This document discusses several key properties of water including its polarity, hydrogen bonding ability, and role in solvation. Water can dissolve polar substances due to hydrogen bonding interactions in a solvation shell. Nonpolar substances are insoluble in water. Hydrogen bonding between water molecules forms a crystal lattice in ice and allows water to have unusual properties like a high boiling point. The pH scale is used to describe hydrogen ion concentration in aqueous solutions. Weak acids and bases only partially dissociate in water according to their acid dissociation constants (Ka) and pKa values. Buffers resist changes in pH through the dynamic equilibrium between a weak acid and its conjugate base as described by the Henderson-Hasselbach equation.

2. Water STRUCTURE

3. WHY DO WE NEED TO DO
THIS AGAIN!!!

4. Properties of water
Very polar
Oxygen is highly electronegative
H-bond donor and acceptor
High b.p., m.p., heat of vaporization, surface tension

6. Water dissolves polar compounds
solvation shell
or
hydration shell

7. Non-polar substances are insoluble in water
Many lipids are amphipathic

8. How detergents work?

9. Hydrogen Bonding of Water
One H2O molecule can associate with 4 other H20 molecules
Ice: 4 H-bonds per water molecule
Water: 2.3 H-bonds per water molecule

11. Crystal lattice of

12. Biological
Hydrogen Bonds

14. non-covalent interactions

15. Relative Bond Strengths
Bond type
kJ/mole
H3C-CH3
88
H-H
104
Ionic
40 to 200
H-bond
2 - 20
Hydrophobic interaction
3 -10
van der Waals
0.4 - 4

16. Ionization of Water

17. Ionization of Water
H20 + H20
H2 0
Keq= [H+] [OH-]
[H2O]
H3O+ + OHH+ + OHKeq=1.8 X 10-16M
[H2O] = 55.5 M
[H2O] Keq = [H+] [OH-]
(1.8 X 10-16M)(55.5 M ) = [H+] [OH-]
1.0 X 10-14 M2 = [H+] [OH-] = Kw
If [H+]=[OH-] then [H+] = 1.0 X 10-7

18. pH Scale
 Devised by Sorenson (1902)
 [H+] can range from 1M and
1 X 10-14M
 using a log scale simplifies notation
 pH = -log [H+]
Neutral pH = 7.0

19. Weak Acids and Bases
Equilibria
•Strong acids / bases – disassociate completely
•Weak acids / bases – disassociate only partially
•Enzyme activity sensitive to pH
• weak acid/bases play important role in
protein structure/function

21. Acid/conjugate base pairs
HA + H2O
HA
A- + H3O+
A- + H+
HA = acid ( donates H+)(Bronstad Acid)
A- = Conjugate base (accepts H+)(Bronstad Base)
Ka = [H ][A ]
[HA]
+
-
pKa = - log Ka
Ka & pKa value describe tendency to loose
H+
large Ka = stronger acid
small Ka = weaker acid

22. pKa values determined by
titration

23. Phosphate has three ionizable H+ and
three pKas

24. Buffers
Buffers are aqueous systems that resist
changes in pH when small amounts of a
strong acid or base are added.
A buffered system consist of a weak acid and
its conjugate base.

25. The most effective buffering occurs at the
region of minimum slope on a titration
curve
(i.e. around the pKa).
Buffers are effective at pHs that are
within +/-1 pH unit of the pKa

26. Henderson-Hasselbach Equation
1) Ka = [H ][A ]
[HA]
+
-
HA = weak acid
A- = Conjugate base
2) [H+] = Ka [HA]
[A-]
3) -log[H+] = -log Ka -log [HA]
[A-]
4) -log[H+] = -log Ka +log [A-]
[HA]
5) pH = pKa +log [A-]
[HA]
* H-H equation describes
the relationship between
pH, pKa and buffer
concentration

27. Case where 10% acetate ion 90% acetic
acid
• pH = pKa + log10
[0.1 ]
¯¯¯¯¯¯¯¯¯¯
[0.9]
• pH = 4.76 + (-0.95)
• pH = 3.81

28. Case where 50% acetate ion 50% acetic
acid
• pH = pKa + log10
[0.5 ]
¯¯¯¯¯¯¯¯¯¯
[0.5]
• pH = 4.76 + 0
• pH = 4.76 = pKa

29. Case where 90% acetate ion 10% acetic
acid
• pH = pKa + log10
[0.9 ]
¯¯¯¯¯¯¯¯¯¯
[0.1]
• pH = 4.76 + 0.95
• pH = 5.71

30. Cases when buffering fails
• pH = pKa + log10
[0.99 ]
¯¯¯¯¯¯¯¯¯¯
[0.01]
• pH = 4.76 + 2.00
• pH = 6.76
• pH = pKa + log10
[0.01 ]
¯¯¯¯¯¯¯¯¯
[0.99]
• pH = 4.76 - 2.00
• pH = 2.76