Reginald H. GarrettCharles M. Grisham

www.cengage.com/chemistry/garrett

Reginald Garrett & Charles Grisham • University of Virginia

Chapter 2 Water: the Medium of Life

Essential Question

• What are the properties of water that render it so suited to its role as 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• 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

2.1 What Are the Properties 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)

2.1 What Are the Properties of Water?

2.1 What Are the Properties of Water?

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• 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"

The Solvent Properties of Water Derive from Its Polar Nature

Hydration shells surrounding ions in solution.

The Solvent Properties of Water Derive from Its Polar Nature

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

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

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.

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.

The Solvent Properties of Water Derive from Its Polar Nature

The Solvent Properties of Water Derive from Its Polar Nature

Micelle formation by amphiphilic molecules in aqueous solution.

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.

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

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

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?

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?

2.1 What Are the Properties of Water?

• Water Can Ionize to Form H+ and OH-

H2O ⇄ H+ + OH-

Water Can Ionize to Form H+ and OH-

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?

2.2 What is pH?

Dissociation of Weak Electrolytes

Consider a weak acid, HA

• The acid dissociation constant is given by:

HA ⇄ H+ + A-

2.2 What is pH?

Titration curves illustrate the progressive dissociation of a weak acid

2.2 What is pH?

Titration curves illustrate the progressive dissociation of a weak acid

The Dissociation Behavior of Weak Electrolytes

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:

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:

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:

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?

The Dissociation Behavior of Weak Electrolytes

The titration curves of several weak acids.

Titration Curves Illustrate the Progressive Dissociation of a Weak Acid

The titration curve for phosphoric acid.

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

2.3 What Are Buffers, and What Do They Do?

A buffer system consists of a weak acid, HA and its conjugate base, A-

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.

2.3 What are Buffers and What Do They Do?

Anserine is a dipeptide buffer that helps maintain intracellular pH in some tissues.

2.3 What are Buffers and What Do They Do?

The structure of HEPES, in its fully protonated form.

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

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.