Unit 5: Ionic Equilibrium
Course Code: CHM-DSC-251
Paper Name: Physical Chemistry - II
Table of Contents
1. Electrolytes and Ionization
Ionic equilibrium involves the equilibrium between unionized molecules and ions in solution.
Classification of Electrolytes
- Strong Electrolytes: Substance that ionizes almost completely in aqueous solution (e.g., HCl, NaOH, NaCl).
- Weak Electrolytes: Substance that ionizes only to a small extent (e.g., CH3COOH, NH4OH).
Degree of Ionization (α)
The fraction of the total number of molecules of an electrolyte that ionizes in solution. It is affected by:
- Nature of solute and solvent.
- Concentration (α increases with dilution).
- Temperature (α increases with temperature).
- Common Ion Effect: Suppression of degree of ionization of a weak electrolyte by adding a strong electrolyte containing a common ion.
2. pH Scale and Ionic Product of Water
Ionic Product of Water (Kw)
Water is a very weak electrolyte that undergoes self-ionization.
Kw = [H+][OH-] = 1.0 x 10^-14 (at 298 K)
The pH Scale
A logarithmic scale used to specify the acidity or basicity of an aqueous solution.
pH = -log [H+]
At 298 K: pH + pOH = 14.
3. Salt Hydrolysis
Salt hydrolysis is the reaction of an anion or cation of a salt with water to produce acidity or alkalinity.
| Salt Type | Nature of Solution | Example |
|---|---|---|
| Strong Acid + Strong Base | Neutral (pH = 7) | NaCl, KNO3 |
| Weak Acid + Strong Base | Alkaline (pH > 7) | CH3COONa |
| Strong Acid + Weak Base | Acidic (pH < 7) | NH4Cl |
| Weak Acid + Weak Base | Depends on Ka/Kb | CH3COONH4 |
4. Buffer Solutions & Henderson Equation
A buffer solution resists changes in pH when small amounts of acid or base are added.
Types of Buffers
- Acidic Buffer: Mixture of a weak acid and its salt with a strong base (e.g., CH3COOH + CH3COONa).
- Basic Buffer: Mixture of a weak base and its salt with a strong acid (e.g., NH4OH + NH4Cl).
Henderson-Hasselbalch Equation
Used to calculate the pH of a buffer solution.
pH = pKa + log ([Salt] / [Acid])
Applications of Buffers
- Analytical Chemistry: To maintain pH for precipitation of cations.
- Biochemistry: Maintaining blood pH (approx. 7.4) in the human body.
5. Acid-Base Titrations and Indicators
Titration curves show the change in pH as a titrant is added.
Acid-Base Indicators
Weak organic acids or bases that change color within a specific pH range.
- Phenolphthalein: Colorless in acid, pink in base (Range: 8.3 - 10.0).
- Methyl Orange: Red in acid, yellow in base (Range: 3.1 - 4.4).
6. Exam Corner: FAQs & Tips
Exam Tips
- Formula Mastery: Be ready to calculate pH for strong/weak acids and buffer solutions using the Henderson equation.
- Common Ion Effect: This is a favorite theoretical question; explain it using Le Chatelier's Principle.
- Indicator Selection: Know which indicator to use for different types of titrations (e.g., Phenolphthalein for strong base titrations).
Frequently Asked Questions
Q: What is the buffer capacity?
A: It is a measure of the effectiveness of a buffer in resisting pH changes.
Q: Define Salt Hydrolysis.
A: It is the interaction of salt ions with water, leading to an excess of H+ or OH- ions.
Q: Why does NH4Cl give an acidic solution?
A: NH4+ ions undergo hydrolysis to produce H3O+ ions, making the solution acidic.