Transcript Previously in Chem104: Today in Chem104: • K a, K b K w • p-functions (pH, pKa, pKw) • using the Kw / pKw circle •structural aspects.

Previously in Chem104:
Today in Chem104:
• K a, K b K w
• p-functions (pH, pKa,
pKw)
• using the Kw / pKw
circle
•structural aspects of
acids & bases
•Neutralization
reactions
•Calc’g pH and pOH
• more acid/base
reactions:
• weak / weak
• strong / strong
• strong / weak
Note: Lewis
Acids/Bases next week
with solubility
• calculations
• Polyprotic acids
Weak Acid /Weak Base Reactions
First type: you start on one side, with one acid and one base.
What does the reaction HAVE TO do?


methylamine
citric acid
methylammonium citrate
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.
XX
Weak Acid /Weak Base Reactions
First type: you start on one side, with one acid and one base.
What does the reaction HAVE TO do?


methylamine
(pKb 3.35)
Kb = 4.47 x 10-4
citric acid
methylammonium citrate
(pKa 3)
Ka = 10-3
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
Second type: you start with all four species, both acid/base pairs.
Which side does the equilibrium favor?


Methylamine
pKb 3.35
citric acid
pKa 3.0
methylammonium citrate
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.
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-
+ H2O
HPO4 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