Hydrolysis of Salts
How salts influence solution pH through interactions with water
What is Hydrolysis?
Salt hydrolysis refers to the reaction where ions from a dissolved salt interact with water molecules, causing a shift in pH. These reactions either release H⁺ ions, making the solution acidic, or OH⁻ ions, making it basic.
This phenomenon plays a critical role in environmental systems, biological processes, industrial reactions, and pharmaceutical formulations.
Why Does Salt Hydrolysis Happen?
When salts dissolve in water, they dissociate into cations and anions. If either ion originates from a weak acid or a weak base, it will react with water in an attempt to reach equilibrium, leading to hydrolysis. This reaction modifies the pH of the solution.
For example:
- NH₄⁺ (from weak base NH₃) reacts with water to release H⁺ → acidic solution
- CH₃COO⁻ (from weak acid CH₃COOH) reacts with water to release OH⁻ → basic solution
Types of Salt Hydrolysis
- Strong Acid + Strong Base (e.g., NaCl): No hydrolysis → Neutral pH (pH = 7)
- Strong Acid + Weak Base (e.g., NH₄Cl): Cation hydrolysis → Acidic solution (pH < 7)
- Weak Acid + Strong Base (e.g., CH₃COONa): Anion hydrolysis → Basic solution (pH > 7)
- Weak Acid + Weak Base (e.g., NH₄CH₃COO): Both ions hydrolyze → pH depends on Ka and Kb values
Hydrolysis Equilibrium Constant (Kh)
Let’s take the example of CH₃COO⁻ hydrolysis:
CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
Kh = Kw / Ka
- Kw: Ionic product of water = 1 × 10⁻¹⁴ at 25°C
- Ka: Acid dissociation constant of the conjugate acid
This formula helps us calculate the extent of hydrolysis for salts derived from weak acids or bases.
Equilibrium Constant for Hydrolysis
The hydrolysis reaction can be represented as:
CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻ KH = Kw / Ka
Where:
- Kw: Ionic product of water (1 × 10⁻¹⁴)
- Ka: Acid dissociation constant
Importance and Applications
| Application | Use |
|---|---|
| Buffer Systems | Helps maintain pH stability in solutions and blood |
| Soil Chemistry | Affects nutrient availability and plant health |
| Water Treatment | Controls pH for corrosion prevention |
| Pharmaceutical Industry | Maintains drug stability and solubility |
Understanding hydrolysis of salts is crucial for mastering chemical equilibrium and pH control in real-world systems.
Real-Life Applications
- Buffer Systems: Maintain constant pH in biological systems
- Soil Chemistry: Affects nutrient availability
- Water Treatment: Controls corrosion and scaling
हिंदी में भी समझें
लवण का हाइड्रोलिसिस एक रासायनिक प्रतिक्रिया है जिसमें पानी में घुले लवण के आयन जल के साथ प्रतिक्रिया करते हैं और घोल का pH बदल देते हैं। यह प्रक्रिया घोल को अम्लीय, क्षारीय या तटस्थ बना सकती है।
उदाहरण:
- NaCl – तटस्थ
- NH₄Cl – अम्लीय
- CH₃COONa – क्षारीय
MCQs on Hydrolysis of Salts
- Which salt gives an acidic solution on hydrolysis?
A. NaCl
B. NH₄Cl ✅
C. CH₃COONa
D. KNO₃
Explanation: NH₄⁺ comes from a weak base and hydrolyzes to produce H⁺ ions. - Which salt does not undergo hydrolysis?
A. NH₄Cl
B. CH₃COONa
C. NaCl ✅
D. NH₄NO₃
Explanation: NaCl is formed from a strong acid and strong base — no hydrolysis.
True or False
- Salts formed from strong acid and strong base undergo hydrolysis. — ❌ False
- Hydrolysis can affect soil fertility. — ✅ True
- All basic salts increase the pH of water. — ✅ True
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