Chem 2 - Acid-Base Equilibria VIII: The Conjugate See-Saw and Analyzing Ka and Kb for Acid or Base Relative Strength

1. Acid-Base Equilibria (Pt. 8)
The Conjugate See-Saw and
Analyzing Ka and Kb for Acid
or Base Relative Strength
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: Conjugate Acid-Base Pairs
Acetic acid (CH3COOH) and its conjugate base…
acetic acid conjugate base for
acetic acid (acetate)
“loseable” H+

3. Recall: Ka and Kb for Conjugate
Acid-Base Pairs
𝐊 𝐚 ∙ 𝐊 𝐛 = 𝐊 𝐖
acetic acid acetate (base)
Ka = 1.76  10-5 Kb = 5.68  10-10

4. Analyzing the Magnitude of Ka
The relative strength of the weak acid
(compared to others weak acids) can be
determined by comparing Ka values.
If Ka is relatively large, the weak acid is
relatively strong.

5. Analyzing the Magnitude of Ka
A larger Ka means the weak acid has a
larger percent dissociation (more
products).
𝐇𝐀 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 ⇌ 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐀−
(𝐚𝐪)
𝐊 𝐚 =
𝐇 𝟑 𝐎+
𝐀−
𝐇𝐀
𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬
𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

6. Example: Analyzing Ka Values
Which is the stronger weak acid?
HF Ka = 3.5  10-4
CH3COOH Ka = 1.76  10-5
Ka ∝
𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬
𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

7. Example: Analyzing Ka Values
Which is the stronger weak acid?
HF Ka = 3.5  10-4
CH3COOH Ka = 1.76  10-5
A larger Ka value means the acid is more
dissociated (stronger).
Ka ∝
𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬
𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

8. Example: Comparing pKa Values
How does pKa relate to weak acid strength?
HF Ka = 3.5  10-4
CH3COOH Ka = 1.76  10-5
Calculate pKa =  log Ka

9. Example: Comparing pKa Values
HF Ka = 3.5  10-4
pKa =  log (3.5  10-4) = 3.46
CH3COOH Ka = 1.76  10-5
pKa =  log (1.76  10-5) = 4.75
Lower pKa means stronger weak acid!

10. Central Concept: The Conjugate
See-Saw
The stronger the acid,
the weaker the
conjugate base.
The stronger the base,
the weaker the
conjugate acid.

11. Conjugate See-Saw Example:
Comparing Ka and Kb Values
For HF, the acid dissociation constant
Ka is 3.5  10-4.
What is the conjugate base?
Calculate Kb for the conjugate base, and
explain what that value tells you.

12. Conjugate See-Saw Example:
Comparing Ka and Kb Values
HF Ka = 3.5  10-4. Kb for the
conjugate base F…
𝐊 𝐛 =
𝐊 𝐖
𝐊 𝐚
=
𝟏. 𝟎 × 𝟏𝟎−𝟏𝟒
𝟑. 𝟓 × 𝟏𝟎−𝟒
= 𝟐. 𝟗 × 𝟏𝟎−𝟏𝟏
Ka is much larger than Kb, so F is a much
weaker base than HF is an acid (i.e., it’s
a very wimpy base.)

13. Conjugate See-Saw Example 2:
Comparing Ka and Kb Values
For NH4
+, the acid dissociation constant
Ka is 5.6  10-10.
What is the conjugate base?
Calculate Kb for the conjugate base, and
explain what that value tells you.

14. Conjugate See-Saw Example 2:
Comparing Ka and Kb Values
NH4
+ Ka = 5.6  10-10.
Kb for the conjugate base NH3 (ammonia)
𝐊 𝐛 =
𝐊 𝐖
𝐊 𝐚
=
𝟏. 𝟎 × 𝟏𝟎−𝟏𝟒
𝟓. 𝟔 × 𝟏𝟎−𝟏𝟎
= 𝟏. 𝟖 × 𝟏𝟎−𝟓
Kb is much larger than Ka
NH4
+ is a much weaker acid than NH3 is a base
(i.e., it’s a very wimpy acid.)

15. Conjugate See-Saw for
Strong Acids
Strong acids dissociate completely in
water.
𝐇𝐂𝐥 𝐚𝐪 + 𝐇 𝟐 𝐎 𝐥 → 𝐇 𝟑 𝐎+
𝐚𝐪 + 𝐂𝐥−
(𝐚𝐪)
The conjugate base of a strong acid does
not affect the pH of the water.
The conjugate base is NOT basic in water.

16. Conjugate See-Saw for
Strong Bases
Strong bases dissociate completely in
water.
𝐍𝐚𝐎𝐇 → 𝐍𝐚+
𝐚𝐪 + 𝐎𝐇−
(𝐚𝐪)
The conjugate acid of a strong base does
not affect the pH of the water.
The conjugate acid is NOT acidic in water.

17. Conjugate See-Saw Summary
Acid/Conjugate Base
If the Ka for the weak acid is
relatively large, the Kb for the
conjugate base will be relatively
small.
If the weak acid is relatively
strong, the conjugate base will be
very weak.

18. Conjugate See-Saw Summary
Base/Conjugate Acid
If the Kb for the weak base is
relatively large, the Ka for the
conjugate acid will be relatively
small.
If the weak base is relatively
strong, the conjugate acid will be
very weak.

19. Next up,
Salts that Hydrolyze in Water
to Affect pH
(Pt 9)