SALT HYDROLYSIS
There are three properties of salt solution, that are acidic, basic, and neutral. The properties of salt solution can explained by hydrolysis concept. Hydrolysis is general technical term used to reaction of substance and water. Based on this concept, component of salt solution (cation or anion) from weak acid or weak base react with water (hydrolyzed) form H3O+ ion (H+) if the cation hydrolyzed or OH- ion if the anion hydrolyzed.
Concept of salt hydrolysis include acid base reaction of Bronsted-Lowry. So, component of salt solution from weak acid or weak base will be strong conjugate base or strong conjugate acid and can react with water. While component of salt solution from strong acid or strong base will be weak conjugate base or weak conjugate acid and can’t react with water. Based on that explanation, salt hydrolysis happened if one of that component is weak acid or weak base.
There are three type from four type of salt solution can hydrolyzed :
Salt solution from weak acid and strong base
Salt solution from weak base and strong acid
Salt solution from weak acid and weak base
But, will be explained all type of salt solution and related with hydrolysis concept.
Salt solution from strong acid and strong base
Sodium chloride (NaCl) consist of Na+ ion from strong base and Cl- ion from strong acid. So, it behaves as a weak acid and weak base and can’t react with water. Because of can’t react with water, NaCl haven’t change the ratio of H+ ion and OH- ion concentration in water (the properties of NaCl is neutral).
2. Salt solution from weak acid and strong base
Sodium acetate (NaCH3COO) consist of Na+ ion from strong base and CH3COO- ion from weak acid. CH3COO- ion behaves as a strong base and can react with water. Because the anion of that compound hydrolyzed and produce OH- ion so the properties of that salt solution is basic. NaCH3COO have partial hydrolysis because just CH3COO- ion can react with water.
CH3COO-(aq)+ H2O(l) CH3COOH(aq) + OH-(aq)
3. Salt solution from strong acid and weak base
Ammonium chloride (NH4Cl) consist of NH4+ ion from weak base and Cl- ion from strong acid. NH4+ ion behaves as a strong acid and can react with water. Because the cation of that compound hydrolyzed and produce H3O+ ion or H+ ion so the properties of that salt solution is acid. NH4Cl have partial hydrolysis because just NH4+ ion can react with water.
NH4+(aq)+ H2O(l) NH3(aq) + H3O+(aq)
4. Salt solution from weak acid and weak base
Ammonium acetate (NH4CH3COO) consist of NH4+ ion from weak base and CH3COO- ion from weak acid. So, it behaves as a strong acid and strong base.
This salt solution will be complete hydrolyzed because of NH4+ ion or CH3COO- ion can react with water.
CH3COO-(aq)+ H2O(l) CH3COOH(aq) + OH-(aq)
NH4+(aq)+ H2O(l) NH3(aq) + H+(aq)
Each of hydrolysis reaction produce H+ ion and OH- ion. pH of the solution depend on the ratio of H+ ion concentration and OH- ion concentration or depend on ionization constant of weak acid lemah (Ka) and weak base (Kb).
Ka > Kb so [H+] > [OH-] and the properties is acidic
Ka < Kb so [H+] < [OH-] and the properties is basic
Ka = Kb so [H+] = [OH-] and the properties is neutral
pH of salt solution
Concept of salt hydrolysis include acid base reaction of Bronsted-Lowry. So, component of salt solution from weak acid or weak base will be strong conjugate base or strong conjugate acid and can react with water. While component of salt solution from strong acid or strong base will be weak conjugate base or weak conjugate acid and can’t react with water. Based on that explanation, salt hydrolysis happened if one of that component is weak acid or weak base.
There are three type from four type of salt solution can hydrolyzed :
Salt solution from weak acid and strong base
Salt solution from weak base and strong acid
Salt solution from weak acid and weak base
But, will be explained all type of salt solution and related with hydrolysis concept.
Salt solution from strong acid and strong base
Sodium chloride (NaCl) consist of Na+ ion from strong base and Cl- ion from strong acid. So, it behaves as a weak acid and weak base and can’t react with water. Because of can’t react with water, NaCl haven’t change the ratio of H+ ion and OH- ion concentration in water (the properties of NaCl is neutral).
2. Salt solution from weak acid and strong base
Sodium acetate (NaCH3COO) consist of Na+ ion from strong base and CH3COO- ion from weak acid. CH3COO- ion behaves as a strong base and can react with water. Because the anion of that compound hydrolyzed and produce OH- ion so the properties of that salt solution is basic. NaCH3COO have partial hydrolysis because just CH3COO- ion can react with water.
CH3COO-(aq)+ H2O(l) CH3COOH(aq) + OH-(aq)
3. Salt solution from strong acid and weak base
Ammonium chloride (NH4Cl) consist of NH4+ ion from weak base and Cl- ion from strong acid. NH4+ ion behaves as a strong acid and can react with water. Because the cation of that compound hydrolyzed and produce H3O+ ion or H+ ion so the properties of that salt solution is acid. NH4Cl have partial hydrolysis because just NH4+ ion can react with water.
NH4+(aq)+ H2O(l) NH3(aq) + H3O+(aq)
4. Salt solution from weak acid and weak base
Ammonium acetate (NH4CH3COO) consist of NH4+ ion from weak base and CH3COO- ion from weak acid. So, it behaves as a strong acid and strong base.
This salt solution will be complete hydrolyzed because of NH4+ ion or CH3COO- ion can react with water.
CH3COO-(aq)+ H2O(l) CH3COOH(aq) + OH-(aq)
NH4+(aq)+ H2O(l) NH3(aq) + H+(aq)
Each of hydrolysis reaction produce H+ ion and OH- ion. pH of the solution depend on the ratio of H+ ion concentration and OH- ion concentration or depend on ionization constant of weak acid lemah (Ka) and weak base (Kb).
Ka > Kb so [H+] > [OH-] and the properties is acidic
Ka < Kb so [H+] < [OH-] and the properties is basic
Ka = Kb so [H+] = [OH-] and the properties is neutral
pH of salt solution
Hydrolysis reaction includes equilibrium reaction. This reaction produces OH- ion and H+ ion. [OH-] and [H+] in equilibrium of hydrolysis reaction will be determine the properties of solution.
The equilibrium of salt solution hydrolysis showed quantitatively by hydrolysis constant (Kh). Value of Kh relevance with Ka of weak acid or Kb of weak base. The hydrolysis constant used to determine the pH of salt solution.
1. Salt solution from strong acid and strong base
This salt solution can’t occur hydrolysis and haven’t change the ratio of H+ ion and OH- ion concentration in water. So, the properties of the salt solution is neutral (pH=7).
2. Salt solution from strong base and weak acid
Salt solution from strong base and weak acid occur partial hydrolysis (anion hydrolysis). The reaction equation is:
A-(aq)+ H2O(l) HA(aq) + OH-(aq)
So the hydrolysis constant is
Concentration of OH- ion same with concentration of HA. While, concentration of A- ion is M so and .
The relationship of Kh, Kw, and Ka.
Kw is water ionization constant = 1 x 10-14 mol2/L2. And Kw=[H+][OH-].
Ka get from ionized weak acid reaction:
HA H+ + A-
So
If Kw divided with Ka produced Kh so,
3. Salt solution from strong acid and weak base
Salt solution from strong acid and weak base occur partial hydrolysis because just the cation can hydrolyzed. So, the reaction equation is:
BH+(aq)+ H2O(l) B(aq) + H3O+(aq)
So the hydrolysis constant is:
If [B]=[H3O+]=[H+] and the concentration of BH+ ion is M, so:
so .
The relationship of n Kh, Kw, and Kb.
Kw is water ionization constant = 1 x 10-14 mol2/L2. And Kw=[H+][OH-].
Kb get from reaction of ionization weak base:
B + H2O BH+ + OH-
So
Jika Kw dibagi dengan Ka diperoleh Kh maka
4. Salt solution from weak acid and weak base
Salt solution from weak acid and weak base occur complete hydrolysis. pH of solution can determined by this formula:
The equilibrium of salt solution hydrolysis showed quantitatively by hydrolysis constant (Kh). Value of Kh relevance with Ka of weak acid or Kb of weak base. The hydrolysis constant used to determine the pH of salt solution.
1. Salt solution from strong acid and strong base
This salt solution can’t occur hydrolysis and haven’t change the ratio of H+ ion and OH- ion concentration in water. So, the properties of the salt solution is neutral (pH=7).
2. Salt solution from strong base and weak acid
Salt solution from strong base and weak acid occur partial hydrolysis (anion hydrolysis). The reaction equation is:
A-(aq)+ H2O(l) HA(aq) + OH-(aq)
So the hydrolysis constant is
Concentration of OH- ion same with concentration of HA. While, concentration of A- ion is M so and .
The relationship of Kh, Kw, and Ka.
Kw is water ionization constant = 1 x 10-14 mol2/L2. And Kw=[H+][OH-].
Ka get from ionized weak acid reaction:
HA H+ + A-
So
If Kw divided with Ka produced Kh so,
3. Salt solution from strong acid and weak base
Salt solution from strong acid and weak base occur partial hydrolysis because just the cation can hydrolyzed. So, the reaction equation is:
BH+(aq)+ H2O(l) B(aq) + H3O+(aq)
So the hydrolysis constant is:
If [B]=[H3O+]=[H+] and the concentration of BH+ ion is M, so:
so .
The relationship of n Kh, Kw, and Kb.
Kw is water ionization constant = 1 x 10-14 mol2/L2. And Kw=[H+][OH-].
Kb get from reaction of ionization weak base:
B + H2O BH+ + OH-
So
Jika Kw dibagi dengan Ka diperoleh Kh maka
4. Salt solution from weak acid and weak base
Salt solution from weak acid and weak base occur complete hydrolysis. pH of solution can determined by this formula:
