Hydrolysis of Salts
Hydrolysis of Salts
Hydrolysis of salts is a chemical reaction involving the breakdown of a salt when dissolved in water, leading to the formation of an acidic or basic solution. This process is a part of ionic equilibrium and is essential for understanding the pH of solutions, buffer systems, and various chemical reactions.
Understanding Hydrolysis
When a salt dissolves in water, it dissociates into its constituent cations and anions. These ions can react with water (H₂O) to produce acidic or basic solutions. The extent to which this reaction occurs depends on the strength of the corresponding acids and bases from which the ions are derived.
Types of Salts and Their Hydrolysis
Salts can be categorized based on the nature of their parent acids and bases:
Salts of Strong Acid and Strong Base: These salts do not undergo hydrolysis because both the cation and the anion come from strong acids and bases, which do not react with water. Examples include NaCl, KNO₃, and Na₂SO₄.
Salts of Weak Acid and Strong Base: These salts undergo anion hydrolysis, producing a basic solution. Example: Sodium acetate (CH₃COONa).
Salts of Strong Acid and Weak Base: These salts undergo cation hydrolysis, resulting in an acidic solution. Example: Ammonium chloride (NH₄Cl).
Salts of Weak Acid and Weak Base: Both the cation and the anion can undergo hydrolysis, and the solution's pH depends on the relative strengths of the acid and base. Example: Ammonium acetate (CH₃COONH₄).
Hydrolysis Equations
For a salt derived from a weak acid (HA) and a strong base (MOH), the hydrolysis can be represented as:
$$ A^- + H_2O \rightleftharpoons HA + OH^- $$
For a salt derived from a strong acid (HX) and a weak base (BOH), the hydrolysis can be represented as:
$$ B^+ + H_2O \rightleftharpoons BOH + H^+ $$
Degree of Hydrolysis
The degree of hydrolysis (h) is the fraction of the total salt that undergoes hydrolysis. It is given by:
$$ h = \frac{\text{Concentration of hydrolyzed salt}}{\text{Initial concentration of salt}} $$
Hydrolysis Constant and pH Calculation
The hydrolysis constant (Kh) is related to the equilibrium constant of the hydrolysis reaction. For anions (A⁻) from a weak acid (HA), the hydrolysis constant is given by:
$$ K_h = \frac{K_w}{K_a} $$
Where (K_w) is the ion-product constant of water and (K_a) is the acid dissociation constant of the weak acid.
The pH of the solution can be calculated using the hydrolysis constant and the concentration of the salt.
Table: Comparison of Salt Hydrolysis
| Type of Salt | Parent Acid | Parent Base | Hydrolysis Reaction | Solution pH | Example |
|---|---|---|---|---|---|
| Strong Acid + Strong Base | Strong | Strong | No hydrolysis | Neutral (pH = 7) | NaCl |
| Weak Acid + Strong Base | Weak | Strong | Anion hydrolysis | Basic (pH > 7) | CH₃COONa |
| Strong Acid + Weak Base | Strong | Weak | Cation hydrolysis | Acidic (pH < 7) | NH₄Cl |
| Weak Acid + Weak Base | Weak | Weak | Both cation and anion hydrolysis | Depends on relative strengths | CH₃COONH₄ |
Examples of Hydrolysis
Hydrolysis of Sodium Acetate (CH₃COONa)
Sodium acetate is a salt of a weak acid (acetic acid, CH₃COOH) and a strong base (NaOH). Its hydrolysis can be represented as:
$$ CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^- $$
Since the solution becomes basic, the pH can be calculated using the hydrolysis constant and the concentration of CH₃COONa.
Hydrolysis of Ammonium Chloride (NH₄Cl)
Ammonium chloride is a salt of a strong acid (HCl) and a weak base (NH₄OH). Its hydrolysis can be represented as:
$$ NH_4^+ + H_2O \rightleftharpoons NH_4OH + H^+ $$
The solution becomes acidic, and the pH can be calculated accordingly.
Conclusion
Understanding the hydrolysis of salts is crucial for predicting the pH of solutions and for various applications in chemistry, such as buffer solutions and titrations. The degree of hydrolysis and the hydrolysis constant are key factors in determining the extent of hydrolysis and the resulting pH of the solution.