Acid-Base Equilibria (Pt. 8)

The Conjugate See-Saw and Analyzing Ka and Kb for Acid or Base Relative StrengthBy 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.

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Recall: Conjugate Acid-Base Pairs

Acetic acid (CH3COOH) and its conjugate base…

acetic acid

conjugate base for acetic acid (acetate)

“loseable” H+

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Recall: Ka and Kb for Conjugate Acid-Base Pairs

acetic acid

acetate (base)

Ka = 1.76 10-5 Kb = 5.68 10-10

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.

Analyzing the Magnitude of Ka

A larger Ka means the weak acid has a larger percent dissociation (more products).

𝐊 𝐚=¿¿ 𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

Example: Analyzing Ka Values

Which is the stronger weak acid?

HF Ka = 3.5 10-4

CH3COOH Ka = 1.76 10-5

K a∝𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

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).

K a∝𝐩𝐫𝐨𝐝𝐮𝐜𝐭𝐬𝐫𝐞𝐚𝐜𝐭𝐚𝐧𝐭𝐬

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

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!

Central Concept: The Conjugate See-Saw

The stronger the acid, the weaker the conjugate base.

The stronger the base, the weaker the conjugate acid.

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.

Conjugate See-Saw Example: Comparing Ka and Kb Values

HF Ka = 3.5 10-4. Kb for the conjugate base F…

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.)

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.

Conjugate See-Saw Example 2: Comparing Ka and Kb Values

NH4+ Ka = 5.6 10-10.

Kb for the conjugate base NH3 (ammonia)

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.)

Conjugate See-Saw for Strong Acids

Strong acids dissociate completely in water.

The conjugate base of a strong acid does not affect the pH of the water.The conjugate base is NOT basic in water.

Conjugate See-Saw for Strong Bases

Strong bases dissociate completely in water.

The conjugate acid of a strong base does not affect the pH of the water.The conjugate acid is NOT acidic in water.

Conjugate See-Saw SummaryAcid/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.

Conjugate See-Saw SummaryBase/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.

Next up, Salts that Hydrolyze in

Water to Affect pH(Pt 9)