1. Acid-Base Properties of Salts

2. Acid-Base Properties of Salt Solutions
Because ions can exhibit acid or base properties
(i.e., donate or accept protons), salt solutions
can be acidic or basic.
-- hydrolysis:
occurs when ions “react” w/H2O;
the H2O is split into H+ and OH–
Hydrolysis occurs when the acetate ion
in sodium acetate reacts with water to
produce acetic acid and hydroxide ion.
OAc– + H2O HOAc + OH–

3. Anions of weak acids react with water
to form OH–, and are thus basic.
Example: Consider the weak acid
HX(aq) H+(aq) X–(aq)
By the very definition of a weak acid,
X– does NOT like to walk alone down
the Boulevard of Broken Dreams.
Now, say we have X–(aq) in solution from a salt.
We’ll get...
X– + H2O HX + OH–
i.e.,
X– accepts protons, acting like a base

4. Ka Kb Anions of strong acids do NOT influence pH. e.g.,
NO3– (from, say, KNO3) wants NOTHING
to do with H+.
Anions that still have ionizable protons (e.g.,HSO3–)
are amphoteric. --
For such substances, the larger of Ka or
Kb determines the pH.
Equation for Ka:
HSO3– H+ + SO32–
Equation for Kb:
HSO3– + H2O H2SO3 +OH– Ka Kb
ACID Ka Kb
BASE
For
“amphoterics…”

5. Does Na2HPO4 form an acidic
or a basic solution in water?
Real question is: “How does HPO42– behave?”
Like an acid...
HPO42– H+ + PO43–
...or a base?
HPO42– + H2O H2PO4– + OH–
From a reference table, we find...
(a) H3PO H+ + H2PO4–
Ka1 = 7.5 x 10–3
(b) H2PO4– H+ + HPO42–
Ka2 = 6.2 x 10–8
(c) HPO42– H+ + PO43–
Ka3 = 4.2 x 10–13

6. BIGGER. Kb2 Ka1 = 7.5 x 10–3 (a) H3PO4 H+ + H2PO4– Ka2 = 6.2 x 10–8
(b) H2PO4– H+ + HPO42–
Ka3 = 4.2 x 10–13
(c) HPO42– H+ + PO43–
Compare (c) going forward with (b) going in reverse.
Ka3
“conjugatized” Kb2
Kb2
Ka3
4.2 x 10–13
1.6 x 10–7
BIGGER.
smaller.
Na2HPO4 acts as a base.

7. Acid-Base Properties of Salts
Type of Salt
Examples
Comment
pH of solution
Cation is from a strong base, anion from a strong acid
KCl, KNO3
NaCl
NaNO3
Both ions are neutral
Neutral
These salts simply dissociate in water:
KCl(s)  K+(aq) + Cl-(aq)

8. Acid-Base Properties of Salts
Type of Salt
Examples
Comment
pH of solution
Cation is from a strong base, anion from a weak acid
NaC2H3O2
KCN, NaF
Cation is neutral,
Anion is basic
Basic
The basic anion can accept a proton from water:
C2H3O H2O  HC2H3O2 + OH base acid acid base

9. Acid-Base Properties of Salts
Type of Salt
Examples
Comment
pH of solution
Cation is the conjugate acid of a weak base, anion is from a strong acid
NH4Cl,
NH4NO3
Cation is acidic,
Anion is neutral
Acidic
The acidic cation can act as a proton donor:
NH4+(aq)  NH3(aq) + H+(aq)
Acid Conjugate Proton
base

10. Acid-Base Properties of Salts
Type of Salt
Examples
Comment
pH of solution
Cation is the conjugate acid of a weak base, anion is conjugate base of a weak acid
NH4C2H3O2
NH4CN
Cation is acidic,
Anion is basic
See below
IF Ka for the acidic ion is greater than Kb for the basic ion, the solution is acidic
IF Kb for the basic ion is greater than Ka for the acidic ion, the solution is basic
IF Kb for the basic ion is equal to Ka for the acidic ion, the solution is neutral

11. Acid-Base Properties of Salts
Type of Salt
Examples
Comment
pH of solution
Cation is a highly charged metal ion; Anion is from strong acid
Al(NO3)2
FeCl3
Hydrated cation acts as an acid;
Anion is neutral
Acidic
Step #1:
AlCl3(s) + 6H2O  Al(H2O)63+(aq) + Cl-(aq)
Salt water Complex ion anion
Step #2:
Al(H2O)63+(aq)  Al(OH)(H2O)52+(aq) + H+(aq)
Acid Conjugate base Proton