1. Previously in Chem104: How K a relates to K b and pK a to pK b More ways to use the K w circle Group worksheet on The Most Important Equilibrium on the Planet (Part 1) Today in Chem104: structural characteristics of acids & bases acid/base reactions: the easy, the difficult, the impossible

2. How can you recognize an acid or a base if one sneaks up behind you … in a dark alley??? AH + BA - + BH+ acid conjugate base conjugate acid We know the characteristic behavior of an acid is: So, think about it: what must be true about the A-H bond? A-H must be easily broken, weak.

3. What kinds of structures make A-H weak? First consider HOW the A-H bond breaks: A:H  A: - + H + heterolytically

4. So what kinds of atoms for A will cause the A-H bond to easily heterolytically break? A:H  A: - + H + Highly electronegative atoms, Like: H-Cl H-Br HNO 3 H 2 SO 4 H-O-NO 2 (H-O) 2 SO 2 HClO 4 H 3 C-CO 2 -HH4C2O2H4C2O2

5. What kinds of structures make A-H a strong acid or a weak one? A:H  A: - + H + Anything that increases A’s ability to pull e- pair towards it, like: High Ox. State HClO 4 (pKa -15) perchloric HClO 3 (pKa -1) chloric HClO 2 (pKa 1.98) chlorous HClO (pKa 7.46) hypochlorous

6. What kinds of structures make A-H a strong acid or a weak one? A:H  A: - + H + Anything that increases A’s ability to pull e- pair towards it, like: High Ox. State HClO 4 (pKa -7) HClO 3 (pKa -1) HClO 2 (pKa 1.98) HClO (pKa 7.46) Electronegative atoms H 3 C-COOH (pKa 4.74) F 3 C-COOH (pKa 0.59)

7. Now, apply the same principles to Bases …. AH + BA - + BH+ acid conjugate base conjugate acid We know the characteristic behavior of a base is: So, think about it: what must be true about B ? B must be able to make a bond to H+, and it must have the e- pair to make the bond:  B must have a lone pair of electrons

8. What kinds of structures make B: a strong or a weak base? The more electron density available to B: The better it can attract and hold a H+ Electron Rich molecules are better bases: H 3 N (pK b 4.74) ammonia (H 3 C)NH 2 (pK b 3.35) methylamine stronger base (H 3 C) 2 NH (pK b 3.13) dimethylamine (HO)NH 2 (pK b 8) hydroxylamine weaker base!

9. Let’s do some chemistry: First, a practical culinary demo … Needing a volunteer …. To be blindfolded…. (H 3 C)NH 2 (pK b 3.35) methylamine XX (H 3 C)NH 2 (pK 3 3) citric acid

10. Adding lemon juice to fish is an Acid-Base Neutralization Reaction methylamine XX citric acid methylammoniumcitrate 

11. Let’s do some chemistry: Acid / Base Reactions There’s 3 types of acid /base reaction problems: Easy: strong acid + strong base Difficult: strong acid + weak base or (strong base) + (weak acid) Impossible: weak acid + weak base

12. Let’s do some problems !!