Chem 2 - Acid-Base Equilibria VII: Conjugate Acid/Base Pairs and Relationships Between Ka, Kb, and Kw

Chem 2 - Acid-Base Equilibria VII: Conjugate Acid/Base Pairs and Relationships Between Ka, Kb, and Kw

1. Acid-Base Equilibria (Pt . 7) Conjugate Acid/Base Pairs and Relationships Between K a , K b , 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: 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

3. The equilibrium constant K is “renamed” for acids to K a The Equilibrium Constant K a for Weak Acids An equilibrium exists between the weak acid (HA) and its products.    

4. K a and K b for Conjugate Acid-Base Pairs Suppose acetic acid (CH 3 COOH) is dissolved in water…   acetic acid conjugate base for acetic acid “ loseable ” H +

5. K a and K b for Conjugate Acid-Base Pairs Suppose acetate (CH 3 COO - ) is dissolved in water…   acetate conjugate acid for acetate acetate gained H + from water

6. K a and K b for Conjugate Acid-Base Pairs CH 3 COOH and CH 3 COO - are conjugate acid-base pairs acetic acid acetate (base) K a = 1.76  10 -5 K b = 5.68  10 -10

7. Multiply K a and K b for Conjugate Acid-Base Pairs Use this relationship to interconvert between K a and K b .  

8. Calculating (Converting Between) K a and K b using K w Use K W to interconvert between K a and K b .  

9. Calculating (Converting Between) K a and K b using K w Use K W to interconvert between K a and K b .  

10. pK a and pK b Calculate the pK a for an acid as p K a =  log K a Calculate the pK b for a base as pK b =  log K b

11. Calculating pK a and pK b Calculate the pK a pK a =  log K a Calculate the pK b pK b =  log K b  

12. Inverse logs for pK a , pK a , and pK w pK a =  log K a 10 pKa = K a pK b =  log K b 10  pKb = K b pK w =  log K w = 14 10 pKw = K w Recall: K w (and pK w ) will have different values at temperatures other than 25  C

13. A Few More Relationships Between pK a , pK b , and pK w pK a + pK b = pK w pK w =  log [ K w ] = 14 ( at 25  C) pK a + pK b = 14 (at 25 C)

14. Next up, The Conjugate See-Saw and Analyzing K a and K b for Acid or Base Relative Strength (Pt. 8)