Water and PolarityWater and Polarity

Chapter 2Chapter 2

What is electronegativity?What is electronegativity?

The tendency of atoms to attract electrons to The tendency of atoms to attract electrons to itself in a chemical bond – itself in a chemical bond – electronegativityelectronegativity

A measure of the force of an atom’s attraction A measure of the force of an atom’s attraction for electrons it shares in a chemical bond with for electrons it shares in a chemical bond with another atom – another atom – electronegativityelectronegativity

ElectronegativityElectronegativity Oxygen and Nitrogen - more electronegative Oxygen and Nitrogen - more electronegative

than Carbon and Hydrogenthan Carbon and Hydrogen Fluorine is most electronegativeFluorine is most electronegative (4)(4)

Formation of nonpolar bondsFormation of nonpolar bonds

Atoms of same element share electrons equally Atoms of same element share electrons equally in a bond – equal electronegativityin a bond – equal electronegativity

When atoms with same electronegativity form When atoms with same electronegativity form a bond – there is equal sharing of electrons - a bond – there is equal sharing of electrons - nonpolar bondsnonpolar bonds

C-H bond in CH4C-H bond in CH4

Formation of polar bondsFormation of polar bonds

Atoms of different elements may not share Atoms of different elements may not share electrons equally – unequal electronegativityelectrons equally – unequal electronegativity

When atoms with different electronegativity When atoms with different electronegativity form a bond – there is unequal sharing of form a bond – there is unequal sharing of electrons - polar bondselectrons - polar bonds

Water - of polar bondWater - of polar bond

What is polarity?What is polarity? Oxygen is an Oxygen is an

"electronegative" or electron "electronegative" or electron "loving" atom. "loving" atom.

Water is a "polar" molecule - Water is a "polar" molecule - there is an uneven distribution there is an uneven distribution of electron densityof electron density giving rise giving rise to partial positive charge to partial positive charge ∂+ ∂+ and ∂-and ∂-

Hydrogen bondsHydrogen bonds (dipole- (dipole-dipole bond) are weak dipole bond) are weak attractions between the attractions between the partially negative oxygen of partially negative oxygen of one water molecule and the one water molecule and the partially positive hydrogen of partially positive hydrogen of a a differentdifferent water molecule water molecule ((PolarityPolarity))

What are dipoles?What are dipoles?What is a polar bond:What is a polar bond:

• • Electrons are unequally shared, more negative charge found closer to one Electrons are unequally shared, more negative charge found closer to one atom.atom.

• • Due to difference in electronegativity of atoms involved in bond.Due to difference in electronegativity of atoms involved in bond.

Polar Bonds & MoleculesPolar Bonds & Molecules

COCO22 have have polarpolar

bonds bonds

Geometry - Geometry - nonpolarnonpolar molecules molecules

COCO22 have a zero have a zero

dipole momentsdipole moments

Solvent Properties of HSolvent Properties of H22OO

HydrophilicHydrophilic: water-loving: water-loving tend to dissolve in watertend to dissolve in water

HydrophobicHydrophobic: water-fearing: water-fearing tend not to dissolve in watertend not to dissolve in water

AmphipathicAmphipathic: has characteristics of both : has characteristics of both propertiesproperties molecules that contain one or more hydrophobic molecules that contain one or more hydrophobic

and one or more hydrophilic regions, e.g., sodium and one or more hydrophilic regions, e.g., sodium palmitatepalmitate

Why do some chemicals dissolve in Why do some chemicals dissolve in water?water?

Hydrophillic (water Hydrophillic (water loving/polar)loving/polar)

Polar covalent Polar covalent compounds (eg: compounds (eg: alcohols - ethanol and alcohols - ethanol and ketones – acetoneketones – acetone

SugarsSugars Ionic compounds-KClIonic compounds-KCl Amino acids, phosphate Amino acids, phosphate

estersesters

Hydrophobic (water Hydrophobic (water fearing/nonpolar)fearing/nonpolar)

Nonpolar covalent Nonpolar covalent compounds (eg: compounds (eg: hydrocarbons hexane)hydrocarbons hexane)

Fatty acidsFatty acids CholesterolCholesterol

Ion-dipole and Dipole-dipole InteractionsIon-dipole and Dipole-dipole Interactions

Electrostatic attraction of unlike charges Electrostatic attraction of unlike charges

- ion-dipole interaction: e.g., KCl dissolved in - ion-dipole interaction: e.g., KCl dissolved in HH22OO

- dipole-dipole interactions: e.g., ethanol or dipole-dipole interactions: e.g., ethanol or acetone dissolved in Hacetone dissolved in H22OO

- dipole induced-dipole interactions: weak dipole induced-dipole interactions: weak and generally do not lead to solubility in and generally do not lead to solubility in waterwater

• • Ion-dipole and dipole-dipole interactions help ionic and polar Ion-dipole and dipole-dipole interactions help ionic and polar compounds dissolve in watercompounds dissolve in water

Ion-dipole and Dipole-dipole InteractionsIon-dipole and Dipole-dipole Interactions

Amphipathic moleculesAmphipathic molecules• • Polar (carboxylic acid group ) and nonpolar Polar (carboxylic acid group ) and nonpolar

(hydrocarbon group) (hydrocarbon group)

Why do oil and water mixed together Why do oil and water mixed together separate into layers?separate into layers?

Ionized polar groups are Ionized polar groups are in contact with aqueous in contact with aqueous environment and environment and nonpolar tails are nonpolar tails are sequestered from water sequestered from water --Micelles formationMicelles formation

Oils and salad dressing Oils and salad dressing form oil/sphere dropletsform oil/sphere droplets

Van der Waals Van der Waals interactions interactions

Why does water have such interesting Why does water have such interesting and unique properties?and unique properties?

Tetrahedral arrangementTetrahedral arrangement Two pairs of partial Two pairs of partial

positive charge and two positive charge and two of partial negative of partial negative chargecharge

Each water molecule Each water molecule can be involved in 4 can be involved in 4 hydrogen bonds: 2 as hydrogen bonds: 2 as donor, and 2 as acceptordonor, and 2 as acceptor

Strength of hydrogen BondingStrength of hydrogen Bonding Hydrogen bonds are weaker than covalent Hydrogen bonds are weaker than covalent

bondsbonds

Why does water have such interesting Why does water have such interesting and unique properties?and unique properties?

High melting and boiling pointHigh melting and boiling point

Solid water is less dense than liquid water.Solid water is less dense than liquid water. Aquatic organisms survive in cold climates - Aquatic organisms survive in cold climates -

density differencedensity difference

Good solvent – polar solutes can serve as Good solvent – polar solutes can serve as donor or an acceptor of hydrogen bondsdonor or an acceptor of hydrogen bonds

Other Biologically Important Hydrogen bondsOther Biologically Important Hydrogen bonds

• • Hydrogen bonding is important in stabilization Hydrogen bonding is important in stabilization of 3-D structures of biological molecules such of 3-D structures of biological molecules such as: DNA, RNA, proteins.as: DNA, RNA, proteins.

Acids and BasesAcids and Bases

Acids are proton (hydrogen Acids are proton (hydrogen ion) donors and bases are ion) donors and bases are proton acceptors. proton acceptors.

When an acid is dissolved in When an acid is dissolved in water, its strength is water, its strength is measured by the amount of measured by the amount of hydrogen ions released by hydrogen ions released by acid – acid – Acid dissociation Acid dissociation constant (Ka)constant (Ka)

Higher the Ka, stronger the Higher the Ka, stronger the acid and more the hydrogen acid and more the hydrogen ions releasedions released

What is pH?What is pH?

HH22O O OH OH-1-1 + + HH+1+1

hydroxide ion hydroxide ion hydrogen ionhydrogen ion Kw= (HKw= (H+1+1 ) (OH ) (OH-1-1 ) = (10 ) = (10-7-7) (10-7) = 10-14) (10-7) = 10-14 pH – negative logarithm of hydrogen ion pH – negative logarithm of hydrogen ion

concentration, pH = -log10 (H+)concentration, pH = -log10 (H+) pH- measure of acidity of solutionpH- measure of acidity of solution pH change of one unit – tenfold change in pH change of one unit – tenfold change in

hydrogen ion concentrationhydrogen ion concentration

Why do we want to know the pH?Why do we want to know the pH?

Various biological reactions require a vast Various biological reactions require a vast range of pHrange of pH

pH should be controlled to make an pH should be controlled to make an experiment function properlyexperiment function properly

Subcellular organelles undergo variations in Subcellular organelles undergo variations in pH – maintain neutrality to stay alivepH – maintain neutrality to stay alive

Naturally occurring pH buffers – phosphate Naturally occurring pH buffers – phosphate and carbonate buffers maintain pH 7.0and carbonate buffers maintain pH 7.0

Titration CurvesTitration Curves Titration:Titration: an experiment in which measured amounts of acid (or base) are an experiment in which measured amounts of acid (or base) are

added to measured amounts of base (or acid)added to measured amounts of base (or acid) Equivalence point:Equivalence point: the point in an acid-base titration at which enough the point in an acid-base titration at which enough

acid has been added to exactly neutralize the base (or vice versa)acid has been added to exactly neutralize the base (or vice versa) a monoprotic acid releases one Ha monoprotic acid releases one H++ per mole per mole a diprotic acid releases two Ha diprotic acid releases two H++ per mole per mole a triprotic acid releasesa triprotic acid releases three Hthree H++ per mole per mole

BuffersBuffers

Buffer:Buffer: a solution whose pH resists change a solution whose pH resists change upon addition of either more acid or more baseupon addition of either more acid or more base consists of a weak acid and its conjugate baseconsists of a weak acid and its conjugate base

Examples of acid-base buffers are solutions Examples of acid-base buffers are solutions containingcontaining CHCH33COOH and CHCOOH and CH33COONaCOONa

HH22COCO33 and NaHCO and NaHCO33

NaHNaH22POPO44 and Na and Na22HPOHPO44

How do buffers work?How do buffers work?

Based on nature of weak acids and their Based on nature of weak acids and their conjugate bases – buffers workconjugate bases – buffers work

Addition of extra hydrogen ion – reacts with Addition of extra hydrogen ion – reacts with conjugate base to form weak acidconjugate base to form weak acid

Addition of hydroxide ion – reacts with weak Addition of hydroxide ion – reacts with weak acid to form water and conjugate baseacid to form water and conjugate base

Addition of either H+ or OH- buffers solution-Addition of either H+ or OH- buffers solution-maintains pHmaintains pH

Naturally Occurring BuffersNaturally Occurring Buffers

HH22POPO44--/HPO/HPO44

2-2- is the principal buffer in cells is the principal buffer in cells

HH22COCO33/HCO/HCO33-- is an important (but not the is an important (but not the

only) buffer in bloodonly) buffer in blood

How do we make buffers in the How do we make buffers in the laboratory?laboratory?

Add weak acid form or weak base form of the Add weak acid form or weak base form of the buffer to a container + add water + measure buffer to a container + add water + measure pH pH

Should be low or highShould be low or high Add strong acid or strong base till the desired Add strong acid or strong base till the desired

buffer pH is obtainedbuffer pH is obtained Bring the solution up to final volume – correct Bring the solution up to final volume – correct

concentrationconcentration Based upon experiment - choose pH Based upon experiment - choose pH

Henderson-Hasselbalch Henderson-Hasselbalch

Henderson-Hasselbalch equationHenderson-Hasselbalch equation

When the concentrations of weak acid and its When the concentrations of weak acid and its conjugate base are equal, the pH of the conjugate base are equal, the pH of the solution equals the pKsolution equals the pKaa of the weak acid of the weak acid when pH < pKwhen pH < pKaa, the weak acid predominates, the weak acid predominates when pH > pKwhen pH > pKaa, the conjugate base predominates, the conjugate base predominates

[Weak acid]

[Conjugate base]log=pH pKa +

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