Chapter 9: Unit 11. Salts Hydrolysis

Salt Hydrolysis

Salt hydrolysis involves the dissociation of the salt in water forming an aqueous solution.

Hydrolysis of the salt in water should yield weak acid or weak base. Hydrolysis of the salt will not yield strong acid or strong base.

The hydrolysis of the salt determines if the salt is acidic, neutral or basic when dissolved in water.

Examples:

Determine if the salts below are acidic, neutral or basic. Explain your choice.

• a. CH3COOK
• b. NH4Cl
• c. NaCl
• d. NaNO2
• e. NH4CH3COO

CH3COOK (Potassium Acetate)

The first step is to dissolve the salt in water to produce the cation and the anion of the salt:

CH 3 COOK(s) + H 2 O(l) ⇔ CH 3 COO–(aq) + K + (aq) + H2O(l)

The second step is to hydrolyze the cation and the anion of the salt with H 2 O:

CH3COO–(aq) + H2O(l) ⇔ CH3COOH(aq) + OH–(aq)[Hydrolysis: CH 3 COOH is a weak acid].

K + (aq) + 2 H2O(l) ⇔ KOH(aq) + H3O + (aq) [No hydrolysis: KOH is strong base]

CH 3 COOH is a weak acid and hence hydrolysis does occur. The hydrolysis yields the weak acid CH 3 COOH and OH–. Thus CH 3 COOK is said to basic and it yields the basic OH–.

NH4Cl(Ammonium Chloride)

The first step is to dissolve the salt in water to produce the cation and the anion of the salt:

NH4Cl(s) + H 2 O(l) ⇔ NH4 + (aq) + Cl–(aq) + H2O(l)

The second step is to hydrolyze the cation and the anion of the salt with H 2 O:

NH4 + (aq) + 2H2O(l)  ⇔NH4OH(aq) + H3O + (aq)[Hydrolysis: NH 4 OHis a weak base].

Cl–(aq) + H 2 O(l) ⇔ HCl(aq) + OH–(aq) [No hydrolysis: HCl is strong acid]

NH 4 OH is a weak base and hence hydrolysis does occur. The hydrolysis yields weak base NH4OH and

H3O + . Thus NH 4 Cl is said to be acidic and it yields the acidic H3O + .

NaCl(Sodium Chloride)

The first step is to dissolve the salt in water to produce the cation and the anion of the salt:

NaCl(s) + H 2 O(l)  ⇔ Na + (aq) + Cl–(aq) + H 2 O(l)

The second step is to hydrolyze the cation and the anion of the salt with H 2 O:

Na + (aq) + 2 H2O(l)  ⇔ NaOH + H3O + (aq) [No hydrolysis: NaOH is strong base]

Cl–(aq) + H2O(l)  ⇔ HCl(aq) + OH–(aq) [No hydrolysis: HCl is strong acid]

Both Na + and Cl– cannot be hydrolyzed. Then what is the pH of NaCl?

The pH of the NaCl in water is actually the pH of water itself because water is the only compound in the solution of NaCl can dissociate and can hydrolyze (auto ionization of water). Thus the pH of NaCl solution is the pH of water dissociation which equals to 7.0. Thus NaCl is said to be neutral.

NaNO2(Sodium Nitrite)

The first step is to dissolve the salt in water to produce the cation and the anion of the salt:

NaNO2 (s) + H2O(l)  ⇔ Na + (aq) + NO 2–(aq) + H2O(l)

The second step is to hydrolyze the cation and the anion of the salt with H 2 O:

Na + (aq) + 2 H2O(l) ⇔ NaOH + H3 O + (aq) [No hydrolysis: NaOH is strong base]

NO 2–(aq) + H2O(l)  ⇔ HNO2 + OH–(aq) [Hydrolysis: HNO2is a weak acid].

HNO 2 is a weak acid and hence hydrolysis does occur. The hydrolysis yields weak acid HNO 2 and OH–. Thus NaNO 2is said to be basic and it yields the basic OH–.

NH4CH3COO

The first step is to dissolve the salt in water to produce the cation and the anion of the salt:

NH4CH 3 COO(s) + H 2 O(l) ⇔ NH 4 + (aq) + CH3COO–(aq) + H2O(l)

The second step is to hydrolyze the cation and the anion of the salt with H 2 O:

NH 4 + (aq) + 2 H 2 O(l)⇔NH 4 OH(aq) + H 3 O + (aq)[Hydrolysis: NH4OHis a weak base].

CH 3 COO–(aq) + H 2 O(l)⇔CH 3 COOH(aq) + OH–(aq)[Hydrolysis: CH3COOH is a weak acid].

Both NH 4 + and CH 3 COO– are hydrolyzed. NH 4 + hydrolysis yields the acidic H 3 O + , and CH 3 COO– yields the basic OH–. Then what is the pH of NH 4 CH 3 COO?

If both the cation and anion are hydrolyzed, then one has to inspect the K a and K b values of the produced weak acid and weak base respectively.

NH 4 OH(aq) + H 2 O(l) ⇔NH 4 + (aq) + OH–(aq) [K b= 1.8 x 10–5 ].

CH3COOH(aq) + H 2 O(l)  ⇔ CH3COO–(aq) + H3O + (aq) [K a= 1.8 x 10–5 ].

Comparing the values of K a with K b :

If K a > K b , then the salt hydrolyzed solution is acidic

If K a < K b , then the salt hydrolyzed solution is basic

If K a = K b , then the salt hydrolyzed solution neutral

NH4CH3COO is hydrolyzed and it does produce H3O + and OH– with Ka= Kb. Thus NH4CH3COO is said to be neutral.