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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.
