Chem 2 - Acid-Base Equilibria II - The Auto-Ionization of Water

Chem 2 - Acid-Base Equilibria II - The Auto-Ionization of Water

1. Acid-Base Equilibria (Pt . 2) The Autoionization of Water and K w 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 + .   acid base

3. Water and Acid-Base Chemistry Water can act as either an acid or a base.   Water (H 2 O) accepts H + from HCl (acts as a base) Water donates H + to NH 3 (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 + )   base conjugate acid for NH 3 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:   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.   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. D oes water react with itself?   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. D oes water react with itself? YES!   base 1 conjugate base 2 conjugate acid 1 acid 2

10. Autoionization of Water and K w Let’s write an equilibrium constant expression for the autoionization of water:   Recall heterogeneous equilibria…the activity is “1” for pure liquids.

11. Autoionization of Water and K w In pure water at 25 C, K w = 1.0  10 14 Therefore, [OH  ] = [H 3 O + ] = 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 pK w (Pt 3)