Life’s Water: Necessary and Abundant

Human H2O content:

Life on Europa (Jupiter moon) only considered if H2O is present Even in unusually

cold hearty Antarctic bacteria (e.g. -2C) H2O is essential

Compound MW BP MP

H2S Sulfide 34 -60 -85H2Se Selenide 81 -4 -50H2Te Telluride 130 -2 -49

H2O Water 18 100 0

Water Unlike its Molecular Neighbors

As the molecule becomes heavier how should this affect boiling point and melting point?

Unusual Physical Properties of H2O

Adhesion – sticks to other molecules

Cohesion – sticks to itself

High surface tension

Water Hydrogen Bonding: Weak But Abundant

Distance for:

Covalent

H-bonding

Non-interacting

Van der Waals radius

Van der Waals Radius: distance from nucleus to effective electron surface

Where is H-bonding on this graph?

Oxygen Electronic configuration? Bond angle?

Hydrogen Bonding in Water

DNA Base Pair Hydrogen Bonding

100

400

200.1

Relative Bond Strengths

Where do dipole-dipole interactions fit into this ranking?

Hydrogen-Bonding Requirement

Differential electronegativity

S – H - - - - - SName the following interactions:

Water: the Universal Solvent

Polar solvents weaken electrostatic interactions

Dielectric Solvent Constants1

1Dielectric solvent constant is a measure of the ability of a solvent to diminish electrostatic attraction between dissolved ions

Exclusion of non-polar substances from a water phase

Entropy driven ∆G = ∆H - TΔS

Hydrophobic Effect

Single Tail Amphipathic Molecules: Micelles

What is the driving force for this reaction?

Double Tail Amphipathic Molecules: Bilayer

A Bilayer Limits Movement of Polar Substances

Amphipathic molecules contain both hydrophobic and hydrophilic portions

Driving Force for Protein Folding: Hydrophobic Effect

Is a molecule always charged or uncharged?

pH Versus pKA

Ka/pKa Values

Le Chatelier’s Principle

When a system in a state of dynamic equilibrium is acted upon by some outside stress, the system will, if possible, shift to a new position of equilibrium in order to minimize the effect of the stress.

Acetic Acid Titration Curve

What is the effective buffering range?

Conceptionally: pKA is the pH at which the ionized

and unionized forms are equal. HA ↔ H+ + A-

KA = [A-][H+]/[HA]

KA[HA]/[A-] = [H+]

log KA + log [HA]/[A-] = log [H+]

pH = pKA – log [HA]/[A-]

Henderson-Hasselbalch Equation

pH = pKA – log [HA]/[A-]

pKA = -log KA and pH = -log [H+]

pH Value of Biological Fluids

Inverse Relationship: [H+] and [OH-]

H2O ↔ H+ + OH-

K = [H+][OH-]/[H2O]

KW = K[H2O] = [H+][OH-]

KW = 10-14 = [H+][OH-]

CO2 + H2O ↔ HCO3- + H+

Water Acidification

Acids to Buffer at Any pH

Chapter 2 Problems:

1-15, 21, 33, 35, 41, 53, 55 and 60