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Lesson 2

Water and Acid-Base Chemisty

TermDefinition
Important Features of Water - dipole (O is partial negative, H is partial positive) - tetrahedral arrangement, allows for H bonds -H bonds are weak, "flickering bonds"
Number of H bonds in Water -3.4 in liquid -4 in ice
H-bond donors N-H, O-H
H-bond acceptors O:, N:
Where are H-bonds used in the body? Peptide bonds (proteins) and DNA base pairing
Ionic Interactions Negative and Positive will attract each other Same charge will repel
Hydrophobic multiple hydrophobic molecules will cluster together to stay away from water, this frees water
van der Waals happen between any molecule that get close
Weakest interactions (.4-4 kJ/mol) hydrophobic, pi-stacking, van der waals
Intermediate interactions (4-40 kJ/mol) electrostatic (ionic) and H-bonding
Strongest interactions (40-400 kJ/mol) salt bridges (H bonding plus ionic)
Amphipathic Molecule has both hydrophobic (nonpolar) and hydrophilic (polar or charged) regions
How does entropy relate to hydrophobic interactions? Water forms cages around hydrophobic molecules. Water would rather move freely (randomly = more entropy). Hydrophobic molecules minimize the surface area in contact with water.
Water equilibrium H2O <=> H+ and OH-
Keq of water [H+][OH-]/[H2O], = 1.8 x 10^-16 at 25 degrees C
Kw of water Keq x [H2O], = 1 x 10^-14
Concentration of H+ and OH- at neutral pH? [H+] = [OH-] = 1 x 10^-7
Equilibrium Reaction of an H+ donor HA + H2O <-> A- + H3O+
Ka [H3O+][A-] / [HA]
When does pKa = pH at half equivalence, when [A-] = [HA]
What is the buffering region of a weak acid? +/- 1 of the pKa
Henderson Hasselbach Equation pH = pKa + log[A-]/[HA]
Pepsin digestive enzyme in gastric acid, operates at pH 1.5
Trypsin digestive enzyme in small intestine, operates at pH 6.5
Created by: CGraybosch
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