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
www.cengage.com/chemistry/garrett
Reginald Garrett & Charles Grisham • University of Virginia
Chapter 2 Water: the Medium of 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 H-bond acceptor• Potential to form four H-bonds per water
molecule
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2.1 What Are the Properties 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)
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2.1 What Are the Properties of Water?
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2.1 What Are the Properties of Water?
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
Hydration shells surrounding ions 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 interact favorably with both polar and nonpolar environments
• Amphipathic molecules contain both polar and nonpolar groups
• Good examples - fatty acids
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The Solvent Properties of Water Derive from Its Polar Nature
(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
Nonpolar molecules decrease the entropy of solvent water (left). When nonpolar molecules coalesce (arrow), the entropy of the solvent increases.
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The Solvent Properties of Water Derive from Its Polar Nature
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The Solvent Properties of Water Derive from Its Polar Nature
Micelle formation by amphiphilic molecules in aqueous solution.
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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.
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Osmotic Pressure
Osmotic Pressure = ∏ = the force required to resist water movement
van‘t Hoff equation: ∏ = icRT
ic = osmolarity of the solutioni = number of solutes per molecule
(NaCl = 2)c = concentration in molarityR = gas constant 8.315 J/mol
T = temperature in Kelvin
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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
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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?
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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?
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2.1 What Are the Properties of Water?
• Water Can Ionize to Form H+ and OH-
H2O ⇄ H+ + OH-
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Water Can Ionize to Form H+ and OH-
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
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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 Electrolytes
Consider a weak acid, HA
• The acid dissociation constant is given by:
HA ⇄ H+ + A-
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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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The Dissociation Behavior of Weak Electrolytes
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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:
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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:
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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:
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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?
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The Dissociation Behavior of Weak Electrolytes
The titration curves of several weak acids.
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Titration Curves Illustrate the Progressive Dissociation of a Weak Acid
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
• 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?
A buffer system consists of a weak acid, HA and its conjugate base, A-
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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.
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2.3 What are Buffers and What Do They Do?
Anserine is a dipeptide buffer that helps maintain intracellular pH in some tissues.
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2.3 What are Buffers and What Do They Do?
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 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
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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.