reginald h. garrett charles m. grisham chapter 2 water: the medium of life
TRANSCRIPT
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Reginald H. GarrettCharles M. Grisham
Chapter 2 Water: the Medium of Life
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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 ?
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2.1 What Are the Properties of Water?
● Water has unusual properties:• High b.p., m.p., heat of vaporization,
surface tension, dielectric constant.• Bent structure makes it polar.• Non-tetrahedral bond angles.• H-bond donor and acceptor.• Potential to form four H-bonds per water
molecule.
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2.1 What Are the Properties of Water?
Figure 2.1 The structure of water.
Note: this arrow is backwards.
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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"!
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2.1 What Are the Properties of Water?
Figure 2.2 The structure of normal ice.
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2.1 What Are the Properties of Water?
Figure 2.3 The fluid network of H bonds linking water molecules in the liquid state.
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The Solvent Properties of Water Derive from Its Polar Nature
• Water has a high dielectric constant.• Dielectric constant is a measure of the ability
of a solvent to solvate ions.• 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“.
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Review Noncovalent Interactions
• Van der Waals• London forces – instantaneous dipole
• H-Bonds – H must be covalent to N or O• Dipole-Dipole• Ionic• Combinations• Hydrophobic interactions – entropy from
solvent reorganization
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The Solvent Properties of Water Derive from Its Polar Nature
Figure 2.4 Hydration shells surrounding ion in solution.
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The Solvent Properties of Water Derive from Its Polar Nature
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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.
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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.
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The Solvent Properties of Water Derive from Its Polar Nature
Figure 2.6 Nonpolar molecules increase the entropy of solvent water.
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The Solvent Properties of Water Derive from Its Polar Nature
Figure 2.7 (a) Sodium palmitate is an amphiphilic molecule.
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The Solvent Properties of Water Derive from Its Polar Nature
Figure 2.7 (b) Micelle formation by amphiphilic molecules in aqueous solution.
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Water Can Ionize to Form H+ and OH-
Figure 2.9 The ionization of water.
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2.1 What Are the Properties of Water?
• Water Can Ionize to Form H+ and OH-.
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Water Can Ionize to Form H+ and OH-
Figure 2.10 The hydration of H3O+.
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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
10log [ ]
14w
pH H
pK pH pOH
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2.2 What is pH?
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Dissociation of Weak Electrolytes
Consider a weak acid, HA
• The acid dissociation constant is given by:
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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:
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2.2 What is pH?
Titration curves illustrate the progressive dissociation of a weak acid.
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Consider the Dissociation of Acetic Acid
• What is the pH if exactly 0.5 eq of base is added to a solution of the fully protonated acetic acid ?
• Solution: With 0.5 eq OH¯ added:
• So, pH = 4.76 + 0
• pH = 4.76 = pKa
10
(0.5)log
(0.5)apH pK
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Consider the Dissociation of Acetic Acid
Another case:• 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:
• pH = 4.76 + (-0.95)• pH = 3.81
10
(0.1)log
(0.9)apH pK
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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?• Solution: With 0.9 eq OH¯ added:
• So, pH = 4.76 + 0.9• pH = 5.71
10
(0.9)log
(0.1)apH pK
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The Dissociation Behavior of Weak Electrolytes
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The Dissociation Behavior of Weak Electrolytes
Figure 2.12 The titration curves of several weak acids.
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Titration Curves Illustrate the Progressive Dissociation of a Weak Acid
Figure 2.13 The titration curve for phosphoric acid.
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2.3 What Are Buffers, and What Do They Do?
• Buffers are solutions that resist change in pH when either acid or base is added.
• Most buffers consist of a weak acid and its conjugate base or a weak base and its conjugate acid.
• 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 one pH unit of their pKa.
• Within a given molecule each ionization of a weak acid or a weak base represents a buffer.
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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- or a weak base A and its conjugate acid HA+.
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pH affects Enzyme Activity
Figure 2.15 Enzymatic activity vs pH(a) Pepsin is a protein-digesting enzyme active in gastric fluid. (b) Fumarase is a metabolic enzyme found in mitochondria. (c) Lysozyme digests the cell walls of bacteria (found in tears).
pH – rate profile
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2.3 What are Buffers and What Do They Do?
Figure 2.17 The structure of HEPES (an example of a buffer used in the laboratory), in its fully protonated form. pKa = 7.47
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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.
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End Chapter 2 Water: the Medium of Life