Previously in Chem104: • How Ka relates to Kb and pKa to pKb • More ways to use the Kw 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
How can you recognize an acid or a base if one sneaks up behind you … in a dark alley??? We know the characteristic behavior of an acid is: AH + B A-+ BH+ acid base conjugate base conjugate acid So, think about it: what must be true about the A-H bond? A-H must be easily broken, weak.
What kinds of structures make A-H weak? First consider HOW the A-H bond breaks: A:H A:- + H+ heterolytically
So what kinds of atoms for A will cause the A-H bond to easily heterolytically break? Highly electronegative atoms, Like: A:H A:- + H+ H2SO4 (H-O)2SO2 H3C-CO2-H H4C2O2 H-Cl HClO4 HNO3 H-O-NO2 H-Br
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 HClO4 (pKa -15) perchloric HClO3 (pKa 1.0) chloric HClO2 (pKa 1.0) chlorous HClO (pKa 1.0) hypochlorous
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 HClO4 (pKa -15) HClO3 (pKa -1) HClO2 (pKa 1.98) HClO (pKa 7.46) Electronegative atoms H3C-COOH (pKa 4.74) F3C-COOH (pKa 0.59)
Now, apply the same principles to Bases …. We know the characteristic behavior of a base is: AH + B A-+ BH+ acid base conjugate base conjugate acid 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
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: H3N (pKb 4.74) ammonia (H3C)NH2 (pKb 3.35) methylamine stronger base (H3C) 2NH (pKb 3.13) dimethylamine (HO)NH2 (pKb 8) hydroxylamine weaker base!
Let’s do some chemistry: First, a practical culinary demo … Needing a volunteer …. To be blindfolded….
The culinary chemistry: X X (H3C)NH2 (pKb 3.35) methylamine (H3C)NH2 (pK3 3) citric acid
Adding lemon juice to fish is an Acid-Base Neutralization Reaction methylamine citric acid methylammonium citrate X X
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