Previously in Chem104: How K a relates to K b and pK a to pK b

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 base

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.

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 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 ….

AH + B A- + BH+

acid conjugate base

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

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 !!