WATER By KHAIRUL FARIHAN KASIM. Chapter 3: Outline Molecular Nature of Water Noncovalent Bonding...

WATERWATERBy

KHAIRUL FARIHAN KASIM

Chapter 3: OutlineChapter 3: Outline

Molecular Nature of WaterMolecular Nature of Water Noncovalent BondingNoncovalent Bonding

– Ionic interactionsIonic interactions – Hydrogen BondsHydrogen Bonds– van der Waals Forcesvan der Waals Forces

Thermal Properties of WaterThermal Properties of Water Solvent Properties of WaterSolvent Properties of Water

– Hydrophilic, hydrophobic, and amphipathic moleculesHydrophilic, hydrophobic, and amphipathic molecules– Osmotic pressureOsmotic pressure

Ionization of WaterIonization of Water– Acids, bases, and pHAcids, bases, and pH– BuffersBuffers– Physiological buffersPhysiological buffers

WaterWater

Solvent for all chemical reactions.Solvent for all chemical reactions. Transports chemicals from place to Transports chemicals from place to

place.place. Helps to maintain constant body Helps to maintain constant body

temperature.temperature. Part of digestive fluids.Part of digestive fluids. Dissolves excretion products. Dissolves excretion products.

3.1 Molecular Structure of Water3.1 Molecular Structure of Water

The oxygen in water The oxygen in water is spis sp33 hybridized. hybridized. Hydrogens are Hydrogens are bonded to two of the bonded to two of the orbitals. Consequently orbitals. Consequently the water molecule is the water molecule is bent. The H-O-H bent. The H-O-H angle is 104.5angle is 104.5oo..

Water is a Water is a polar molecule. polar molecule.

– A polar molecule is one in which one end A polar molecule is one in which one end is partially positive and the other partially is partially positive and the other partially negative.negative.

– This polarity results from unequal sharing This polarity results from unequal sharing of electrons in the bonds and the specific of electrons in the bonds and the specific geometry of the molecule.geometry of the molecule.

Water molecule with bond ( ) and net Water molecule with bond ( ) and net

( ) dipoles.( ) dipoles.

HO

H+

-

+

Water has an abnormally high boiling Water has an abnormally high boiling point due to intermolecular hydrogen point due to intermolecular hydrogen bonding.bonding.

HO

H

HO

H

HO

H

H bonding is a weak attraction between an electronegative atom in one molecule and an H(on an O or N) in another.

3.2 Noncovalent Bonding3.2 Noncovalent Bonding

Ionic interactionsIonic interactions

Hydrogen bondingHydrogen bonding

Van der Waals forcesVan der Waals forces–Dipole-dipoleDipole-dipole–Dipole-induced dipoleDipole-induced dipole–Induced dipole-induced dipoleInduced 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

Ionic InteractionsIonic Interactions Ionic interactions occur between charged atoms or Ionic interactions occur between charged atoms or

groups.groups.

In proteins, side chains sometimes form ionic In proteins, side chains sometimes form ionic salt salt bridgesbridges, particularly in the absence of water which , particularly in the absence of water which normally hydrates ions.normally hydrates ions.

CH2CH2COO-

CH2CH2NH3

+

Salt bridge

Hydrogen BondingHydrogen Bonding In water molecules each of molecules can In water molecules each of molecules can

form hydrogen bonds with four other water form hydrogen bonds with four other water molecules. molecules.

two through two through thethe

hydrogens and hydrogens and

two through two through the the nonbonding nonbonding

electron pairs.electron pairs.

HO

H

HO

H

HO

H

HO

HH

OH

Van der Waals AttractionsVan der Waals Attractionsa. Dipole-dipolea. Dipole-dipole

b. Dipole-induced dipoleb. Dipole-induced dipole

c. Induced dipole-induced dipolec. Induced dipole-induced dipole

C O C O+-

+-

H

H

HH

H

H

HH

+-

+-

C O H

H

HH

+- +

-

Hydrophobic interactionsHydrophobic interactions

Nonpolar molecules tend to coalesce into Nonpolar molecules tend to coalesce into droplets in water. The repulsions between droplets in water. The repulsions between the water molecules and the nonpolar the water molecules and the nonpolar molecules cause this phenomenon.molecules cause this phenomenon.

The water molecules form a “cage” around The water molecules form a “cage” around the small hydrophobic droplets.the small hydrophobic droplets.

3.3 Thermal Properties3.3 Thermal Properties

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 atmHeat capacityHeat capacity - energy to change the - energy to change the temperataure by 1temperataure 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.

3.4 Solvent Properties3.4 Solvent Properties Water dissolves chemicals that have an Water dissolves chemicals that have an

affinity for it, ie. hydrophilic (water loving) affinity for it, ie. hydrophilic (water loving) materials.materials.– many ionic compoundsmany ionic compounds– polar organic compoundspolar organic compounds

These compounds are soluble in water due These compounds are soluble in water due to three kinds of noncovalent interactions:to three kinds of noncovalent interactions:1.1. ion-dipole ion-dipole

2.2. dipole-dipole dipole-dipole

3.3. hydrogen bondinghydrogen bonding

Ion-dipole InteractionsIon-dipole Interactions Ions are hydrated by water molecules. The water Ions are hydrated by water molecules. The water

molecules orient so the opposite charge end molecules orient so the opposite charge end points to the ion to partially neutralize charge. The points to the ion to partially neutralize charge. The shell of water molecules is a solvation sphere.shell of water molecules is a solvation sphere.

K + Cl-

HO

H

HO

H

HO

H

H

OH

HO

HHO

H

HOH H

OH

Dipole-dipole InteractionsDipole-dipole Interactions The polar water molecule interacts with an The polar water molecule interacts with an

O or N or an H on an O or N on an organic O or N or an H on an O or N on an organic molecule.molecule.

HO

H

HO H

CH3

CCH3

OHOH

+

-

Dipole-dipoleinteractions

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

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

R1 O H

HO

H

HOH

hydrogen bondshown in yellow

Nonpolar MoleculesNonpolar Molecules Nonpolar molecules have no polar Nonpolar molecules have no polar

bonds or the bond dipoles cancel due bonds or the bond dipoles cancel due to molecular geometry.to molecular geometry.

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.: CHeg.: CH33CHCH22CHCH22CHCH22CHCH22CHCH33, hexane, hexane

Nonpolar Molecules-2Nonpolar Molecules-2

Water forms hydrogen-bonded cagelike Water forms hydrogen-bonded cagelike structures around hydrophobic molecules, structures around hydrophobic molecules, forcing them out of solution.forcing them out of solution.

Amphipathic MoleculesAmphipathic Molecules Amphipathic molecules contain both polar Amphipathic molecules contain both polar

and nonpolar groups.and nonpolar groups.

Ionized fatty acids are amphipathic. The Ionized fatty acids are amphipathic. The carboxylate group is water soluble and the carboxylate group is water soluble and the long carbon chain is not.long carbon chain is not.

Amphipathic molecules tend to form Amphipathic molecules tend to form micelles, colloidal aggregates with the micelles, colloidal aggregates with the charged “head” facing outward to the water charged “head” facing outward to the water and the nonpolar “tail” part inside.and the nonpolar “tail” part inside.

A MicelleA Micelle

Osmotic Pressure-2Osmotic Pressure-2

Osmosis is a spontaneous process in which Osmosis is a spontaneous process in which solvent molecules pass through a solvent molecules pass through a semipermeable membrane from a solution semipermeable membrane from a solution of lower solute concentration to a solution of of lower solute concentration to a solution of higher solute concentration.higher solute concentration.

Osmotic pressure is the pressure required Osmotic pressure is the pressure required to stop osmosis.to stop osmosis.


Osmotic pressure (Osmotic pressure () is measured in an ) is measured in an osmometer.osmometer.

Osmotic Pressure-4Osmotic Pressure-4 iMRTiMRT

i = van’t Hoff factor (% as ions)i = van’t Hoff factor (% as ions)

M = molarity (mol/L for dilute solns)M = molarity (mol/L for dilute solns)

R = 0.082 L atm/ mol KR = 0.082 L atm/ mol K

T = Kelvin temperatureT = Kelvin temperature

The concentration of a solution can be expressed in terms of osmolarity.The concentration of a solution can be expressed in terms of osmolarity.

The unit is osmol/ literThe unit is osmol/ liter

Osmolarity = iMOsmolarity = iM

where i (the vant’t hoff factor) represents the degree of ionization of the where i (the vant’t hoff factor) represents the degree of ionization of the solute species.solute species.

The degree of ionization of a 1 M NaCl solution is 90% with 10% of the The degree of ionization of a 1 M NaCl solution is 90% with 10% of the NACl existing as ion pairs.NACl existing as ion pairs.

i = [Nai = [Na++] + [Cl] + [Cl--] + [NaCl]] + [NaCl]unionizedunionized

i = 0.9 + 0.9 + 0.1 = 1.9i = 0.9 + 0.9 + 0.1 = 1.9

Class discussionClass discussion

Problem 1Problem 1

When 0.1 g of urea (M.W = 60) is diluted to When 0.1 g of urea (M.W = 60) is diluted to 100 mL, what is the osmotic pressure of the 100 mL, what is the osmotic pressure of the solution? Assume that the temperature is solution? Assume that the temperature is 25°C25°C

Problem 2Problem 2

Estimate the osmotic pressure of a solution of Estimate the osmotic pressure of a solution of 0.1 M NaCl at 25°C. Assume 100% 0.1 M NaCl at 25°C. Assume 100% ionization of solute?ionization of solute?


Because cells have a higher ion Because cells have a higher ion concentration than the surrounding fluids, concentration than the surrounding fluids, they tend to pick up water through the they tend to pick up water through the semipermeable cell membrane.semipermeable cell membrane.

The cell is said to be hypertonic relative to The cell is said to be hypertonic relative to the surrounding fluid and will burst the surrounding fluid and will burst (hemolyze) if osomotic control is not (hemolyze) if osomotic control is not effected. effected.


Cells placed in a hypotonic solution will lose Cells placed in a hypotonic solution will lose water and shrink (crenate).water and shrink (crenate).

If cells are placed in an isotonic solution If cells are placed in an isotonic solution (conc same on both sides of membrane) (conc same on both sides of membrane) there is no net passage of water.there is no net passage of water.

3.5 Ionization of Water3.5 Ionization of Water

Water dissociates. (self-ionizes)Water dissociates. (self-ionizes)

HH22O + HO + H22O = HO = H33OO++ + OH + OH--

Kw = Ka [H2O]2 = [H3O+ ][OH-]

Ka = [H3O+][OH-]

[H2O]2

Water Ionization-2Water Ionization-2 The conditions for the water The conditions for the water

dissociation equilibrium must hold dissociation equilibrium must hold under all situations at 25under all situations at 25oo..

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

In neutral water, In neutral water,

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

MM

Water: A/B PropertiesWater: A/B Properties 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 K] ) must equal Kww..

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

Water: A/B Properties-2Water: A/B Properties-2

Lowry-BronstedLowry-Bronstedacid = proton donoracid = proton donor

HA + HHA + H22O = HO = H33OO++ + A + A--

A B CA CBA B CA CB

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

Water: A/B Properties-3Water: A/B Properties-3

Lowry-BronstedLowry-Bronstedbase = proton acceptorbase = proton acceptor

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

B A CB CAB A CB CA

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

hydronium ion and bases the concentration of hydronium ion and bases the concentration of hydroxide ion.hydroxide ion.

In acid solutions In acid solutions [H[H33OO++]] > > 1 x 10 1 x 10-7 -7 MM [OH[OH--] ] << 1 x 10 1 x 10-7 -7 MM

In basic solutions In basic solutions [OH[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: pH = - logDefine: pH = - 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 = ?

7.0 7.0

pH Scale - 2pH Scale - 2

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

5 (acidic)5 (acidic)

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

10 (basic)10 (basic)

What if preexponential number is What if preexponential number is not 1?not 1?

pH Scale - 3pH Scale - 3[H[H33OO++]=2.6 x 10]=2.6 x 10-5 -5 M, pH = ?M, pH = ?

4.59 (acidic)4.59 (acidic)

[H[H33OO++]=6.3 x 10]=6.3 x 10-9 -9 M, pH = ?M, pH = ?

8.20 (basic)8.20 (basic)

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

2.11 (acidic)2.11 (acidic)

pH Scale-4pH Scale-4pH to [HpH to [H33OO++]?]?

inverse log of negative pHinverse log of negative pH

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

[H[H33OO++] = 3.2 x 10] = 3.2 x 10-4 -4 MM

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

[H[H33OO++] = 6.3 x 10] = 6.3 x 10-7 -7 MM

Strength of AcidsStrength of Acids

Strength of an acid is measured Strength of an acid is measured by the percent which reacts with by the percent which reacts with water to form hydronium ions.water to form hydronium ions.

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

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

Strength of Acids-2Strength of Acids-2

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

eg. CHeg. CH33COCOOH, pyruvic acidCOCOOH, pyruvic acid

CHCH33CHOHCOOH, lactic acidCHOHCOOH, lactic acid

CHCH33COOH, acetic acid COOH, acetic acid

Strength of Acids-3Strength of Acids-3

Strength of an acid is also Strength of an acid is also measured by its Kmeasured by its Kaa or pK or pKaa values. values.

HA + HHA + H22O = HO = H33OO++ + A + A--

Larger Ka and smaller pKa valuesindicate stronger acids.

Ka = [H3O+][A-]

[HA]

Strength of Acids-4Strength of Acids-4 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 stronger acids. values indicate stronger acids.

3.5 (cont.) Monitoring Acidity3.5 (cont.) Monitoring Acidity

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

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

Monitoring Acidity-2Monitoring Acidity-2 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.

Monitoring Acidity-3Monitoring Acidity-3

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 reached.is reached.

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

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

moles OH- per mole acid

pH

Titration Curve (HOAc with NaOH) - 2Titration Curve (HOAc with NaOH) - 2

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

At the inflection point, equal moles At the inflection point, equal moles of salt and acid are present in of salt and acid are present in solution.solution. [HOAc] = [NaOAc][HOAc] = [NaOAc]

pH = pKpH = pKaa


The pKThe pKaa for acetic acid is 4.76. for acetic acid is 4.76.

1. Calculate the relative percents of acetic 1. Calculate the relative percents of acetic acid and acetate ion when the acid is acid and acetate ion when the acid is titrated with 0.7 equivalents of NaOH.titrated with 0.7 equivalents of NaOH.

2. Use the Henderson-Hasselbalch 2. Use the Henderson-Hasselbalch equation to calculate the pH at this equation to calculate the pH at this point. point.


0.7 equivalents of NaOH neutralizes 0.7 eq 0.7 equivalents of NaOH neutralizes 0.7 eq of acid producing 0.7 eq of salt and leaving of acid producing 0.7 eq of salt and leaving 0.3 eq of unneutralized acid.0.3 eq of unneutralized acid.

pKpKaa of HOAc is 4.76 of HOAc is 4.76

pH = 4.76 + log [0.7] [0.3]

30% acid and 70% salt. pH=5.13

AssignmentAssignment

Buffer SolutionsBuffer Solutions

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

Buffer Solutions-2Buffer Solutions-2 Maximum buffer effect occurs at Maximum buffer effect occurs at

the pKthe pKaa for an acid. for an acid.

Effective buffer range is +/- 1 of the Effective buffer range is +/- 1 of the pK value for the acid or base.pK value for the acid or base.

eg. Heg. H22POPO44--/HPO/HPO44

2-2-, K, Kaa=7.20=7.20

buffer range 6.20-8.20 pHbuffer range 6.20-8.20 pH

Buffer Solutions-3Buffer Solutions-3

High concentrations of acid and High concentrations of acid and conjugate base give a high conjugate base give a high buffering capacity.buffering capacity.

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


Within cells the primary buffer is the Within cells the primary buffer is the phosphate buffer: Hphosphate buffer: H22POPO44

--/HPO/HPO442-2-

The primary blood buffer is the The primary blood buffer is the bicarbonate system: HCObicarbonate system: HCO33

--/H/H22COCO33..

Proteins also provide buffer Proteins also provide buffer capacity. Side chains can accept or capacity. Side chains can accept or donate protons.donate protons.


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

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


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

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


Buffers work by chemically tying up Buffers work by chemically tying up acid and base. Eg.:acid and base. Eg.:

HCO3- + H3O

+ H2CO3 + H2O

H2CO3 + OH- HCO3- + H2O


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

The EndThe End

Water : The Medium of LifeWater : The Medium of Life

WATER By KHAIRUL FARIHAN KASIM. Chapter 3: Outline Molecular Nature of Water Noncovalent Bonding...

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Transcript of WATER By KHAIRUL FARIHAN KASIM. Chapter 3: Outline Molecular Nature of Water Noncovalent Bonding...