Previously in Chem104:

• Ka, Kb Kw

• p-functions (pH, pKa, pKw)

• using the Kw / pKw circle

• structural aspects of acids & bases

• Neutralization reactions

• Calc’g pH and pOH

Note: Lewis Acids/Bases next week with solubility

Today in Chem104:

• more acid/base reactions:

• weak / weak• strong / strong• strong / weak

• calculations

• Polyprotic acids

Weak Acid /Weak Base Reactions

methylamine

X X

citric acidmethylammonium citrate

First type: you start on one side, with one acid and one base. What does the reaction HAVE TO do?

The reaction HAS TO go forward to make some product until it reaches equilibrium.How much product it makes depends on the Ka and Kb.

methylamine citric acidmethylammonium citrate

First type: you start on one side, with one acid and one base. What does the reaction HAVE TO do?

Calculation of amounts is very difficult unless you have information on most of the species. You won’t have problems like that.

You will be asked about which direction the equilibrium favors in a weak acid / weak base problem.

Weak Acid /Weak Base Reactions

(pKb 3.35)Kb = 4.47 x 10-4

(pKa 3) Ka = 10-3

Methylamine pKb 3.35

citric acid pKa 3.0 methylammonium citrate

Second type: you start with all four species, both acid/base pairs. Which side does the equilibrium favor?

The equilibrium favors the side with weakest acid and base pair. Or, the stronger acid and base push the reaction to the side with weaker acid /base.

Weak Acid /Weak Base Reactions

Example weak-weak problems

Equimolar solutions of acetic acid, acetate, cyanide and hydrocyanic acid are mixed. How will the equilibrium shift?

Polyprotic acids…are acids that can donate more than one proton!

Phosphoric acid, H3PO4 … does having 3 H’s automatically mean it acts as a polyprotic acid? Consider H4C2O2

Step 1. H3PO4 + H2O

H2PO4 - + H3O+

Ka1 = 7.6 x 10-3

Step 2. H2PO4- +

H2OHPO4

2- + H3O+

Ka2 = 6.2 x 10-8

Step 3. HPO42-

+ H2O

PO4 3- + H3O+

Ka3 = 2.12 x 10-13

Net. H3PO4 + 3H2O PO4 3- + 3H3O+

Keq = Ka1Ka2Ka3