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: 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.

2.1 What Are the Properties of Water? Note: this arrow is backwards. Figure 2.1 The structure 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). That's "one times ten to the minus eleven second"!

2.1 What Are the Properties of Water? Figure 2.2 The structure of normal ice.

2.1 What Are the Properties of Water? Figure 2.3 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. 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“.

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

The Solvent Properties of Water Derive from Its Polar Nature Figure 2.4 Hydration shells surrounding ion in solution.

The Solvent Properties of Water Derive from Its Polar Nature

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.

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.

The Solvent Properties of Water Derive from Its Polar Nature Figure 2.6 Nonpolar molecules increase the entropy of solvent water.

The Solvent Properties of Water Derive from Its Polar Nature Figure 2.7 (a) Sodium palmitate is an amphiphilic molecule.

The Solvent Properties of Water Derive from Its Polar Nature Figure 2.7 (b) Micelle formation by amphiphilic molecules in aqueous solution.

Water Can Ionize to Form H+ and OH- Figure 2.9 The ionization of water.

2.1 What Are the Properties of Water? Water Can Ionize to Form H+ and OH-.

Water Can Ionize to Form H+ and OH- Figure 2.10 The 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?

Dissociation of Weak Electrolytes Consider a weak acid, HA The acid dissociation constant is given by:

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:

2.2 What is pH? Titration curves illustrate the progressive dissociation of a weak acid.

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

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

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

The Dissociation Behavior of Weak Electrolytes

The Dissociation Behavior of Weak Electrolytes Figure 2.12 The titration curves of several weak acids.

Titration Curves Illustrate the Progressive Dissociation of a Weak Acid Figure 2.13 The titration curve for phosphoric acid.

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.

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+.

pH affects Enzyme Activity 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).

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

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.

End Chapter 2 Water: the Medium of Life