Revised 1/9/2014
Biochemistry I
Dr. Loren Williams
Chapter 2
Water
Colorado River 1999
Colorado River 2013
Water is polar. The oxygen atom in a water is
partially negative (it carries a partial negative
charge). Each hydrogen atom is partially positive.
This phenomenon is called polarity. Water is a
polar molecule.
Oxygen is electronegative. It does not share
electrons equally with hydrogen atoms. The
hydrogen atoms, with no inner shell electrons,
are stripped of much of their electron density
and carry a partial positive charge, the protons
are exposed.
In liquid water and especially in solid water, the
molecules interact strongly with each other, with
preferred orientations.
Water has all sort of strange and unusual properties.
Water:
o has a high boiling point and high heat of vaporization.
o has high surface tension.
o dissolves many salts (like sodium chloride) and polar
molecules (like ethanol).
o does not dissolve non-polar substances (oil and water
don’t mix).
o has high heat capacity.
o expands when it freezes, so ice floats. The density of
water decreases when it freezes.
o forms compartments in the presence of amphipaths.
o is often found on Earth simultaneously as a solid, liquid
and gas.
Capillary action is the tendency of a liquid to flow into
narrow spaces in opposition to external forces. Water is
lifted by surface tension (caused by cohesion between water
molecules) and adhesive forces between the water and the
surface.
Figure 2-1a
Osmosis is the spontaneous net movement of molecules into regions
of lower concentration. The net movement of molecules is in
directions that tend to equalize concentrations.
Osmotic pressure is the pressure required to halt the next flux of
water across the semipermeable membrane.
Figure 2-14
Condensation-Dehydration & Hydrolysis
Autoionization of water:
[H3O+][OH-]
[H2
O(l)]2
2 H2O (l)
H3O+ (aq) + OH- (aq)
Kw = [H3O+][OH-] = [H+][OH-]
Kw = 1.0 x 10-14 (at 25oC)
In pure water [H+] = [OH-]
Kw = [H3O+][OH-]
Figure 2-16
HA
+ H2O
H3O+ + A –
HA = generic acid
• There is competition for the proton between two bases,
H2O and A–
• If H2O is a stronger base than A– the equilibrium lies to
the right.
• If A– is a stronger base than H2O the equilibrium lies to
the left.
Polyprotic
Acids
It is easier to remove the first proton in a polyprotic acid than
the second. That is, Ka1 > Ka2 > Ka3
Note: Phosphoric acid is a strong acid in its first dissociation, and a weaker acid in its
second step and an even weaker acid in its third step.
H3PO4
H+ + H2PO4–
Ka1 = 7.5x10-3
pKa1 = 2.2
H2PO4–
H+ + HPO42-
Ka2 = 6.2 x10-8
pKa1 = 7.2
HPO42-
H+ + PO43-
Ka3 = 4.8x10-13
pKa1 = 12.4
H3PO4
H+ + H2PO4–
Ka1 = 7.5x10-3
pKa1 = 2.2
H2PO4–
H+ + HPO42-
Ka2 = 6.2 x10-8
pKa1 = 7.2
HPO42-
H+ + PO43-
Ka3 = 4.8x10-13
pKa1 = 12.4
Table 2-3
Table 2-4
Figure 2-17