Principles Related to Practical Chemistry

This chapter covers the principles behind common laboratory experiments in JEE Chemistry.

Overview

graph TD
    A[Practical Chemistry] --> B[Qualitative Analysis]
    A --> C[Titrations]
    A --> D[Preparations]
    B --> B1[Cation Analysis]
    B --> B2[Anion Analysis]

Qualitative Salt Analysis

Preliminary Tests

1. Physical appearance: Color, smell
2. Dry heating: Evolution of gases, residue color
3. Flame test: Characteristic colors
4. Solubility: In water, acids

Cation Analysis (Groups)

Group	Reagent	Cations	Precipitate
0	NH₄⁺ test	NH₄⁺	NH₃ gas evolved
I	Dil. HCl	Pb²⁺	PbCl₂ (white)
II	H₂S in acidic	Cu²⁺, Pb²⁺	Sulfides
III	NH₄OH	Fe³⁺, Al³⁺	Hydroxides
IV	H₂S in basic	Zn²⁺, Ni²⁺	Sulfides
V	(NH₄)₂CO₃	Ba²⁺, Ca²⁺	Carbonates
VI	NaOH/NH₃	Mg²⁺	Hydroxide

Anion Analysis

Anion	Test	Observation
CO₃²⁻	Dil. HCl	CO₂ with effervescence
S²⁻	Dil. HCl	H₂S (rotten egg smell)
SO₄²⁻	BaCl₂	White ppt, insoluble in HCl
Cl⁻	AgNO₃	White ppt, soluble in NH₃
Br⁻	AgNO₃	Pale yellow ppt
I⁻	AgNO₃	Yellow ppt, insoluble in NH₃
NO₃⁻	Ring test	Brown ring

Titrations

Acid-Base Titrations

Indicators:

Indicator	pH Range	Acid → Base
Methyl orange	3.1-4.4	Red → Yellow
Phenolphthalein	8.3-10	Colorless → Pink

Choice:

• Strong acid + Strong base: Any indicator
• Strong acid + Weak base: Methyl orange
• Weak acid + Strong base: Phenolphthalein

Redox Titrations

KMnO₄ Titrations:

• Self-indicator (purple to colorless)
• In acidic medium: MnO₄⁻ → Mn²⁺

K₂Cr₂O₇ Titrations:

• Uses external indicator
• Orange to green

Calculations

$$n_1V_1 = n_2V_2$$

where n = normality = Molarity × valence factor

Inorganic Preparations

Mohr’s Salt [(NH₄)₂Fe(SO₄)₂·6H₂O]

• Mix FeSO₄ and (NH₄)₂SO₄ solutions
• Crystallize
• Double salt

Potash Alum [K₂SO₄·Al₂(SO₄)₃·24H₂O]

• Mix K₂SO₄ and Al₂(SO₄)₃ solutions
• Crystallize
• Double salt

Organic Preparations

Acetanilide

$$\text{Aniline} + \text{Acetic anhydride} \rightarrow \text{Acetanilide}$$

p-Nitroacetanilide

$$\text{Acetanilide} + \text{HNO}_3 \rightarrow \text{p-Nitroacetanilide}$$

Aniline Yellow (Dye)

$$\text{Aniline} \xrightarrow{\text{diazotize}} \text{Diazonium salt} \xrightarrow{\text{couple}} \text{Aniline yellow}$$

Iodoform

$$\text{Acetone} + \text{I}_2 + \text{NaOH} \rightarrow \text{CHI}_3$$

Yellow crystals with characteristic smell.

Thermochemistry Experiments

Enthalpy of Solution (CuSO₄)

Measure temperature change when dissolving in water.

Enthalpy of Neutralization

$$\text{Strong acid} + \text{Strong base} → \Delta H \approx -57 \text{ kJ/mol}$$

Practice Problems

1. How would you distinguish between Na₂SO₄ and Na₂CO₃?

2. What indicator would you use for titrating NH₄OH with HCl?

3. Describe the preparation of Mohr’s salt.

Further Reading

• Equilibrium - pH and buffers
• Redox and Electrochemistry - Oxidation-reduction