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Useful Concepts | ![]() |
First, consider the general equation of a simple acid reaction:
The more stable the conjugate base, A-,
is then the more the equilibrium favours the product side.....
The more the equilibrium favours products, the more H+
there is....
The more H+ there is then the stonger H-A
is as an acid....
So looking for factors that stabilise the conjugate base, A-,
gives us a
Basicity
Structure and pKa
The information here is to help you decide
which structure of an acid
or base will dominate at a particular pH. Lets do a general case.
The
equation for an acid is just HA = H+ + A- where
= means equilibrium
pKa is defined as -log10 Ka where Ka = [H+][A-]
/ [HA].
From these expressions it is possible to derive the
Henderson-Hasselbalch
equation which is
pKa = pH + log [HA] / [A-]
This tells us that when the pH = pKa then log [HA] / [A-]
= 0 therefore [HA] = [A-] ie equal amounts of the two
forms.
If we make the solution more acidic, ie lower the pH, then pH
< pKa and log [HA] / [A-] has to be > 0 so
[HA] > [A-].
This makes sense as it tells us that the protonated form dominates in
an
acidic medium.
If instead we make the solution more basic, ie raise the pH,
then pH > pKa and log [HA] / [A-] has to be
< 0 so [HA] <[A-].
This makes sense as it tells us that the deprotonated form dominates in
the basic medium.
These principles can be extended to poly acidic / basic systems (such as amino acids) by thinking of each pKa value in turn.
Lets look at an example.
To the right are the processes for the
amino acid HISTIDINE, which has
three acidic groups of pKa's 1.82 (carboxylic acid) 6.04
(pyrrole NH) and
9.17 (ammonium NH). Histidine can exist in the four forms shown,
depending
on the solution pH, from acidic pH (top) to basic pH. (bottom).
Starting from the top, we can imagine that as we add base, the most
acidic proton is removed first (COOH), then the pyrrole NH then finally
the amino NH. These takes us through each of the forms in turn.
At pH < 1.82, A is the dominant form.
In the range 1.82 < pH < 6.02 B is the dominant form.
In the range 6.02 < pH < 9.17 C is the dominant form, and
when pH > 9.17, D is the major form in solution. OK?
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© Dr. Ian Hunt, Department of Chemistry, University of Calgary | ![]() |
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