Acid-Base Equilibria (Pt. 2)
The Autoionization of Water and Kw
By Shawn P. Shields, Ph.D.
This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Water and Acid-Base Chemistry
Water can act as either an acid or a base.
Water (H2O) accepts H+ from HCl (acts as a base)
Water donates H+ to NH3 (acts as an acid)
Water is Amphiprotic
“Amphiprotic” means a substance can act as either an acid or a base.
Let’s go back to the concept of conjugate acid/base pairs…
Recall: Conjugate Acid-Base Pairs
Conjugate acid-base pairs are two related species differing only by a proton (H+)
base
conjugate acid for NH3
acid conjugate base for HF
Conjugate Acid-Base Pairs for Water
Now, let’s label water conjugate acid-base pairs for a weak acid dissolved in water:
acid 1 conjugate base 1
conj acid 2
base 2
Conjugate Acid-Base Pairs for Water
Now, let’s label water conjugate acid-base pairs for a weak base dissolved in water.
base 1 conjugate base 2
conj acid 1
acid 2
Autoionization of Water
What if there isn’t an acid or base in solution.Does water react with itself?
base 1 conjugate base 2
conjugate acid 1
acid 2
Autoionization of Water
What if there isn’t an acid or base in solution.Does water react with itself? YES!
base 1 conjugate base 2
conjugate acid 1
acid 2
Autoionization of Water and Kw
Let’s write an equilibrium constant expression for the autoionization of water:
Recall heterogeneous equilibria…the activity is “1” for pure liquids.
Autoionization of Water and Kw
In pure water at 25C, Kw = 1.0 1014
Therefore, [OH] = [H3O+] = 1.0 107
What you Should Be Able to Do
Identify whether water is acting as an acid or a base.
Write the chemical equation for the autoionization of water
Identify conjugate acid-base pairs for the autoionization of water.