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Properties of Water
1) Water has a high boiling point and relatively
low melting point; its range in the liquid state is
large
2) The heat of vaporization is high (more heat to
change to vapor state than to raise temperature of the
liquid state)
3) Liquid is more dense than the solid (e.g., most
metals and other substances the solid is more dense)
4) High viscosity relative to its molecular weight
(MW)
5) High surface tension
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Figure 2.15 Surface Tension
60
Properties of Water
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Properties of Water
The 4 S’s for Water:
Shape
Size
Solubility
Stability
Most of the properties of water are explained
or due to its shape, in other words, its
structure
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Shape of Water
• sp3 hybridized orbitals:
2 are bonding
2 are filled non-bonding
• Distorted tetrahedral (equal angles of
109.5°): due to repulsion of electrons
in filled orbitals
Shape of Water
Permanent DIPOLE
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Size of Water
2.0 Å
Very compact structure:
2.0 Å in diameter
though van der Waals
diameter of oxygen atom is
2.8 Å
The O–H bond is
relatively short (0.96 Å)
though van der Waals
diameter of hydrogen atom
is 2.4 Å
What are these discrepancies due to?
ELECTRONEGATIVITY OF OXYGEN
Solubility of Water: Hydrogen Bonding
These dipoles interact with each other in a special kind of non-covalent bond:
the hydrogen bond.
Bit of a misnomer but named as such because it appears though two oxygen
atoms of two water molecules share a hydrogen
Its not at all equal: the water with the covalent bond to hydrogen is called the
DONOR. The opposite water molecule is called the ACCEPTOR
DONOR
ACCEPTOR
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Solubility of Water: Hydrogen Bonding
d+
THREE important aspects
that make H-bonds unique:
1) Electrostatic: dipoledipole interaction
d–
d–
d+
2) Partial electron sharing:
partial covalency
3) Directional: geometry
0.96 Å
1.84 Å
2.8 Å
This is a stronger bond
than this (due to optimizing
the covalency)
Rings of Water “Flickering Clusters”
Molecules
of Water Molecules
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Structure of Ice
Properties of Water
1) Water has a high boiling point and relatively
low melting point; its range in the liquid state is
large
2) The heat of vaporization is high
•
Ave number of H-bonds in liquid water is ~3, but in
steam ~0
3) Liquid is more dense than the solid
•
Ave number of H-bonds in liquid water is ~3, but in
ice its 4, but the geometry takes over
4) High viscosity relative to its molecular weight
(MW)
5) High surface tension
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Solvation of Ions
Hydrogen Bonding to Polar Molecules
Water to
by Water
ketones/aldehydes
Water to alcohols
Water to
carboxylic acids
Water to amines
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Typical Bond Energies
(kcal · mol-1)
}
95
4.7
Electrostatic Interactions
The strength of all these is governed by
Coulomb’s Law:
Charge – Charge interactions
Na+
Cl–
Force between two charged
particles = k q1q2
D r2
r
Where q is the charge on each particle 1 and 2, D is
the dielectric constant (unit-less; =1.0 in vacuum, ~80
in water), r is the distance between the two charges,
and k is Coulomb’s constant (9x109 J·m/C2, where
C=1.6x10-19 electron charges)
r3
r5
Induced dipole – Induced dipole interactions
(AKA Van der Waals interactions)
Recalling the relationship between Force
and Energy (E = Force x distance):
E = k q1q2
Dr
r6
For partial charges, this distance dependence is exponential
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van der Waals Interactions
Induced dipole – Induced dipole interactions
(AKA Van der Waals interactions)
E = k q1q2
D r6
10 Å / 1 nm
van der Waals Interactions
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van der Waals Interactions
Typical Bond Energies
(kcal · mol-1)
}
@ ~3 Å
95
20
4.7
Van der Waals interactions
0.07
Hydrophobic interactions
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Hydrophobic Interactions
Hydrophobic interactions: atoms “attracted”
together due to water structure.
Nonpolar Solutes Aggregate in Water
Hydrophobic Interactions
Transfer of Hydrocarbons to Nonpolar Solvents is
Entropically Driven
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Hydrophobic Interactions
Structure: Nonpolar Solutes in Water
Like ICE
Hydrophobic Interactions
Figure 2.12 Hydrophilic and
Hydrophobic
69
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Hydrophobic Interactions
Figure 2.12 Hydrophilic and
Hydrophobic
x5
70
Hydrophobic Interactions
3 “degrees” of freedom (DoF)
Statistical mechanics has determined that
each DoF is equal to 5-6 kcal/mol at 25 °C
6 “degrees” of freedom
56
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Hydrophobic Interactions
For EXAMPLE
3 [CH4·10H2O]
[(CH4)3 · 12H2O] + 18H2O
DG is definitely NEGATIVE: How?
Enthalpy
Enthalpy
30 H2O x 4 H-bonds
each x 5 kcal/mol = 600
kcal/mol to break H-bonds
18 H2O x 3 H-bonds
each x 5 kcal/mol = 270 kcal/mol,
plus 18x4x5 = 240 kcal/mol;
Total = 510 kcal/mol to break Hbonds
Entropy
Entropy
3 degrees of
freedom (DoF) x 6 kcal/mol per
DoF = 18 kcal/mol
19 degrees of
freedom (DoF) x 6 kcal/mol per
DoF = 114 kcal/mol
DG = DH – TDH
= +90 – 96
= –6 kcal/mol
DH = +90
kcal/mol
added to
break Hbonds
TDS = +96
kcal/mol
added to
break Hbonds
ENTROPY DRIVEN
55
Bond Energies of Important Noncovalent Interactions
(kcal · mol-1)
}
95
20
@ ~3 Å
4.7
Van der Waals interactions
0.07
Hydrophobic interactions (per –CH2–)
26
6
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Amphipathic Interactions
(both hydrophobic and hydrophilic)
=O
Example: amphiphiles like Fatty Acid Anions,
detergents
–O–S–O– · +Na
=O
Sodium dodecyl sulfate (C12H25O4S) (SDS)
Amphiphiles Form Micelles & Bilayers
Structure of a Micelle
Protein interiors
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Disrupters of the Hydrophobic Effect
=O
Urea
H2N–C–NH2
Guanidinium-HCl
NH2
H2N+=C–NH2
· Cl–
Physical Properties of Water
Learning Goals: Concepts about water
• Water molecules, which are polar, can form hydrogen bonds
with other molecules.
• In ice, water molecules are hydrogen bonded in a crystalline
array, but in liquid water, hydrogen bonds rapidly break and
re-form in irregular networks.
• The attractive forces acting on biological molecules include
ionic interactions, hydrogen bonds, and van der Waals
interactions.
• Polar and ionic substances can dissolve in water.
• The hydrophobic effect explains the exclusion of nonpolar
groups as a way to maximize the entropy of water molecules.
• Amphiphilic substances form micelles or bilayers that hide
their hydrophobic groups while exposing their hydrophilic
groups to water.
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The 4 S’s for Water:
Shape
Size
Solubility
Stability
The discussion of water’s stability is a
discussion of its ionization.
H2 O
2H2O
D
D
H+ + OH–
H3O+ + OH–
Ionization of Water
Proton Jumping Occurs Rapidly
H
The Positive
Charge jumps
H
H
H
H
H
H+
Proton transfers are very rapid!
2H2O
D
H3O+ + OH–
What is the concentration of OH–?
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Ionization of Water
What is the concentration of OH–?
2H2O
D
H3O+ + OH–
Equilibrium constant (Keq) is 1.8 x 10-16 M
Keq = 1.8 x 10-16 M =
[H+] [OH–]
[H2O]
[H2O] = 55.5 M*
[H+] [OH–]
+
–
[H ] [OH ] since [H+] = [OH–]
[H+]2
[H+]2
[H+]
call the –log[H+] pH
7 = pH
[H2O]1.8 x 10-16 M =
55.5 M x 1.8 x 10-16 M =
55.5 M x 1.8 x 10-16 M =
1.0 x 10-14 M2 =
1.0 x 10-7 M =
Take the –log of both sides:
*1 L water = 1000 g ÷ 18 g/mol = 55.5 M
Ionization of Water
Relationship of pH, [H+], and [OH-]
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Ionization of Water
pH Values of Common Substances
Ionization of Water
Definitions of Acids and Bases:
HA
HB
D
D
H+ + A–
H+ + B–
If dissociation is >water: ACID
If dissociation is <water: BASE
When B– associates with protons, it leaves a net difference in [OH–]
In other words:
A proton donor is an Acid (HA)
A proton acceptor is a Base (B–)
Likewise:
The resulting anion from acid dissociation is
called the conjugate base (A–)
The resulting protonated base is called a
conjugate acid (HB)
These are Bronsted/Lowry definitions
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Ionization of Water
Definitions of Acids and Bases:
HA
Strong Acid
Strong Base
® H+ + A–
H+ + B– ® HB
Protons come from pulling the water dissociation equilibrium
Weak Acid
Weak Base
HA
HB
D
D
H+ + A–
H+ + B–
For weak acids/bases, all 3 species is in measurable
Concentrations.
How do you calculate these concentrations and how
are they related to the pH?
Ionization of Water
How do you calculate these concentrations and how are they
related to the pH?
Use the Equilibrium Constant (Keq) for the dissociation reaction
HA
D
H+ + A–
[H+] [A–]
[HA]
since [H+] is best expressed as pH, can take –log of each side:
[A–]
pKa = pH – log
[HA]
[A–]
pH = pKa + log
[HA]
Keq = Kd = Ka =
This is called the HendersonHasselbalch equation
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Ionization of Water
Ionization of Water
Dissociation Constants and pK Values
of Some Acids
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Ionization of Water
Buffers: Titration Curves of Weak Acids
Ionization of Water
Buffers: Titration of a Polyprotic Acid
H3PO4
D H+ + H2PO4– D H+ + HPO42– D H+ + PO43–
pK1
pK2
pK3
12.4
6.8
2.2
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Ionization of Water
Does this make sense? …. Go back to the reaction:
HA
D
H+ + A–
At low pH, [H+] is high, forcing the equilibrium to the left=more HA
Ionization of Water
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Chapter 2
Chemical Properties of Water
Checkpoint2.2
•Whataretheproductsofwater’sionization?Howaretheir
concentrationsrelated?
•PredictthepHofasampleofwaterifKwwere1010or
1020.
•DescribehowtocalculatepHfromtheconcentrationofH
orOH.
•Defineacidandbase.
•Whatistherelationshipbetweenthestrengthofanacid
anditspKvalue?
Chapter 2
Chemical Properties of Water
Checkpoint2.2
•ExplainwhyitismorecomplicatedtocalculatethepHofa
solutionofweakacidorbasethantocalculatethepHofa
solutionofstrongacidorbase.
•Beabletosketchatitrationcurve,andlabelitsparts,fora
monoproticandapolyproticacid.
•Whatmustabuffersolutionincludeinordertoresist
changesinpHonadditionofacidorbase?
•Whyisitimportanttomaintainbiologicalmoleculesina
bufferedsolution?
25