Reginald H. Garrett
Charles M. Grisham
Chapter 2
Water: the Medium of Life
Chapter 2
“If there is magic on
this planet, it is
contained in water.”
Loren Eisley
Where there’s
water, there’s life.
Essential Question
• What are the properties of water that render it so
suited to its role as the medium of life?
Outline
•
•
•
•
What are the properties of water?
What is pH?
What are buffers, and what do they do?
Does water have a unique role in the fitness of the
environment?
2.1 What Are the Properties of Water?
● Water has unusual properties:
• High b.p., m.p., heat of vaporization, surface
tension
• Bent structure makes it polar
• Non-tetrahedral bond angles
• H-bond donor and acceptor
• Potential to form four H-bonds per water
molecule
2.1 What Are the Properties of Water?
Figure 2.1 The structure of water.
2.1 What Are the Properties of Water?
A comparison of ice and water, in terms of
H-bonds and Motion
• Ice: 4 H-bonds per water molecule
• Water: 2.3 H-bonds per water molecule
• Ice: H-bond lifetime - about 10 microsec
• Water: H-bond lifetime - about 10 psec
• (10 psec = 0.00000000001 sec)
• That's "one times ten to the minus eleven
second"!
2.1 What Are the Properties of Water?
Figure 2.2 The structure of normal ice.
2.1 What Are the Properties of Water?
Figure 2.3 The fluid
network of H bonds
linking water
molecules in the liquid
state.
2.1 What Are the Properties of Water?
Figure 2.3 The fluid
network of H bonds
linking water
molecules in the
liquid state.
The Solvent Properties of Water Derive
from Its Polar Nature
• Water has a high dielectric constant
• Ions are always hydrated in water and
carry around a "hydration shell"
• Water forms H-bonds with polar solutes
• Hydrophobic interactions - a "secret of life"
The Solvent Properties of Water Derive
from Its Polar Nature
Figure 2.4
Hydration shells
surrounding ion
in solution.
The Solvent Properties of Water Derive
from Its Polar Nature
Hydrophobic Interactions
• A nonpolar solute "organizes" water
• The H-bond network of water reorganizes to
accommodate the nonpolar solute
• This is an increase in "order" of water
• This is a decrease in ENTROPY
Amphiphilic/Amphipathic Molecules
• “Amphiphilic” and “amphipathic” are
essentially synonymous terms
• Amphiphilic molecules are attracted to
both polar and nonpolar environments
• Amphipathic molecules that contain both
polar and nonpolar groups
• Good examples - fatty acids
The Solvent Properties of Water Derive
from Its Polar Nature
Figure 2.5
(left) A disordered
network of Hbonded water
molecules.
(right) A clathrate
cage of ordered, Hbonded water
molecules around
a nonpolar solute
molecule.
The Solvent Properties of Water Derive
from Its Polar Nature
Figure 2.6 Nonpolar molecules decrease the entropy of solvent water.
The Solvent Properties of Water Derive
from Its Polar Nature
Figure 2.7 (a) Sodium palmitate is an amphiphilic molecule.
The Solvent Properties of Water Derive
from Its Polar Nature
Figure 2.7 (b)
Micelle formation by
amphiphilic
molecules in
aqueous solution.
The Solvent Properties of Water Derive
from Its Polar Nature
Figure 2.8 The osmotic pressure of a 1 molal (m) solution is
equal to 22.4 atmospheres.
2.1 What Are the Properties of Water?
• Water Can Ionize to Form H+ and OH-
Water Can Ionize to Form H+ and OH-
Figure 2.9 The ionization of water.
Water Can Ionize to Form H+ and OH-
Figure 2.10
The hydration
of H3O+.
2.2 What is pH?
• Søren Sørensen of Denmark devised the pH scale
• pH is the negative logarithm of the hydrogen ion
concentration
• If [H+] = 1 x 10 -7 M
• Then pH = 7

pH   log10 [ H ]
pK w  pH  pOH  14
2.2 What is pH?
2.2 What is pH?
Dissociation of Weak Electrolytes
Consider a weak acid, HA
• The acid dissociation constant is given by:
The Henderson-Hasselbalch Equation
Know this! You'll use it constantly.
• For any acid HA, the relationship between
the pKa, the concentrations existing at
equilibrium and the solution pH is given by:
2.2 What is pH?
Titration curves
illustrate the
progressive
dissociation of a
weak acid
2.2 What is pH?
Titration curves
illustrate the
progressive
dissociation of
a weak acid
Consider the Dissociation of Acetic Acid
• Assume 0.1 eq base has been added to a
fully protonated solution of acetic acid
• The Henderson-Hasselbalch equation
can be used to calculate the pH of the
solution:
With 0.1 eq OH¯ added:
(0.1)
pH  pK a  log10
(0.9)
• pH = 4.76 + (-0.95)
• pH = 3.81
Consider the Dissociation of Acetic Acid
Another case:
• What happens if exactly 0.5 eq of base is
added to a solution of the fully protonated
acetic acid?
• With 0.5 eq OH¯ added:
(0.5)
pH  pK a  log10
(0.5)
• pH = 4.76 + 0
• pH = 4.76 = pKa
Consider the Dissociation of Acetic Acid
A final case to consider:
• What is the pH if 0.9 eq of base is added to a
solution of the fully protonated acid?
• With 0.9 eq OH¯ added:
(0.9)
pH  pK a  log10
(0.1)
• pH = 4.76 + 0.9
• pH = 5.71
The Dissociation Behavior of Weak
Electrolytes
The Dissociation Behavior of Weak
Electrolytes
Figure 2.12 The
titration curves of
several weak
acids.
Titration Curves Illustrate the Progressive
Dissociation of a Weak Acid
Figure 2.13 The
titration curve
for phosphoric
acid.
2.3 What Are Buffers, and What Do They Do?
• Buffers are solutions that resist changes in
pH as acid and base are added
• Most buffers consist of a weak acid and its
conjugate base
• Note in Figure 2.14 how the plot of pH
versus base added is flat near the pKa
• Buffers can only be used reliably within a
pH unit of their pKa
2.3 What Are Buffers, and What Do They Do?
Figure 2.14 A buffer system
consists of a weak acid, HA
and its conjugate base, A-
Enzyme Activity is
Influenced by pH
Figure 2.15 pH versus
enzymatic activity. Pepsin is
a protein-digesting enzyme
active in gastric fluid.
Fumarase is a metabolic
enzyme found in
mitochondria. Lysozyme
digests the cell walls of
bacteria. It is found in tears.
2.3 What are Buffers and What Do They Do?
Figure 2.16 Anserine is an important dipeptide
buffer in the maintenance of intracellular pH in
some tissues.
2.3 What are Buffers and What Do They Do?
Figure 2.17 The structure of HEPES, in its
fully protonated form.
2.4 What Properties of Water Give It a
Unique Role in the Environment?
• Water is a very good solvent for a variety of
substances
• Water is a very poor solvent for nonpolar substances
• Due to hydrophobic interactions, lipids coalesce,
membranes form, and the cellular nature of life is
established
• Due to its high dielectric constant, water is a suitable
medium for the formation of ions
• The high heat capacity of water allows effective
temperature regulation in living things