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.

2. Recall: Brønsted-Lowry Acids and Bases
Acids donate protons H+
Bases accept protons H+.
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
𝐍𝐇 𝟑 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐍𝐇 𝟒
+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
acid
base

3. Water and Acid-Base Chemistry
Water can act as either an acid or a
base.
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
𝐍𝐇 𝟑 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐍𝐇 𝟒
+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
Water (H2O) accepts H+
from HCl (acts as a base)
Water donates H+ to NH3
(acts as an acid)

4. 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…

5. Recall: Conjugate Acid-Base Pairs
Conjugate acid-base pairs are two
related species differing only by a
proton (H+)
𝐇𝐅 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐅−
(𝐚𝐪)
𝐍𝐇 𝟑 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐍𝐇 𝟒
+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
base conjugate acid for NH3
acid conjugate
base for HF

6. Conjugate Acid-Base Pairs for
Water
Now, let’s label water conjugate
acid-base pairs for a weak acid
dissolved in water:
𝐇𝐅 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐅−
(𝐚𝐪)
acid 1 conjugate
base 1
conj
acid 2
base 2

7. Conjugate Acid-Base Pairs for
Water
Now, let’s label water conjugate acid-
base pairs for a weak base dissolved
in water.
𝐍𝐇 𝟑 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐍𝐇 𝟒
+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
base 1 conjugate
base 2
conj
acid 1
acid 2

8. Autoionization of Water
What if there isn’t an acid or base in
solution.
Does water react with itself?
𝐇 𝟐 𝐎 𝐥 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
base 1 conjugate
base 2
conjugate
acid 1
acid 2

9. Autoionization of Water
What if there isn’t an acid or base in
solution.
Does water react with itself? YES!
𝐇 𝟐 𝐎 𝐥 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
base 1 conjugate
base 2
conjugate
acid 1
acid 2

10. Autoionization of Water and Kw
Let’s write an equilibrium constant
expression for the autoionization of water:
𝐇 𝟐 𝐎 𝐥 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
𝑲 𝑾 =
𝐎𝐇−
[𝐇 𝟑 𝐎+
]
𝟏 [𝟏]
Recall heterogeneous equilibria…the
activity is “1” for pure liquids.

11. Autoionization of Water and Kw
In pure water at 25C, Kw = 1.0  1014
𝟐 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
𝐊 𝐖 = 𝐎𝐇−
[𝐇 𝟑 𝐎+
]
𝟏. 𝟎 × 𝟏𝟎−𝟏𝟒
= 𝐎𝐇−
[𝐇 𝟑 𝐎+
]
Therefore, [OH] = [H3O+] = 1.0  107

12. 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.

13. Next up,
pH, pOH, and pKw
(Pt 3)