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Chapter 3: Outline-1Molecular Nature of Water

Noncovalent Bonding

Ionic interactions Hydrogen Bonds

van der Waals Forces

Thermal Properties of Water

Solvent Properties of Water

Hydrophilic, hydrophobic, and amphipathic molecules

Osmotic pressure

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Chapter 3: Outline-2Ionization of Water

Acids, bases, and pH

Buffers

Physiological buffers

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Water

Solvent for all chemical reactions.

Transports chemicals from place to place.

Helps to maintain constant body temperature.

Part of digestive fluids.Dissolves excretion products.

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3.1 Molecular Structure of WaterThe oxygen in water

is sp3 hybridized. Hydrogens are bonded to two of the orbitals. Consequently the water molecule is bent. The H-O-H angle is 104.5o.

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WaterWater is a polar molecule.

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

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

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WaterWater molecule with bond ( ) and net

( ) dipoles.

HO

H+

-

+

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WaterWater has an abnormally high

boiling point due to intermolecular hydrogen 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.

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3.2 Noncovalent BondingIonic interactions

Hydrogen bonding

Van der Waals forces

Dipole-dipole

Dipole-induced dipole

Induced dipole-induced dipole

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Typical “Bond” Strengths

Type kJ/mol

Covalent >210

Noncovalent

Ionic interactions 4-80

Hydrogen bonds 12-30

van der Waals 0.3-9

Hydrophobic interactions 3-12

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Ionic InteractionsIonic interactions occur between

charged atoms or groups.

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

CH2CH2COO-

CH2CH2NH3

+

Salt bridge

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Hydrogen BondingWater molecules hydrogen bond with

one another. Four hydrogen bonding attractions are possible per molecule:

two through the

hydrogens and two

through the nonbonding electron pairs.

HO

H

HO

H

HO

H

HO

HH

OH

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Van der Waals Attractionsa. Dipole-dipole

b. Dipole-induced dipole

c. Induced dipole-induced dipole

C O C O+-

+-

H

H

HH

H

H

HH

+-

+-

C O H

H

HH

+- +

-

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Hydrophobic interactionsNonpolar molecules tend to coalesce

into droplets in water. The repulsions between the water molecules and the nonpolar molecules cause this phenomenon.

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

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3.3 Thermal PropertiesHydrogen bonding keeps water in the

liquid phase between 0 oC and 100 oC.

Liquid water has a high:

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

Heat capacity-energy to change the temperataure by 1 oC

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

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3.4 Water-solvent PropertiesWater dissolves chemicals that have an

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

These compounds are soluble in water due to three kinds of noncovalent interactions:1. ion-dipole 2. dipole-dipole 3. hydrogen bonding

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Ion-dipole InteractionsIons are hydrated by water molecules. The water molecules orient so the opposite charge end points to the ion to partially neutralize charge. The shell of water molecules is a solvation sphere.

K + Cl-

HO

H

HO

H

HO

H

H

OH

HO

HHO

H

HOH H

OH

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Dipole-dipole Interactions

The polar water molecule interacts with an O or N or an H on an O or N on an organic molecule.

HO

H

HO H

CH3

CCH3

OHOH

+

-

Dipole-dipoleinteractions

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Hydrogen BondingA 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.

R1 O H

HO

H

HOH

hydrogen bondshown in yellow

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Nonpolar MoleculesNonpolar molecules have no polar

bonds or the bond dipoles cancel due to molecular geometry.

These molecules do not form good attractions with the water molecule. They are insoluble and are said to be hydrophobic (water hating).

eg.: CH3CH2CH2CH2CH2CH3, hexane

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Nonpolar Molecules-2Water forms hydrogen-bonded cagelike

structures around hydrophobic molecules, forcing them out of solution.

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Amphipathic MoleculesAmphipathic molecules contain both

polar and nonpolar groups.

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

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

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A Micelle

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Osmotic Pressure-2Osmosis is a spontaneous process in

which solvent molecules pass through a semipermeable membrane from a solution of lower solute concentration to a solution of higher solute concentration.

Osmotic pressure is the pressure required to stop osmosis.

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Osmotic Pressure-3Osmotic pressure () is measured in an

osmometer.

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Osmotic Pressure-4iMRT

i = van’t Hoff factor (% as ions)

M = molarity (mol/L for dilute solns)

R = 0.082 L atm/ mol K

T = Kelvin temperature

1 M NaCl is 90% ionized and 10% ion pairs.

i = 0.9 + 0.9 + 0.1 = 1.9

Osmolarity (osm/L) = iM

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Osmotic Pressure-5Because cells have a higher ion

concentration than the surrounding fluids, they tend to pick up water through the semipermeable cell membrane.

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

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Osmotic Pressure-6Cells placed in a hypotonic solution will

lose water and shrink (crenate).

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

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3.5 Ionization of WaterWater dissociates. (self-ionizes)

H2O + H2O = H3O+ + OH-

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

Ka = [H3O+][OH-]

[H2O]2

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Water Ionization-2

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

Kw= [H3O+][OH-]=1 x 10-14

In neutral water,

[H3O+ ] = [OH-] = 1 x 10-7 M

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Water: A/B PropertiesWhen external acids or bases are

added to water, the ion product

([H3O+ ][OH-] ) must equal Kw.

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

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Water: A/B Properties-2

Lowry-Bronsted

acid = proton donor

HA + H2O = H3O+ + A-

A B CA CB

C: conjugate (product) A/B

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Water: A/B Properties-3

Lowry-Bronsted

base = proton acceptor

RNH2 + H2O = OH- + RNH3+

B A CB CA

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Measuring AcidityAdded acids increase the concentration

of hydronium ion and bases the concentration of hydroxide ion.

In acid solutions [H3O+] > 1 x 10-7 M [OH-] < 1 x 10-7 MIn basic solutions [OH-] > 1 x 10-7 M

[H3O+] < 1 x 10-7 MpH scale measures acidity without using

exponential numbers.

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pH Scale

Define: pH = -log(10)[H3O+]

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

acidic basic

[H3O+]=1 x 10-7 M, pH = ?

7.0

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pH Scale-2

[H3O+]=1 x 10-5 M, pH = ?

5 (acidic)

[H3O+]=1 x 10-10 M, pH = ?

10 (basic)

What if preexponential number is not 1?

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pH Scale-3

[H3O+]=2.6 x 10-5 M, pH = ?

4.59 (acidic)

[H3O+]=6.3 x 10-9 M, pH = ?

8.20 (basic)

[H3O+]=7.8 x 10-3 M, pH = ?

2.11 (acidic)

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pH Scale-4

pH to [H3O+]?

inverse log of negative pHorange juice, pH 3.5. [H3O+]=?

[H3O+] = 3.2 x 10-4 Murine, pH 6.2. [H3O+]=?

[H3O+] = 6.3 x 10-7 M

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Strength of AcidsStrength of an acid is

measured by the percent which reacts with water to form hydronium ions.

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

eg. HCl, HBr, HNO3, H2SO4

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Strength of Acids-2Weak acids (or bases) ionize

typically in the 1-5% range .eg. CH3COCOOH, pyruvic acid

CH3CHOHCOOH, lactic acid

CH3COOH, acetic acid

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Strength of Acids-3Strength of an acid is also

measured by its Ka or pKa values.

HA + H2O = H3O+ + A-

Larger Ka and smaller pKa valuesindicate stronger acids.

Ka = [H3O+][A-]

[HA]

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Strength of Acids-4 Ka pKa

CH3COCOOH 3.2x10-3 2.5

CH3CHOHCOOH 1.4x10-4 3.9

CH3COOH 1.8x10-5 4.8

Larger Ka and smaller pKa values

indicate stronger acids.