Transcript Equilibria:Polyprotic weak acids

Equilibria:Polyprotic weak acids
For example, phosphoric acid, H3PO4
H3PO4  H2PO4-1 + H+1
KA1= 7.5*10-3
H2PO4-1  HPO4-2 + H+1
KA2= 6.2*10-8
HPO4-2  PO4-3 + H+1
KA3= 4.8*10-13
H+1T = H+11 + H+12 + H+13 ~ H+11
Equilibria: Salts
Illustrated using a simple =1/-1 salt
There are 4 reactions to consider.
1.
2.
3.
4.
CatAn  Cat+1 + An-1
Cat+1 + H2O  CatOH + H+1 KA = KW/KB
An-1 + H2O  Han + OH-1 KB = KW/KA
H2O  H+1 + OH-1
KW
Equilibria: Salts
We may classify salts into 4 groups, depending on
the source of the cation and anion. SBSA (NaCl),
WBSA (NH4Cl), SBWA (NaF), WB WA (NH4F).
The pH of the final solution will be influenced by
one or more of the previous reactions.
Reactions influencing solution pH
Rxn #
SBSA
2
3
4
WBSA
SBWA
X
X
X
WBWA
X
X
X
X
X
Equilibria: Salts
WAWB salts are especially interesting.
The pH is influenced by both the cation
and anion hydrolysis reactions.
1. Cat+1 + H2O  CatOH + H+1 KA = KW/KB
2. An-1 + H2O  Han + OH-1 KB = KW/KA
The pH of the final solution will depend on
the size of KA & KB. KA > KB, acidic, etc.
Equilibria: Titrations
1.
2.
3.
3.
If one titrates an acetic acid solution with
a sodium hydroxide solution. There are
4 stages in the titration.
Only acetic acid, pH = -log(CAKA)/2
Buffer,
pH = pKA + log [A-]
[HA]
WASB salt,
pOH = -log(CAKB)/2
note, must find new “acid” concentration
Strong base
pH = 14 - pOH
Equilibria: Solubility &
Solubility Products
If a salt is “insoluble” or sparingly soluble,
one may write an equilibrium expression for
the reaction with water.
MXAY(S)  XM+Y(aq) + YA-X(aq)
KSP = [M+Y]X[A-X]Y
e.g.
CaF2(S)  Ca+2(aq) + 2F-1(aq) KSP = [Ca+2]1[F-1]2
KSP = 4.2*10-11
Equilibria: Solubility &
Solubility Products
We prepare a solution which is 1.0*10-4M in
copper(I) and 2.0*10-3M in lead(II). To this
solution we slowly add sodium iodide. Which
will precipitate first, CuI or PbI2? The KSP’s
are respectively, 5.3*10-12 and 1.4*10-8.