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.

Recall: Brønsted-Lowry Acids and Bases

Acids donate protons H+

Bases accept protons H+.

acid

base

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.

Next up,

pH, pOH, and pKw(Pt 3)