Class 12 Chemistry Notes Chapter 9 (Preparation of Inorganic Compounds) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 9 from your Lab Manual: 'Preparation of Inorganic Compounds'. This is an important topic, not just for your practical exams, but questions based on the principles, reactions, and properties of these compounds frequently appear in various government competitive exams. Pay close attention to the details.

Chapter 9: Preparation of Inorganic Compounds - Detailed Notes

This chapter typically involves the synthesis, purification (usually by crystallization), and characterization of specific inorganic compounds. The key is to understand the underlying chemical principles, stoichiometry, and properties.

1. Preparation of Potash Alum (Potassium Aluminium Sulphate) - K₂SO₄·Al₂(SO₄)₃·24H₂O

• Aim: To prepare pure crystals of potash alum from potassium sulphate and aluminium sulphate.
• Chemical Formula: K₂SO₄·Al₂(SO₄)₃·24H₂O or KAl(SO₄)₂·12H₂O
• Type of Compound: Double Salt.
  • Key Concept: A double salt is formed by the combination of two different simple salts which crystallize together from a saturated aqueous solution as a single substance. In solution, they dissociate completely into their constituent ions.
  • Potash Alum gives ions: K⁺, Al³⁺, and SO₄²⁻ in aqueous solution.
• Principle: Potash alum is prepared by dissolving an equimolar mixture of potassium sulphate (K₂SO₄) and aluminium sulphate (Al₂(SO₄)₃) in a minimum amount of hot water containing a small amount of dilute sulphuric acid. The solution is then cooled to allow crystallization.
  • K₂SO₄ (aq) + Al₂(SO₄)₃ (aq) + 24H₂O (l) → K₂SO₄·Al₂(SO₄)₃·24H₂O (s)
• Role of Dilute H₂SO₄: To prevent the hydrolysis of aluminium sulphate. Al³⁺ ions tend to hydrolyze in water, forming aluminium hydroxide [Al(OH)₃], which would make the solution turbid and contaminate the product.
  • Al³⁺(aq) + 3H₂O(l) ⇌ Al(OH)₃(s) + 3H⁺(aq)
  • Adding acid shifts this equilibrium to the left, keeping Al³⁺ ions in solution.
• Procedure Highlights:
  • Dissolve calculated amounts of K₂SO₄ and Al₂(SO₄)₃ (in 1:1 molar ratio) in hot water with dilute H₂SO₄.
  • Concentrate the solution by heating if necessary, but avoid boiling to dryness.
  • Cool the solution slowly and undisturbed to get well-defined crystals.
  • Filter the crystals, wash with a small amount of ice-cold water (to remove mother liquor without dissolving the crystals significantly), and dry them between filter paper folds.
• Observations: Colourless, transparent, octahedral crystals are formed.
• Uses: Purification of water (coagulant), dyeing industry (mordant), leather tanning, styptic agent (to stop bleeding), component of baking powder.

2. Preparation of Mohr's Salt (Ferrous Ammonium Sulphate) - (NH₄)₂SO₄·FeSO₄·6H₂O

• Aim: To prepare pure crystals of Mohr's salt from ferrous sulphate and ammonium sulphate.
• Chemical Formula: (NH₄)₂SO₄·FeSO₄·6H₂O
• IUPAC Name: Ammonium Iron(II) Sulphate Hexahydrate
• Type of Compound: Double Salt.
  • It dissociates in water to give Fe²⁺, NH₄⁺, and SO₄²⁻ ions.
• Principle: Mohr's salt is prepared by dissolving an equimolar mixture of hydrated ferrous sulphate (FeSO₄·7H₂O) and ammonium sulphate ((NH₄)₂SO₄) in a minimum amount of hot water containing a small amount of dilute sulphuric acid. The solution is cooled for crystallization.
  • FeSO₄ (aq) + (NH₄)₂SO₄ (aq) + 6H₂O (l) → (NH₄)₂SO₄·FeSO₄·6H₂O (s)
• Role of Dilute H₂SO₄: To prevent the hydrolysis of ferrous sulphate (Fe²⁺ ions) and also to prevent the oxidation of Fe²⁺ (ferrous) ions to Fe³⁺ (ferric) ions by atmospheric oxygen.
  • Hydrolysis: Fe²⁺(aq) + 2H₂O(l) ⇌ Fe(OH)₂(s) + 2H⁺(aq) (Acid shifts left)
  • Oxidation: 4Fe²⁺(aq) + O₂(g) + 4H⁺(aq) → 4Fe³⁺(aq) + 2H₂O(l) (Acid helps prevent this in neutral/basic medium, although it's a reactant here, the overall acidic environment stabilizes Fe²⁺).
• Procedure Highlights:
  • Dissolve calculated equimolar amounts of FeSO₄·7H₂O and (NH₄)₂SO₄ in hot water containing dilute H₂SO₄.
  • Filter if necessary (if starting FeSO₄ is impure/oxidized).
  • Concentrate the solution gently.
  • Cool slowly for crystallization.
  • Filter the crystals, wash with a very small amount of ice-cold water or alcohol (alcohol is preferred as it reduces solubility loss and aids drying), and dry.
• Observations: Light green, transparent, monoclinic crystals are formed.
• Key Property: Mohr's salt is preferred over ferrous sulphate in volumetric analysis (titrations) because it is less readily oxidized by air compared to FeSO₄·7H₂O and is not efflorescent (does not lose water of crystallization easily). It's a stable primary standard for Fe²⁺ ions.
• Uses: Primary standard in volumetric analysis (especially in redox titrations like permanganometry and dichrometry).

3. Preparation of Potassium Trioxalatoferrate(III) - K₃[Fe(C₂O₄)₃]·3H₂O

• Aim: To prepare crystals of potassium trioxalatoferrate(III).
• Chemical Formula: K₃[Fe(C₂O₄)₃]·3H₂O
• IUPAC Name: Potassium trioxalatoferrate(III) trihydrate
• Type of Compound: Coordination Compound (Complex Salt).
  • Key Concept: A complex salt contains a central metal ion bonded to ligands (neutral molecules or anions) through coordinate bonds. It does not dissociate completely into simple constituent ions in solution.
  • In solution, it gives K⁺ ions and the complex anion [Fe(C₂O₄)₃]³⁻. The complex anion remains intact.
• Principle: This preparation involves three main steps:
  1. Preparation of Ferrous Oxalate: Ferrous ammonium sulphate (Mohr's salt) is treated with oxalic acid in an acidic medium to precipitate ferrous oxalate.
     (NH₄)₂SO₄·FeSO₄·6H₂O + H₂C₂O₄ → FeC₂O₄(s) + (NH₄)₂SO₄ + H₂SO₄ + 6H₂O
  2. Oxidation of Ferrous Oxalate to Ferric Hydroxide: The ferrous oxalate precipitate is treated with potassium oxalate solution and then oxidized using hydrogen peroxide (H₂O₂) in the presence of potassium hydroxide (formed in situ or added). The Fe²⁺ in ferrous oxalate is oxidized to Fe³⁺, which precipitates as ferric hydroxide.
     • FeC₂O₄ + 3KOH + H₂O₂ → Fe(OH)₃(s) + K₂C₂O₄ + KHCO₃ (Simplified representation, mechanism is complex)
     • Alternatively: 2FeC₂O₄(s) + H₂O₂ (aq) + 2H₂O(l) + K₂C₂O₄(aq) → 2Fe(OH)₃(s) + 2KHC₂O₄(aq) (Reaction pathway can vary based on conditions)
     • A more common lab route involves reacting Mohr's salt with oxalic acid, then reacting the ferrous oxalate precipitate with potassium oxalate and H₂O₂.
     • Let's use a clearer, standard route: React Ferric Chloride with Potassium Oxalate.
     • Revised Standard Principle: Ferric hydroxide (freshly prepared from FeCl₃ + KOH/NaOH) is treated with potassium oxalate solution and oxalic acid.
       FeCl₃(aq) + 3KOH(aq) → Fe(OH)₃(s) + 3KCl(aq)
       Fe(OH)₃(s) + 3KHC₂O₄(aq) → K₃Fe(C₂O₄)₃ + 3H₂O(l)
       (Potassium hydrogen oxalate KHC₂O₄ is often formed by mixing K₂C₂O₄ and H₂C₂O₄)
       OR
       Fe(OH)₃(s) + 3H₂C₂O₄(aq) + 3KOH(aq) → K₃Fe(C₂O₄)₃ + 6H₂O(l)
  3. Crystallization: The resulting solution containing the complex is concentrated and cooled. Ethanol is often added to reduce the solubility of the complex salt and promote crystallization.
• Coordination Details:
  • Central Metal Ion: Fe³⁺ (Iron in +3 oxidation state)
  • Ligand: Oxalate ion (C₂O₄²⁻), which is a bidentate ligand (donates through two oxygen atoms).
  • Coordination Number: 6 (since there are 3 bidentate ligands, 3 x 2 = 6).
  • Geometry: Octahedral.
• Procedure Highlights:
  • Prepare fresh Fe(OH)₃.
  • Dissolve Fe(OH)₃ in a hot solution containing stoichiometric amounts of potassium oxalate and oxalic acid.
  • Filter the hot solution if necessary.
  • Cool the filtrate. Add ethanol slowly to induce crystallization.
  • Filter the crystals, wash with a mixture of water and ethanol, then finally with ethanol. Dry in the dark (the complex is photosensitive).
• Observations: Bright green, well-defined crystals (often described as emerald green). The shape is typically prismatic or octahedral depending on crystallization conditions.
• Key Property: It is photosensitive and decomposes in light. It's used in actinometry (measurement of light intensity).
• Uses: Actinometry, blueprinting (less common now), photography.

Key Differences: Double Salt vs. Complex Salt

Multiple Choice Questions (MCQs)

1. Potash Alum is an example of a:
   a) Simple Salt
   b) Complex Salt
   c) Double Salt
   d) Mixed Salt

2. The chemical formula for Mohr's Salt is:
   a) K₂SO₄·Al₂(SO₄)₃·24H₂O
   b) (NH₄)₂SO₄·Fe₂(SO₄)₃·6H₂O
   c) (NH₄)₂SO₄·FeSO₄·6H₂O
   d) K₃[Fe(C₂O₄)₃]·3H₂O

3. Dilute sulphuric acid is added during the preparation of Mohr's salt primarily to:
   a) Increase the solubility of ammonium sulphate
   b) Prevent the hydrolysis of Fe²⁺ and its oxidation to Fe³⁺
   c) Act as a catalyst for the reaction
   d) Ensure complete precipitation of the double salt

4. In the complex K₃[Fe(C₂O₄)₃], the oxidation state and coordination number of iron are, respectively:
   a) +2, 4
   b) +3, 6
   c) +2, 6
   d) +3, 4

5. Which of the following compounds gives tests for Fe²⁺, NH₄⁺, and SO₄²⁻ ions in aqueous solution?
   a) K₄[Fe(CN)₆]
   b) K₃[Fe(C₂O₄)₃]
   c) (NH₄)₂SO₄·FeSO₄·6H₂O
   d) FeSO₄·7H₂O

6. The geometry of the complex anion [Fe(C₂O₄)₃]³⁻ is:
   a) Tetrahedral
   b) Square planar
   c) Octahedral
   d) Trigonal bipyramidal

7. Which reagent is commonly used to oxidize Fe²⁺ to Fe³⁺ during the preparation of potassium trioxalatoferrate(III) from ferrous precursors?
   a) Concentrated HNO₃
   b) KMnO₄ solution
   c) K₂Cr₂O₇ solution
   d) Hydrogen peroxide (H₂O₂)

8. The crystals of Potash Alum are typically:
   a) Light green and monoclinic
   b) Colourless and octahedral
   c) Emerald green and prismatic
   d) White and amorphous

9. Mohr's salt is often preferred over ferrous sulphate in titrations because it is:
   a) More soluble in water
   b) Less susceptible to oxidation by air
   c) A complex salt
   d) Anhydrous

10. The ligand C₂O₄²⁻ (oxalate) in K₃[Fe(C₂O₄)₃] is best described as:
    a) Monodentate
    b) Bidentate
    c) Tridentate
    d) Ambidentate

Answer Key for MCQs:

1. c) Double Salt
2. c) (NH₄)₂SO₄·FeSO₄·6H₂O
3. b) Prevent the hydrolysis of Fe²⁺ and its oxidation to Fe³⁺
4. b) +3, 6
5. c) (NH₄)₂SO₄·FeSO₄·6H₂O
6. c) Octahedral
7. d) Hydrogen peroxide (H₂O₂)
8. b) Colourless and octahedral
9. b) Less susceptible to oxidation by air
10. b) Bidentate

Remember to correlate these preparations with concepts from 'Coordination Compounds' and 'p-block elements' (for alum) and 'd & f block elements' (for iron compounds) chapters in your theory textbook. Understanding the 'why' behind each step is crucial for competitive exams. Good luck with your preparation!