Chem 2 - Acid-Base Equilibria II - The Auto-Ionization of Water
1. 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.
3. Water and Acid-Base Chemistry
Water can act as either an acid or a
base.
πππ₯ ππͺ + π π π π₯ β π π π+
ππͺ + ππ₯β
(ππͺ)
ππ π ππͺ + π π π π₯ β ππ π
+
ππͺ + ππβ
(ππͺ)
Water (H2O) accepts H+
from HCl (acts as a base)
Water donates H+ to NH3
(acts as an acid)
4. Water is Amphiprotic
βAmphiproticβ means a substance can
act as either an acid or a base.
Letβs go back to the concept of
conjugate acid/base pairsβ¦
5. Recall: Conjugate Acid-Base Pairs
Conjugate acid-base pairs are two
related species differing only by a
proton (H+)
ππ ππͺ + π π π π₯ β π π π+
ππͺ + π β
(ππͺ)
ππ π ππͺ + π π π π₯ β ππ π
+
ππͺ + ππβ
(ππͺ)
base conjugate acid for NH3
acid conjugate
base for HF
6. Conjugate Acid-Base Pairs for
Water
Now, letβs label water conjugate
acid-base pairs for a weak acid
dissolved in water:
ππ ππͺ + π π π π₯ β π π π+
ππͺ + π β
(ππͺ)
acid 1 conjugate
base 1
conj
acid 2
base 2
7. Conjugate Acid-Base Pairs for
Water
Now, letβs label water conjugate acid-
base pairs for a weak base dissolved
in water.
ππ π ππͺ + π π π π₯ β ππ π
+
ππͺ + ππβ
(ππͺ)
base 1 conjugate
base 2
conj
acid 1
acid 2
8. Autoionization of Water
What if there isnβt an acid or base in
solution.
Does water react with itself?
π π π π₯ + π π π π₯ β π π π+
ππͺ + ππβ
(ππͺ)
base 1 conjugate
base 2
conjugate
acid 1
acid 2
9. Autoionization of Water
What if there isnβt an acid or base in
solution.
Does water react with itself? YES!
π π π π₯ + π π π π₯ β π π π+
ππͺ + ππβ
(ππͺ)
base 1 conjugate
base 2
conjugate
acid 1
acid 2
10. Autoionization of Water and Kw
Letβs write an equilibrium constant
expression for the autoionization of water:
π π π π₯ + π π π π₯ β π π π+
ππͺ + ππβ
(ππͺ)
π² πΎ =
ππβ
[π π π+
]
π [π]
Recall heterogeneous equilibriaβ¦the
activity is β1β for pure liquids.
11. Autoionization of Water and Kw
In pure water at 25ο°C, Kw = 1.0 ο΄ 10ο14
π π π π π₯ β π π π+
ππͺ + ππβ
(ππͺ)
π π = ππβ
[π π π+
]
π. π Γ ππβππ
= ππβ
[π π π+
]
Therefore, [OHο] = [H3O+] = 1.0 ο΄ 10ο7
12. What you Should Be Able to Do
ο΄Identify whether water is acting as
an acid or a base.
ο΄Write the chemical equation for
the autoionization of water
ο΄Identify conjugate acid-base pairs
for the autoionization of water.