Hydrolysis
A salt is produced as the product of a neutralization reaction (DD) between
an acid and a base.
HCl + NaOH  NaCl + HOH (H2O)
NaCl is the product of a strong acid and a strong base.
The reverse reaction is possible especially since weak acids or bases involve
reversible equilibrium equations.
CH3CO2H (aq) + H2O (l)
H3O+ (aq) + CH3CO2- (aq)
When a salt is mixed with water, it dissociates. Hydrolysis describes the
reaction between the salt’s cation + or anion- or both with water.
The resulting solution can be neutral, acidic, or basic, which of course can
affect the pH of the solution.
This resulting solution’s pH is dependent on the strength of the acid and base
creating the salt.
DO YOU KNOW YOUR STRONG ACIDS AND BASES?
Salts are divided into 4 types based on classifying the salts from their
production from an acid and base:
1.
2.
3.
4.
strong acid and strong base
strong acid and weak base
weak acid and strong base
weak acid and weak base
Salts that Produce Neutral Solutions:
If the salt can be formed from a strong acid and a strong base, the salt
formed will form a neutral solution when added to water.
Anion (strong acid) and the cation (strong base) will not hydrolyze.
KCl:
H2O (l)
KCl 
K+ + CL-
K+ + H2O  KOH (aq) + H+ (aq)
Cl- + H2O  HCl (aq) + OH- (aq)
Also can be written or expressed as:
KCl + H2O  HCl + KOH
H+ Cl-
K+ OH-
Both strong acid and base so 100 % dissociation so [H+] = [OH-]
Neither ion is in excess so the solution remains neutral with a pH of 7.
Salts That Produce Basic Solutions:
H2O (l)
NaNO2

Na+ (aq) + NO2- (aq)
Na + + H2O  NaOH (aq) + H+ (aq)
strong
base
NO2- (aq) + H2O (l)
HNO2 (aq) + OH- (aq)
weak
excess OH- produced
acid
Solution is basic pH > 7.0
SALTS FORMED FROM WEAK ACIDS AND STROND BASES
PRODUCE A BASIC SOLUTION.
Salts That Produce Acidic Solutions:
H2O (l)
NH4Br (s)  NH4+ (aq) + Br- (aq)
NH4+ (aq) + H2O (l)
NH3 (aq) + H3O+ (aq)
weak
excess H3O+
base
Br- (aq) + H2O (l)  HBr (aq) + OH-
(aq)
strong
acid
Solution is acidic pH < 7.0
SALTS FORMED FROM STRONG ACIDS AND WEAK BASES
PRODUCE AN ACIDIC SOLUTION.
Salts in Which Both the Cation and Anion Hydrolyze
H2O (l)
NH4+ (aq) + F- (aq)
NH4F (s) 
NH4+ (aq) + H2O (l)
NH3 (aq) + H3O+ (aq)
weak
base
F- (aq) + H2O (l)
HF (aq) + OH- (aq)
weak
acid
Since both of these reactions favor the products, we need to check the Ka and
Kb to see which is stronger.
Ka HF = 6.6 x 10 -4
Kb NH3 = 1.7 x 10 -5
Kb
<
Ka
1.7 x 10 -5 < 6.6 x 10 -4
weaker base
weak acid therefore more H3O+ will be produced than OH-
so the solution will be acidic pH < 7.
Salts formed from a weak acid and a weak base produce a solution that is:
Acidic
Ka > Kb
Basic
Kb > Ka
Almost Neutral
Ka ≈ Kb