Reginald H. Garrett
Charles M. Grisham
www.cengage.com/chemistry/garrett
Chapter 2
Water: the Medium of Life
Reginald Garrett & Charles Grisham • University of Virginia
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 H-bond acceptor
• Potential to form four H-bonds per water
molecule
2.1 What Are the Properties 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)
2.1 What Are the Properties of Water?
2.1 What Are the Properties of Water?
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
Hydration shells
surrounding ions
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 interact favorably
with both polar and nonpolar environments
• Amphipathic molecules contain both polar
and nonpolar groups
• Good examples - fatty acids
The Solvent Properties of Water Derive
from Its Polar Nature
(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
Nonpolar molecules decrease the entropy of solvent water (left). When
nonpolar molecules coalesce (arrow), the entropy of the solvent increases.
The Solvent Properties of Water Derive
from Its Polar Nature
The Solvent Properties of Water Derive
from Its Polar Nature
Micelle formation by
amphiphilic
molecules in
aqueous solution.
The Solvent Properties of Water Derive
from Its Polar Nature
The osmotic pressure of a 1 molal (m) solution is equal to
22.4 atmospheres.
Osmotic Pressure
Osmotic Pressure = ∏ = the force required to resist water
movement
van‘t Hoff equation: ∏ = icRT
ic = osmolarity of the solution
i = number of solutes per molecule
(NaCl = 2)
c = concentration in molarity
R = gas constant 8.315 J/mol
T = temperature in Kelvin
Practice
• Which of the following solutions has an osmolarity of
3?
•
•
•
•
•
3M Na3PO4
0.43M Na3PO4
0.75 M Na3PO4
3 M NaCl
1.5 M NaCl
Practice
• You want to isolate a lysosome to study it in the lab.
Assuming that the only components inside a
lysosome are KCl (0.1M) and NaCl (0.03M), how
much sucrose (342 g/mol) do you need to make 1
liter of an isotonic solution to isolate the lysosomes?
Practice
• You want to isolate a lysosome to study it in the lab.
Assuming that the only components inside a
lysosome are KCl (0.1M) and NaCl (0.03M), how
much glycogen (18,000 g/mol) do you need to make
1 liter of an isotonic solution to isolate the
lysosomes?
2.1 What Are the Properties of Water?
• Water Can Ionize to Form H+ and OH-
H2O ⇄ H+ + OH-
Water Can Ionize to Form H+ and OHThe 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
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:
HA ⇄ H+ + A-
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
The Dissociation Behavior of Weak
Electrolytes
Consider the Dissociation of Acetic Acid
• Assume 0.1 equivalents (eq) of 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:
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:
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:
Practice
• What is the buffer concentration and pH of a mixture
of 0.042M NaH2PO4 (pK = 6.86) and 0.058 M
Na2HPO4?
• What is the pH of a mixture of 75 mL of 0.042M
NaH2PO4 (pK = 6.86) and 150 mL of 0.058 M
Na2HPO4?
The Dissociation Behavior of Weak
Electrolytes
The titration
curves of
several weak
acids.
Titration Curves Illustrate the Progressive
Dissociation of a Weak Acid
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
• Buffers can only be used reliably within a
pH unit of their pKa
2.3 What Are Buffers, and What Do They Do?
A buffer system consists of a
weak acid, HA and its
conjugate base, A-
Enzyme Activity is
Influenced by pH
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?
Anserine is a dipeptide buffer that helps maintain
intracellular pH in some tissues.
2.3 What are Buffers and What Do They Do?
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 ionic and polar
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
Real World Practice
• You are working in the lab and need to make a
physiological phosphate buffer (pH=7.2). You have
the following chemicals at your disposal: Phosphoric
acid, monosodium phosphate, and disodium
phosphate whose corresponding pKa’s are 2.15,
6.86, and 12.32. The molecular weight of each
species is 98.0 g/mol, 119.98 g/mol, and 141.96
g/mol. Calculate the mass of each species that you
would use to make a 100 mL of a 0.5M phosphate
buffer at pH=7.2.