solven levelling.ppt

acid and Base

1. 9/3/2023 1
Acids and Bases

2. 9/3/2023 2
B-L definitions
For strong acids,
pK
a< 0
pK
a(HCl) ≈ -7
Bronsted-Lowry definitions:
Acid = proton donor; Base = proton acceptor
HF (aq) + H
2O H
3O
+
(aq)+ F
-
(aq)
BL acid BL base
Fluoride ion is the conjugate base of HF
Hydronium ion is the conjugate acid of H
2O
K
a, the acidity constant, measures acid
strength as:
K
a= [H
3O
+
] [A
-
] / [HA]
pK
a= -log K
a
When pH = pK
a, [HA] = [A
-
]

3. 9/3/2023 3
K
w
K
w= water autopyrolysis constant
2 H
2O = H
3O
+
(aq)+ OH
-
(aq)
K
w = [H
3O
+
] [OH
-
] = 1 x 10
-14
(at 25°C)
Using the above, you should prove that for any conjugate
acid-base pair:
pK
a+ pK
b= pK
w = 14

4. 9/3/2023 4
Polyprotic acids
H
3PO
4+ H
2O = H
2PO
4
-
+ H
3O
+
pK
a1= 2.1
H
2PO
4
-
+ H
2O = HPO
4
2-
+ H
3O
+
pK
a1= 7.4
HPO
4
2-
+ H
2O = PO
4
3-
+ H
3O
+
pK
a1= 12.7
Since pK
avalues
are generally well-
separated, only 1 or
2 species will be
present at
significant
concentration at
any pH

5. 9/3/2023 5
Solvent levelling
Strongest acid possible in aqueous soln is H
3O
+
Ex: HCl + H
2O H
3O
+
(aq) + Cl
-
(aq)
no equilibrium, this reaction goes quantitatively
HCl does not exist in aqu soln
Ex: KNH
2+ H
2O K
+
(aq) + OH
-
(aq) + NH
3(aq)
we call this a solvent levelling effect, the strongest
acidic and basic species that can exist are the BL acid-
base pair of the solvent itself.
NH
2
-
= imide anion
NR
2
-
, some
substituted imide ions
are less basic and can
exist in aq soln

6. 9/3/2023 6
Solvent levelling
Only species with 0 <
pK
a< 14 can exist in
aq soln
The water pH stability
window = pK
w= 14
Other solvents have
different windows and
different levelling
effects

7. 9/3/2023 7
pK
avs z
2
/ (r
++ d)

8. 9/3/2023 8
Oxide acidity/basicity trends

9. 9/3/2023 9
pK
a‘s for oxoacids

10. 9/3/2023 10
pH effect on solubility
Al
2O
3
[Al(OH
2)
6]
3+
[Al(OH)
4]
-
[AlO
4(Al(OH)
2)
12]
7+
pH ≈ 4

11. 9/3/2023 11
log K vs ligand type