Chapter 3: Outline-1
Molecular 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
3P1-1
Chapter 3: Outline-2
Ionization of Water
Acids, bases, and pH
Buffers
Physiological buffers
3P1-2
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.
3P1-3
3.1 Molecular Structure of Water
The 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.
3P1-4
Water
Water 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.
3P1-5
Water
Water molecule with bond (
( ) dipoles.
) and net
d-
H
d+
O
Hd+
3P1-6
Water
Water has an abnormally high
boiling point due to intermolecular
d
H
hydrogen bonding.
d
O
dH
d
O
H
d
H
d
H bonding is a
weak attraction
between an
electronegative
d
H atom in one
molecule
and
an
H
O
d H (on an O or N) in
d another.
3P1-7
3.2 Noncovalent Bonding
Ionic interactions
Hydrogen bonding
Van der Waals forces
Dipole-dipole
Dipole-induced dipole
Induced dipole-induced dipole
3P1-8
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
3P1-9
Ionic Interactions
Ionic 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
Salt bridge
+
H3N CH2CH2
3P1-10
Hydrogen Bonding
Water molecules hydrogen bond with
one another. Four hydrogen bonding
attractions are possible per molecule:
H
two through the
H
O
O
hydrogens and two
H
H
through the
O
nonbonding
H H
H
H electron pairs.
O
H
O
H
3P1-11
Van der Waals Attractions
a. Dipole-dipole
b. Dipole-induced dipole
c. Induced dipole-induced dipole
d+ C
d-
O
d+ C
d-
O
d+
d+
d+ C
d-
H
H
d+
O H
H
H
H
H
H d- H
H
H d-
H d3P1-12
Hydrophobic interactions
Nonpolar 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.
3P1-13
3.3 Thermal Properties
Hydrogen 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.
3P1-14
3.4 Water-solvent Properties
Water 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
3P1-15
Ion-dipole Interactions
Ions 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.
H
H
O
O
H
H
H
O K+ O
H
H
O
H
H
H
O
H
- H
Cl
H
H
H
O
H
O
3P1-16
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.
H O
O
d-
H O H
H3C
H
C
d+
O
CH3
H
H
Dipole-dipole
interactions
3P1-17
Hydrogen Bonding
A 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 O
bond.
H
H
hydrogen bond
shown in yellow
R1 O H
H
O
H
3P1-18
Nonpolar Molecules
Nonpolar 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
3P1-19
Nonpolar Molecules-2
Water forms hydrogen-bonded cagelike
structures around hydrophobic
molecules, forcing them out of
solution.
3P1-20
Amphipathic Molecules
Amphipathic 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.
3P1-21
A Micelle
3P1-22
Osmotic Pressure-2
Osmosis 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.
3P1-23
Osmotic Pressure-3
Osmotic pressure (p) is measured in an
osmometer.
3P1-24
Osmotic Pressure-4
p = iMRT
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
3P1-25
Osmotic Pressure-5
Because 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.
3P1-26
Osmotic Pressure-6
Cells 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.
3P1-27
3.5 Ionization of Water
Water dissociates. (self-ionizes)
H2O + H2O =
Ka =
+
H3O + OH
+
[H3O ][OH ]
2
[H2O]
2
+
Kw = Ka [H2O] = [H3O ][OH ]
3P1-28
Water Ionization-2
The conditions for the water
dissociation equilibrium must
o
hold under all situations at 25 .
+
-14
Kw= [H3O ][OH ]=1 x 10
In neutral water,
+
-7
[H3O ] = [OH ] = 1 x 10 M
3P1-29
Water: A/B Properties
When 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.
3P1-30
Water: A/B Properties-2
Lowry-Bronsted
acid = proton donor
+
HA + H2O = H3O + A
A
B
CA
CB
C: conjugate (product) A/B
3P1-31
Water: A/B Properties-3
Lowry-Bronsted
base = proton acceptor
RNH2 + H2O =
B
A
OH
+ RNH3
CB
+
CA
3P1-32
Measuring Acidity
Added acids increase the concentration
of hydronium ion and bases the
concentration of hydroxide ion.
+
-7
In acid solutions [H3O ] > 1 x 10 M
-7
[OH ] < 1 x 10 M
-7
In basic solutions [OH ] > 1 x 10 M
+
-7
[H3O ] < 1 x 10 M
pH scale measures acidity without using
exponential numbers.
3P1-33
pH Scale
+
Define: pH = -log(10)[H3O ]
0---------------7---------------14
acidic
basic
+
-7
[H3O ]=1 x 10 M, pH = ?
7.0
3P1-34
pH Scale-2
+
-5
[H3O ]=1 x 10 M, pH = ?
5 (acidic)
+
-10
[H3O ]=1 x 10
M, pH = ?
10 (basic)
What if preexponential
number is not 1?
3P1-35
pH Scale-3
+
-5
[H3O ]=2.6 x 10 M, pH = ?
4.59 (acidic)
+
-9
[H3O ]=6.3 x 10 M, pH = ?
8.20 (basic)
+
-3
[H3O ]=7.8 x 10 M, pH = ?
2.11 (acidic)
3P1-36
pH Scale-4
+
O ]?
pH to [H3
inverse log of negative pH
orange juice, pH 3.5.
[H3
+
O]
= 3.2 x
+
[H3O ]=?
-4
10 M
urine, pH 6.2. [H3O+]=?
[H3
+
O]
= 6.3 x
-7
10 M
3P1-37
Strength of Acids
Strength 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
3P1-38
Strength of Acids-2
Weak acids (or bases) ionize
typically in the 1-5% range .
eg. CH3COCOOH, pyruvic acid
CH3CHOHCOOH, lactic acid
CH3COOH, acetic acid
3P1-39
Strength of Acids-3
Strength of an acid is also
measured by its Ka or pKa
values.
HA + H2O =
+
H3O
+
A
Ka = [H3O+][A-]
[HA]
Larger Ka and smaller pKa values
indicate stronger acids.
3P1-40
Strength of Acids-4
Ka
pKa
-3
CH3COCOOH
3.2x10
2.5
-4
CH3CHOHCOOH 1.4x10 3.9
-5
CH3COOH
1.8x10
4.8
Larger Ka and smaller pKa values
indicate stronger acids.
3P1-41