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Chapter 2 Water: 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:

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Chapter 2 water the medium of life l.jpg

Chapter 2 Water: the Medium of Life


Outline l.jpg
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?

Note: this arrow is backwards.

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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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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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



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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


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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

Consider the Dissociation of Acetic Acid


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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



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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? a Weak Acid

  • 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? a Weak Acid

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 a Weak Acid

pH – rate profile

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).


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2.3 What are Buffers and What Do They Do? a Weak Acid

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 Environment?Water: the Medium of Life


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