7. Autoprotolysis and the Ionization of
Water
Please remember this concept::
- Many processes can be at equilibrium.
But with changes in condition - concentration, temperature - the system will no
longer be at equilibrium and will adjust to try to get there again.
Is water an acid or a base? Why both, of course, and neither!
According to the Bronsted-Lowry definition, water can act as both an acid and a base,
because it is able to both accept a proton to become the hydronium ion, and to donate a
proton, becoming the hydroxyl ion. Let’s take a look!
2H2O <=> H3O+
+ OH-
This reaction is called autoprotolysis. Notice
that the arrow goes both to the left and to the right. This reaction is an equilibrium
reaction. Water, of course, is a liquid, and the hydronium and hydroxyl ions are dissolved
in the aqueous solvent. By the law of mass action, we can calculate the equilibrium
constant by: |
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Remember this: this autoprotolysis of water always
occurs when water is present. This reaction occurs in all dilute aqueous solutions. In
dilute solutions, the molar concentration of undissociated water can be taken as a
constant, since so little of the water is dissociated to the ionized form. This
simplifyies the equation to the forward reaction, or the dissociation of water to the
hydronium and hydroxyl ions. Thus, the reaction becomes
Experimentally it has been determined that, in water at 25° C, the
molar concentrations of the hydronium and hydroxyl ions are 1x10-7 each.
(Remember that they must be in equal concentrations!)
So, plug in the numbers!
The relationship between these numbers is very important.
Things to remember about this
section:
- Protonization of water occurs rapidly.
- Most of the water molecules that exist , do so as H2O
molecules, that is, in the unionized state.
- For every two molecules of water, one hydronium ion and one hydroxyl
ion are produced SIMULTANEOUSLY.
- The concentrations of H3O+ and OH- are
vanishingly small in pure water.
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