What Is Hydrolysis Of Salts Explanation Types And Ka Kb Formula
Salt Hydrolysis definition chemistry is Neutralization reaction between an acid and a base results in the formation of salt. Most salts ionize in water to form acids and bases. These salts dissociate into their respective ions, which exist as hydrated ions in aqueous solutions or can interact with the hydrogen or hydroxyl ion of the solvent to form the resulting bases and acids. The mechanism of interaction between the ions of the salts with that of water is called hydrolysis of salts. As per the salt hydrolysis definition and the extent of hydrolysis, salts can be categorized as:
Basic salt
Acidic Salt
Neutral or amphoteric salts.
The formation of these salts depends on the type of salt hydrolysis. They are:
Salts of a Strong Base and a Strong acid
Salts that are produced by the reaction between a strong base and a strong acid are neutral in nature. The bonds formed between the anion and the cation are strong, and they do not break apart in solution. The electronic distribution of both the cation and the anion after the formation of the bond between them is such that both the ions complete their most stable electronic configuration. These salts are formed by ionic or electrostatic bonds, which cannot be broken in solution. These salts do not get hydrolyzed, but they can get hydrated. Since these salts do not carry any charge, they are called neutral or amphoteric salts. Sodium chloride (NaCl) is a classic example of neutral salt.
Salts of a Weak Acid and a Strong Base
Salts that are formed by the neutralization of weak acid and strong base have a positive charge. These salts are called a basic salt. These salts hydrolyze easily in water. For example, Sodium acetate (CH3COONa). The salt hydrolysis formula of Sodium acetate is as follows:
CH3COONa (aq) → CH3COO- (aq) + Na+ (aq)
The acetate ion, thus formed, interacts with the hydrogen ion in water to form acetic acid and hydroxyl ion.
CH3COO- (aq) + HOH → CH3COOH+ OH- (aq)
Since acetic acid is a weak acid, it does not get ionized in water. However, the increase in the OH- ion results in the increased basicity of the solution. Therefore these salts are called basic salts. The pH of the aqueous solution of these salts is always more than 7.
Salts of a Weak Base and a Strong Acid
Salts formed from neutralization reaction between weak bases and strong acids are found to be acidic in nature. Ammonium chloride is an example of acidic salt.
NH4Cl(aq) → Cl−(aq) + NH4+(aq)
Ammonium ions thus formed then interacts with the hydroxyl ions in the water molecule to form ammonium hydroxide. The hydrogen ions thus formed results in the acidic nature of the solution. Therefore an aqueous solution of these salts is acidic and has a pH of less than 7.
NH+4(aq) + H2O ⇋ NH4OH(aq) + H+(aq)
Salts of Weak Base and Weak Acid
Salts resulting from the neutralization reaction between a weak acid and weak base can be mildly basic, mildly acidic, or neutral depending on the nature of the bases and acids involved. The degree of hydrolysis and ionization of water is independent of the concentration of the solution. An example of such salts is CH3COONH4. The mechanism of formation of such ions are as follows:
CH3COO− + NH+4 + H2O ⇋ CH3COOH + NH4OH
The pH of the aqueous solution of such salts are denoted as
pH= 7 + 12 (pKa – pKb)
That is, the pH depends on the pKa and pKb of the acid and the base, respectively.
FAQs on Hydrolysis Of Salts In Chemistry
1. What is hydrolysis of salts?
The hydrolysis of salts is the reaction of the ions of a salt with water to produce an acidic or basic solution. When a salt dissolves in water, its cation and anion may react with H2O to form H3O+ or OH- ions, changing the pH.
- Salts of strong acid + strong base → no hydrolysis (neutral solution).
- Salts of strong acid + weak base → acidic solution.
- Salts of weak acid + strong base → basic solution.
- Salts of weak acid + weak base → depends on relative values of Ka and Kb.
2. Why do some salts undergo hydrolysis while others do not?
Salts undergo hydrolysis when at least one of their ions is the conjugate of a weak acid or weak base. Ions derived from strong acids (like Cl-) or strong bases (like Na+) do not react with water.
- NaCl: no hydrolysis because NaOH and HCl are strong.
- NH4Cl: hydrolysis occurs because NH4+ is from weak base NH3.
- CH3COONa: hydrolysis occurs because CH3COO- is from weak acid CH3COOH.
3. What are the types of hydrolysis of salts?
The main types of hydrolysis of salts are cationic, anionic, and both-ion hydrolysis. These are classified based on which ion reacts with water.
- Cationic hydrolysis: Salt of strong acid and weak base (e.g., NH4Cl).
- Anionic hydrolysis: Salt of weak acid and strong base (e.g., CH3COONa).
- Both-ion hydrolysis: Salt of weak acid and weak base (e.g., NH4CH3COO).
4. How do you calculate the pH of a salt solution undergoing hydrolysis?
The pH of a salt solution is calculated using equilibrium constants such as Ka, Kb, and Kw depending on the type of hydrolysis. The formula varies with the nature of the salt.
- For salt of weak acid + strong base:
pH = 7 + (1/2)(pKa + log C) - For salt of weak base + strong acid:
pH = 7 − (1/2)(pKb + log C) - For salt of weak acid + weak base:
pH = 7 + (1/2)(pKa − pKb)
5. Can you give an example of hydrolysis of ammonium chloride?
Yes, NH4Cl undergoes cationic hydrolysis because NH4+ reacts with water to form an acidic solution. The hydrolysis reaction is:
NH4+(aq) + H2O(l) ⇌ NH3(aq) + H3O+(aq)
- Cl- does not hydrolyze (from strong acid HCl).
- Formation of H3O+ makes the solution acidic.
- pH of the solution is less than 7.
6. What is the hydrolysis constant (Kh) of a salt?
The hydrolysis constant (Kh) is the equilibrium constant for the hydrolysis reaction of a salt in water. It relates to Ka, Kb, and Kw.
- For salt of weak acid + strong base:
Kh = Kw / Ka - For salt of weak base + strong acid:
Kh = Kw / Kb - For salt of weak acid + weak base:
Kh = Kw / (Ka × Kb)
7. What is the degree of hydrolysis of a salt?
The degree of hydrolysis (h) is the fraction of the total salt that reacts with water during hydrolysis. It measures how much of the salt is hydrolyzed.
- For salt of weak acid + strong base:
h = √(Kh / C) - For salt of weak acid + weak base:
h = √Kh
8. What is the difference between hydrolysis and hydration?
The key difference is that hydrolysis involves a chemical reaction with water, while hydration involves only physical attachment of water molecules.
- Hydrolysis: Ion reacts with water to form new species (e.g., NH4+ forming NH3).
- Hydration: Water molecules surround ions without chemical change (e.g., Na+(aq)).
9. Why is sodium acetate solution basic?
A solution of CH3COONa is basic because the acetate ion CH3COO- undergoes anionic hydrolysis to produce OH- ions. The reaction is:
CH3COO-(aq) + H2O(l) ⇌ CH3COOH(aq) + OH-(aq)
- Na+ does not hydrolyze.
- Formation of OH- increases pH above 7.
- This is a salt of weak acid and strong base.
10. How does dilution affect the hydrolysis of salts?
Dilution increases the degree of hydrolysis for salts of weak acids or weak bases. As concentration decreases, equilibrium shifts to favor hydrolysis.
- For weak acid + strong base salt: h = √(Kh / C), so h increases when C decreases.
- pH moves further from 7 upon dilution in such cases.
- For weak acid + weak base salt, degree of hydrolysis is independent of concentration.
