Acids and bases can be defined by their chemical properties in several different
ways. However, because acid–base reactions important for biochemistry occur
in aqueous solution (that is, in water), the Bronsted–Lowry definitions
of acids and bases are generally used. In the Bronsted–Lowry scheme:
An acid is a proton (H+) donor A base is a proton (H+) acceptor
A proton is a hydrogen atom that has lost its electron and is designated H+.
Thus, acids are chemical compounds that tend to give up H+ ions when
placed in water. For example, when hydrochloric acid (HCl) is placed in water,
the reaction that occurs is
The HCl has donated its H+ ion, which was accepted by the water
molecule. Notice how the H+ ion, once donated by the HCl molecule,
does not simply float around in the water freely, but instead associates strongly
with water (the proton acceptor) to form a hydronium ion (H3O+).
Thus, using the Bronsted–Lowry convention, the acid proton H+
is often written as H3O+ to reflect this fact.
Similarly, ammonia (NH3) is a base because it acts as a proton acceptor:
As you can see in the examples above, water can act not only as a proton acceptor,
but also as a proton donor. In fact, water actually reacts with itself. The
result is that even in a sample of pure water, not every molecule is in the
form of H2O. A very small proportion of molecules undergo this reaction,
Autoionization of water
Just how common is this reaction? Is a cup of water out of the tap really just a collection of charged molecules rather than H2O?
Let's look closer. The equilibrium constant for the reaction of water molecules
(the square brackets indicate molar concentrations)
By convention, when a reaction involves water, the concentration of water is factored into the value of Keq. For the reaction of water molecules, the resulting equilibrium constant is known as Kw, the ion product constant of water, and its value is 1014.
Kw = Keq[H2O]
= [OH][H3O+] = 1014
Scientists determined experimentally that the equilibrium constant for this
reaction of water molecules is
Kw = [OH][H3O+] = 1014
The value of Kw is known as the ion product constant of water. Note
that by convention, water molecules are not included in the equilibrium constant
expression, because the reaction is happening in water, and the concentration
of water is already factored in to the Kw constant. Therefore Kw
is just the Keq for the ionization of water, with the concentration
of water factored into the constant (Kw = Keq/[H2O]).
Pure water is neutral, having neither an overall positive nor an overall negative
charge. This is because each time a water molecule donates a proton, another
accepts it, so that the concentrations of OH and H3O+
are always equal:
[OH] = [H3O+]
in pure water
Since the Kw expression for pure water states that the product of
these two must equal 10–14:
[H3O+] = 107
M and [OH] = 107 for pure water ONLY
These are very minute concentrations, especially since pure water is 55.5 M.
In other words, the fraction of H3O+ and OH
ions is very tiny compared to the overall H2O concentration. In fact,
there are over 500 million times more H2O molecules than either H3O+
or OH ions in water!
The Kw value is still very important for biochemists, however, since
it aids in the understanding of acid–base chemistry. The Kw value
is useful because it says that in any aqueous solution, the product of [OH]
and [H3O+] ions is a constant value. Thus, an acid solution
contains not only H3O+ ions, but also a lesser amount of
If a 10–1 M solution of HCl was prepared,
what would be the equilibrium concentrations of H3O+ and
Returning to the hydrochloric acid dissociation reaction from above, we can
describe the reaction of the acid with water as:
Because HCl is a strong acid, essentially all of the acid will dissociate,
and the concentration of H3O+ ions would be equal to 101
M. Plugging this into the Kw expression gives the following:
Similarly, a basic solution contains not only OH ions, but
also some H3O+ ions. To further grasp this concept, interact
with the graph below. Note that while the ratio of hydronium ion to hydroxide
ion changes, the Kw (the product of these two) does not change.
Move the slider left to add more acid or right to
add more base to an aqueous solution