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???
AH + B A- + BH+acid conjugate
basebase 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.
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?
A:H A:- + H+
Highly electronegative atoms,Like:
H-Cl
H-Br HNO3
H2SO4
H-O-NO2
(H-O)2SO2
HClO4
H3C-CO2-HH4C2O2
What kinds of structures make A-H astrong 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 astrong 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 ….
AH + B A- + BH+acid conjugate
basebase 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
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:
(H3C)NH2 (pKb 3.35) methylamine
X X
(H3C)NH2 (pK3 3) citric acid
Adding lemon juice to fish is anAcid-Base Neutralization Reaction
methylamine
X X
citric acidmethylammonium citrate
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
Let’s do some problems !!
