Revision Notes: Principles Related to Practical Chemistry | Chemistry Class 12 - NEET PDF Download

Detection of Extra Elements in Organic Compounds

Extra Elements Detected:

• Nitrogen: Detected using Lassaigne's test. The compound is fused with sodium metal in a fusion tube, and the resulting sodium extract is tested for the presence of ammonium ions (NH₄⁺) using Nessler's reagent.
  Test Reaction: Sodium fusion extract + Nessler's reagent → Brown precipitate (Indicates presence of Nitrogen).
• Sulphur: Detected by Lassaigne's test. The sodium extract is acidified and treated with lead acetate solution. A black precipitate of lead sulphide (PbS) confirms the presence of sulphur.
  Test Reaction: Sodium fusion extract + Lead acetate → Black precipitate (PbS, confirms Sulphur).
• Halogens: Halogens (Chlorine, Bromine, Iodine) are detected using Lassaigne's test. The sodium extract is acidified with dilute nitric acid and treated with silver nitrate solution. A white, yellow, or cream precipitate confirms chlorine, iodine, or bromine, respectively.
  Test Reaction (Chlorine): Sodium fusion extract + Silver nitrate → White precipitate (AgCl, confirms Chlorine).

Detection of Functional Groups in Organic Compounds

1. Hydroxyl Group (Alcoholic and Phenolic):

• Alcoholic: Ferric chloride test gives a red coloration with phenols.
  Reaction: Phenol + Ferric chloride → Red color (Indicates Phenol).
• Phenolic: Sodium hydroxide test gives a bright yellow solution with phenols.
  Reaction: Phenol + NaOH → Yellow color (Indicates Phenol).

2. Carbonyl Group (Aldehyde and Ketones):

• Aldehyde: Tollens' Test (silver mirror test) and Fehling’s Test give a red precipitate of copper(I) oxide for aldehydes.
  Reaction: Aldehyde + Tollens’ reagent → Silver mirror (Indicates Aldehyde).
  Reaction: Aldehyde + Fehling’s solution → Red precipitate (Cu₂O).
• Ketones: No reaction with Tollens' or Fehling’s solution.

3. Carboxyl Group:

• Carboxyl group is detected by adding sodium bicarbonate. Effervescence (release of CO₂) indicates the presence of a carboxyl group.
  Reaction: R-COOH + NaHCO₃ → CO₂ effervescence (Indicates Carboxyl group).

4. Amino Group:

• Ninhydrin test is used for amino acids. A blue or purple color indicates the presence of an amino group.
  Reaction: Amino acid + Ninhydrin → Blue/Purple color (Indicates Amino group).
• Reaction with acetic acid gives a characteristic red color.
  Reaction: Amino group + Acetic acid → Red color.

Preparation of Inorganic Compounds

• Mohr’s Salt (Ammonium Iron (II) Sulphate): Prepared by reacting ferrous sulphate (FeSO₄) with ammonium sulphate (NH₄)₂SO₄ in the presence of excess dilute sulfuric acid.
  Reaction: FeSO₄ + (NH₄)₂SO₄ → (NH₄)₂Fe(SO₄)₂ + H₂O
• Potash Alum (Potassium Aluminum Sulfate): Prepared by reacting potassium chloride (KCl) with aluminum sulfate (Al₂(SO₄)₃) in the presence of water.Reaction: Al₂(SO₄)₃ + 2 KCl → 2 KAl(SO₄)₂·12H₂O
  
  

Preparation of Organic Compounds

• Acetanilide: Prepared by reacting aniline with acetic anhydride.
  Reaction: C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ + CH₃COOH
• p-Nitro Acetanilide: Prepared by nitrating acetanilide with concentrated nitric acid and sulfuric acid.
  Reaction: C₆H₅NHCOCH₃ + HNO₃ → C₆H₄(NO₂)NHCOCH₃
• Aniline Yellow: Prepared by the reaction of aniline with nitrous acid.
  Reaction: C₆H₅NH₂ + HONO → C₆H₅N = N - C₆H₅ (Aniline yellow).
• Iodoform (CHI₃): Prepared by the reaction of alcohol or aldehyde with iodine in the presence of sodium hydroxide.
  Reaction: RCH(OH) + I₂ + NaOH → CHI₃ + RCOONa + H₂O

Titrimetric Exercises

• Acids and Bases:
  Strong Acid - Strong Base Titration: The pH indicator (such as phenolphthalein) is used to find the end point.
  Reaction: HCl + NaOH → NaCl + H₂O
• Oxalic Acid vs. KMnO₄: The reaction between oxalic acid (H₂C₂O₄) and KMnO₄ in acidic medium gives a color change.
  Reaction: 5H₂C₂O₄ + 2KMnO₄ + 6H⁺ → 10CO₂ + 2Mn²⁺ + 8H₂O
• Mohr’s Salt vs. KMnO₄: The reaction involves oxidation of ferrous ion (Fe²⁺) to ferric ion (Fe³⁺) by potassium permanganate (KMnO₄).
  Reaction: 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O

Qualitative Salt Analysis

1. Cations:

• Pb²⁺ (Lead): Forms a yellow precipitate with KI.
  Reaction: Pb²⁺ + 2I⁻ → PbI₂ (yellow precipitate).
• Cu²⁺ (Copper): Forms a blue precipitate with ammonia.
  Reaction: Cu²⁺ + 4NH₃ → [Cu(NH₃)₄]²⁺ (blue solution).
• Fe³⁺ (Iron): Forms a red-brown precipitate with KSCN.
  Reaction: Fe³⁺ + SCN⁻ → Fe(SCN)₃ (red-brown precipitate).
• Zn²⁺ (Zinc): Forms a white precipitate with ammonia.
  Reaction: Zn²⁺ + 2NH₃ + 2H₂O → [Zn(NH₃)₂(H₂O)₂]²⁺ (white precipitate).
• Ca²⁺ (Calcium): Forms a white precipitate with ammonium oxalate.
  Reaction: Ca²⁺ + C₂O₄²⁻ → CaC₂O₄ (white precipitate).

2. Anions:

• CO₃²⁻ (Carbonate): Reacts with HCl to release CO₂ gas.
  Reaction: CO₃²⁻ + 2H⁺ → CO₂ + H₂O (effervescence).
• S²⁻ (Sulfide): Forms a black precipitate of PbS with lead acetate.
  Reaction: S²⁻ + Pb²⁺ → PbS (black precipitate).
• SO₄²⁻ (Sulfate): Forms a white precipitate of BaSO₄ with BaCl₂.
  Reaction: SO₄²⁻ + Ba²⁺ → BaSO₄ (white precipitate).
• NO₃⁻ (Nitrate): Reduced to NO₂ (brown fumes) by sulfuric acid.
  Reaction: 2NO₃⁻ + 10H⁺ + 8e⁻ → 2NO₂ + 4H₂O.
• Cl⁻ (Chloride): Forms a white precipitate of AgCl with AgNO₃.
  Reaction: Cl⁻ + Ag⁺ → AgCl (white precipitate).

Chemical Principles Involved in Experiments

• Enthalpy of Solution of CuSO₄: When CuSO₄ dissolves in water, the heat change is observed, which is the enthalpy change.
• Enthalpy of Neutralization of Strong Acid and Strong Base: The reaction between a strong acid (e.g., HCl) and a strong base (e.g., NaOH) produces water and releases heat, which is measured as enthalpy change.
  Reaction: HCl + NaOH → NaCl + H₂O.
• Preparation of Lyophilic and Lyophobic Sols:
  • Lyophilic sols: Prepared by dissolving substances like gelatin in water, forming stable sols.
  • Lyophobic sols: Prepared by dispersing metals like gold in a suitable liquid.
• Kinetic Study of the Reaction of Iodide Ions with Hydrogen Peroxide: The reaction follows the rate law and can be studied using the change in concentration of the reactants over time:
  Reaction: H₂O₂ + 2I⁻ → I₂ + 2OH⁻