Chapter 2 Water: the Medium of Life
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Transcript of Chapter 2 Water: the Medium of Life
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Reginald H. GarrettCharles M. Grisham
Chapter 2 Water: the Medium of Life
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Chapter 2
“If there is magic on this planet, it is contained in water.”
Loren Eisley
Where there’s water, there’s life.
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Essential Question
• What are the properties of water that render it so suited to its role as 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• 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.
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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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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• 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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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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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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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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The Solvent Properties of Water Derive from Its Polar Nature
Figure 2.5(left) A disordered network of H-bonded water molecules.(right) A clathrate cage of ordered, H-bonded water molecules around a nonpolar solute molecule.
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The Solvent Properties of Water Derive from Its Polar Nature
Figure 2.6 Nonpolar molecules decrease 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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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.
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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 2.9 The ionization of water.
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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 HpK pH pOH
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2.2 What is pH?
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2.2 What is pH?
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Dissociation of Weak ElectrolytesConsider 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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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
• 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
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:
• 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
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:
• 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 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
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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-
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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.
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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.
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2.3 What are Buffers and What Do They Do?
Figure 2.17 The structure of HEPES, in its fully protonated form.
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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