Water is the most abundant compound in living systems.
It provides the solvent phase for all intracellular reactions.
with water determine the three-dimensional
Water has a higher melting point, boiling point, and heat of vaporization than other compounds of similar molecular weight.
Water Ammonia Methane
M.W. 18 17 16
M.P. 0 -77.7 -182
B.P. 100 -33.4 -162
of 40.7 23.2 8.3
Due to attractive forces between the water molecules, known as hydrogen bonds:
These bonds arise due to the polar nature of the water molecule.
There is separation of charge across the molecule, and these charges allow water molecules to interact with each other.
Hydrogen bonds can form between other molecules and water, and between several molecules themselves.
Due to its polar nature, water is an excellent
1) Crystalline salts completely dissociate.OSMOTIC PRESSURE:
2) Neutral organics with polar functional groups.
3) Amphiphilic compounds form micelles,
monolayers, and bilayers.
The addition of solute to water creates osmotic pressure, which is proportional to the number of solute particles.
Water will move from areas of low osmotic pressure to areas of high osmotic pressure (the opposite of diffusion!).
You need to know what the terms hypertonic, hypotonic, and isotonic mean.
LAW OF MASS ACTION
A + B <====> C + D
Given the above system at equilibrium:
Addition of A or B or removal of C or D will
cause the equilibrium to shift to the Right.
Removal of A or B or addition of C or D will
cause the equilibrium to shift to the Left.
Changes in concentration for both
and nonspontaneous reactions:
Keq. = [C] * [D]/[A] * [B]For a spontaneous reaction, Keq. > 1.
For a nonspontaneous reaction, Keq. < 1.
N.B. [ ]
IONIZATION OF WATER
In solution, water will ionize:
The concentration of pure water is 55 M,
so multiplying both sides by 55 yields:
Kw = 1 x 10-14 = [H+] * [OH-]
pH = -log [H+]
There are two important things to remember
about the pH scale:
2) A small change in pH is a big change in [H+]
According to the Brönsted-Lowry theory:
An acid is a proton donor, e.g. HA.
A base is a proton acceptor, e.g. A-.
HA <====> H+ + A-
"acid" "conjugate base"
expression for this is:
Keq. = Ka = [H+] * [A-]/[HA]
If we take the logarithm of both sides, substitute,TAKE HOME MESSAGE I
and rearrange, we get the Henderson-Hasselbalch
pH = pKa + log [A-]/[HA]
What is the pKa?
Acids have low pKas (less than 7).
Bases have high pKas. (more than 7).
The further from 7, the stronger.
At a pH below its pKa, a molecule will
At a pH above its pKa, a molecule will
N.B. The pKa is a physical constant
for a given molecule. It cannot
The pH is a property of the solution,
and it can change.
At pH below its pKa, an acid will be
protonated and have no charge.
At pH above its pKa, an acid will be
unprotonated, and have a negative charge.
At pH below its pKa, a base will be
protonated and have a positive charge.
At pH above its pKa, a base will be
unprotonated, and have no charge.
Definition: A solution that resists a change in pH.
Recipe: A solution of a weak acid
TAKE HOME MESSAGE II
1) Buffers work best at pHs near their pKa.
2) The pKa is the pH where the molecule
shifts from giving up to accepting a
proton, or vice versa.
PHYSIOLOGICALLY IMPORTANT BUFFERS
1) Bicarbonate Buffer
2) Phosphate Buffer