Hydrolysis of acid and base

Hydrolysis of Acidic, Basic, and Neutral Salts

A salt is formed between the reaction of an acid and a base. Usually, a neutral salt
is formed when a strong acid and a strong base is neutralized in the reaction:

Example: NaCl + H2O
1. NaOH + HCl KNO3 + H2O
2. KOH + HNO3 Mg(ClO4)2 + H2O
3. Mg(OH)2 + HClO4
Ions of Neutral Salts K+ Rb+ Cs+
Na+ Cations Ca2+ Sr2+ Ba2+
Mg2+
Anions Br- I-, NO3-
Cl- BrO4- ClO3-
ClO4-

 The bystander ions in an acid-base reaction form a salt solution. Most
neutral salts consist of cations and anions listed in the above table. These
ions have little tendency to react with water. Thus, salts consisting of these
ions are neutral salts. For example: NaCl, KNO3, CaBr2, CsClO4 are neutral
salts.

NH4+ Al3+ Acidic Ions Sn2+
Transition metal ions H2PO4- Pb2+
HSO4- HCO3-
SO42-
Basic Ions

F- C2H3O2- NO2-
CN- CO32- S2-
HPO42- PO43-

 When weak acids and bases react, the relative strength of the conjugated
acid-base pair in the salt determines the pH of its solutions. The salt, or its
solution, so formed can be acidic, neutral or basic.

 A salt formed between a strong acid and a weak base is an acid salt, for
example NH4Cl.

 A salt formed between a weak acid and a strong base is a basic salt, for
example NaCH3COO. These salts are acidic or basic due to their acidic or
basic ions as shown in the tables here

Hydrolysis of Acidic Salts
A salt formed between a strong acid and a weak base is an acid salt.

Ammonia is a weak base, and its salt with any strong acid gives a solution with a
pH < 7.

For example, let us consider the reaction:

HCl + NH4OH = NH4+ + Cl- + H2O

In the solution, the NH4+ ion reacts with water (called hydrolysis) according to the
equation:

NH4+ + H2O = NH3 + H3O+.
The acidity constant can be derived from Kw and Kb for the above reaction.

[H3O+] [NH3] [OH-]
Ka = ---------------- ------

[NH4+] [OH-]
= Kw / Kb
Ka – acid dissociation constant

Kw – water product

Kb – Base dissociation constant

Hydrolysis and Basic Salts
A basic salt is formed between a weak acid and a strong base. The basicity

is due to the hydrolysis of the conjugate base of the (weak) acid used in the
neutralization reaction. For example, sodium acetate formed between the weak
acetic acid and the strong base NaOH is a basic salt. When the salt is dissolved,
ionization takes place:

NaOAc = Na+ + AcO-

In the presence of water, AcO- undergo hydrolysis:
H2O + AcO- = HOAc + OH-

And the equilibrium constant for this reaction is Kb of the conjugate base AcO- of

the acid HOAc. Note the following equilibrium constants:

[HOAc] [OH-] Note
Acetic acid Ka=1.75e-5
Kb = ----------- Ammonia Kb=1.75e-5
[AcO-]

[HOAc] [OH-] [H+]

Kb = ----------- ---

[AcO-] [H+]

[HOAc] [OH-][H+]

Kb = ---------- ---------

[AcO-] [H+]

= Kw / Ka

Thus,
Ka . Kb = Kw

or
pKa + pKb = 14

for a conjugate acid-base pair. Let us look at a numerical problem of this type

Salts of weak acids and weak bases
A salt formed between a weak acid and a weak base can be neutral, acidic, or
basic depending on the relative strengths of the acid and base.

If Ka(cation) > Kb(anion) the solution of the salt is acidic.

If Ka(cation) = Kb(anion) the solution of the salt is neutral.

If Ka(cation) < Kb(anion) the solution of the salt is basic.