scale

QuestionsQuestions

1) [H1) [H33OO++]=1 x 10]=1 x 10-5 -5 M, pH = ?M, pH = ?

2) [H2) [H33OO++]=1 x 10]=1 x 10-10 -10 M, pH = ?M, pH = ?

3) [H3) [H33OO++]=1 x 10]=1 x 10-8 -8 M, pH = ?M, pH = ?

pH Scale pH Scale QuestionsQuestions

1) [H1) [H33OO++]=2.6 x 10]=2.6 x 10-5 -5 M, pH = ?M, pH = ?

2)[H2)[H33OO++]=6.3 x 10]=6.3 x 10-9 -9 M, pH = ?M, pH = ?

3)[H3)[H33OO++]=7.8 x 10]=7.8 x 10-3 -3 M, pH = ?M, pH = ?

pH ScalepH Scale

pH to [HpH to [H33OO++]?]?

inverse log of negative pHinverse log of negative pH

orange juice, pH 3.5. [Horange juice, pH 3.5. [H33OO++]=?]=?

urine, pH 6.2. [Hurine, pH 6.2. [H33OO++]=?]=?

Strength of AcidsStrength of Acids Strength of an acid is measured by the Strength of an acid is measured by the

percent which reacts with water to percent which reacts with water to form hydronium ions.form hydronium ions.

Strong acids (and bases) ionize close Strong acids (and bases) ionize close to 100%.to 100%.

• eg. HCl, HBr, HNOeg. HCl, HBr, HNO33, H, H22SOSO44

• eg. NaOH, KOH, CaOHeg. NaOH, KOH, CaOH

Strength of AcidsStrength of Acids

Weak acids (or bases) ionize typically in Weak acids (or bases) ionize typically in the 1-5% rangethe 1-5% range

eg. Organic acid (contain carboxyl eg. Organic acid (contain carboxyl groups)groups)

CHCH33COCOOH, pyruvic acidCOCOOH, pyruvic acid

CHCH33CHOHCOOH, lactic acid CHOHCOOH, lactic acid CH CH33COOH, acetic acid COOH, acetic acid

Strength of AcidsStrength of Acids Strength of an acid is also measured by Strength of an acid is also measured by

its Kits Kaa or pK or pKaa values values Dissociation of weak acid : Dissociation of weak acid :

HAHA + H + H22O = HO = H33OO++ + + AA--

Weak acid conjugate base of HAWeak acid conjugate base of HA

Strength of weak acid may be Strength of weak acid may be determined : determined : Ka = [H3O

+][A-] [HA]

pKa= -log Ka

Strength of AcidsStrength of Acids KKaa pK pKaa

CHCH33COCOOH 3.2x10COCOOH 3.2x10-3 -3 2.52.5

CHCH33CHOHCOOH 1.4x10CHOHCOOH 1.4x10-4 -4 3.93.9

CHCH33COOH 1.8x10COOH 1.8x10-5 -5 4.84.8

Larger KLarger Kaa and and smallersmaller pK pKaa values indicate values indicate

stronger acids.stronger acids.

Monitoring acidityMonitoring acidity

The The Henderson-Hasselbalch (HH) Henderson-Hasselbalch (HH) equationequation is derived from the is derived from the equilibrium expression for a weak equilibrium expression for a weak acid.acid.

pH = pKa + log [A-] [HA]

HH equationHH equation The HH equation enables us to The HH equation enables us to

calculate the pH during a calculate the pH during a titrationtitration and and to make predictions regarding to make predictions regarding buffer buffer solutions.solutions.

What is a titration?What is a titration?It is a process in which carefully It is a process in which carefully measured volumes of a base are measured volumes of a base are added to a solution of an acid in order added to a solution of an acid in order to determine the acid concentration.to determine the acid concentration.

When chemically equal (equivalent) When chemically equal (equivalent) amounts of acid and base are present amounts of acid and base are present during a titration, the during a titration, the equivalence pointequivalence point is is reached.reached.

The equivalence point is detected by using The equivalence point is detected by using an indicator chemical that changes color or an indicator chemical that changes color or by by following the pH of the reaction versus following the pH of the reaction versus added base, ie. a titration curve.added base, ie. a titration curve.

Titration Curve (HOAc with NaOH)Titration Curve (HOAc with NaOH)

Equivalence point

End point

NaOH (equivalents added)

pH

Titration Curve (HOAc with NaOH)Titration Curve (HOAc with NaOH)

At the end point, only the salt (NaOAc) is At the end point, only the salt (NaOAc) is present in solution.present in solution.

At the equivalence point, equal moles of At the equivalence point, equal moles of salt and acid are present in solution.salt and acid are present in solution.

[HOAc] = [NaOAc][HOAc] = [NaOAc]

pH = pKpH = pKaa

QuestionsQuestions1)1) By using HH equation, calculate the By using HH equation, calculate the

pH of a mixture of 0.25M acetic acid pH of a mixture of 0.25M acetic acid and 0.1M sodium acetate. The pKand 0.1M sodium acetate. The pKaa of of acetic acid is 4.76acetic acid is 4.76

pH = pKpH = pKaa + log [A + log [A--]]

[HA][HA]

pH = 4.76 + log [acetate]pH = 4.76 + log [acetate]

[acetic acid][acetic acid]

pH = 4.76 + log 0.1 = 4.36pH = 4.76 + log 0.1 = 4.36

0.250.25

2) Calculate the ratio of lactic acid to 2) Calculate the ratio of lactic acid to lactate in a buffer at pH 5.00. The pKlactate in a buffer at pH 5.00. The pKaa for for lactic acid is 3.86lactic acid is 3.86

5.00 = 3.86 + log [lactate] [lactic acid]

5.00-3.86 = log [lactate] [lactic acid]

antilog 1.14 = [lactate] [lactic acid]

= 13.8

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

pH = pKa + log [A-]pH = pKa + log [A-]

[HA][HA]

= 2.96 + log [hydrogen tertrate]= 2.96 + log [hydrogen tertrate]

[tartaric acid][tartaric acid]

= 2.96 + log 2= 2.96 + log 2

= 3.26= 3.26

Buffer solutionBuffer solution

Buffer : a solution that resists change Buffer : a solution that resists change in pH when small amounts of strong in pH when small amounts of strong acid or base are added.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

How does buffer work?How does buffer work?

Accepting hydrogen ions from the Accepting hydrogen ions from the solution when they are in excesssolution when they are in excess

Donating hydrogen ions from the Donating hydrogen ions from the solution when they have depletedsolution when they have depleted

Buffer SolutionsBuffer Solutions Maximum buffer effect occurs at the pKMaximum buffer effect occurs at the pKaa for for

an acid.an acid. Effective buffer range is at 1 pH unit above Effective buffer range is at 1 pH unit above

and below the pKand below the pKaa value for the acid or base. value for the acid or base.

eg. Heg. H22POPO44--/HPO/HPO44

2-2-, pK, pKaa=7.20=7.20

buffer range 6.20-8.20 pHbuffer range 6.20-8.20 pH

Buffer SolutionsBuffer Solutions High concentrations of acid and High concentrations of acid and

conjugate base give a high buffering conjugate base give a high buffering capacity.capacity.

Buffer systems are chosen to match Buffer systems are chosen to match the pH of the physiological situation, the pH of the physiological situation, usually around pH 7.usually around pH 7.

Physiological bufferPhysiological buffer 3 most important buffer in body:3 most important buffer in body:

Within cells the primary buffer is the Within cells the primary buffer is the phosphate bufferphosphate buffer: H: H22POPO44

--/HPO/HPO442-2-

The primary blood buffer is the The primary blood buffer is the bicarbonate bicarbonate bufferbuffer: HCO: HCO33

--/H/H22COCO33..

ProteinsProteins also provide buffer capacity. Side also provide buffer capacity. Side chains can accept or donate protons. (eg. chains can accept or donate protons. (eg. Hemoglobin, serum albumins)Hemoglobin, serum albumins)

A zwitterion is a compound with both A zwitterion is a compound with both positive and negative charges.positive and negative charges.

Zwitterionic buffersZwitterionic buffers have become have become common because they are less likely common because they are less likely to cause complications with to cause complications with biochemical reactions.biochemical reactions.

N-tris(hydroxymethyl)methyl-2-N-tris(hydroxymethyl)methyl-2-aminoethane sulfonate (TES) is a aminoethane sulfonate (TES) is a zwitterion buffer example.zwitterion buffer example.

(HOCH(HOCH22))33CNCN++HH22CHCH22CHCH22SOSO33--

Assignment (water)Assignment (water)

Date of submission: Date of submission: 30/7/1030/7/10

1.1. Explain how the changes in temperature Explain how the changes in temperature give effect to hydrogen bonds in water give effect to hydrogen bonds in water molecule. Elaborate the situation with molecule. Elaborate the situation with drawing of water moleculesdrawing of water molecules at every at every temperature level.temperature level.

2. Explain how the acids produced in 2. Explain how the acids produced in metabolism are transported to the liver metabolism are transported to the liver without greatly affecting the pH of the without greatly affecting the pH of the blood.blood.

The EndThe End

Water : The Medium of LifeWater : The Medium of Life