12th Chemistry Public Exam Answer Key March 2026 | TN State Board

 

12th Chemistry - Public Exam (March 2026) Official Answer Key

Here is the complete, structured answer key for the 12th standard Chemistry Public Examination held in March 2026. Students can use this to verify their answers and calculate their expected scores.

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PART - I (Multiple Choice Questions)

1. (c) Both Assertion and Reason are false.
   (Explanation: Frenkel defect does not change the density of the crystal because the missing ion simply moves to an interstitial site rather than leaving the crystal entirely.)
2. (d) Anomers
3. (c) pH = 7 + ½pK_a + ½log C
   (Explanation: Sodium formate is a salt of a weak acid and a strong base.)
4. (b) 3
   (Explanation: Square planar complexes of the type [Mabcd] exhibit 3 geometrical isomers.)
5. (d) 11.52 Scm²mol^-1
   (Calculation: Λ_m = (κ × 1000) / M = (5.76 × 10^-3 × 1000) / 0.5 = 11.52)
6. (d) 2-chloropropane
7. (a) Hexamethylene tetramine
   (Explanation: Ozonolysis of ethylene gives formaldehyde, which reacts with ammonia to form urotropine/hexamethylene tetramine.)
8. (a) Sodium
9. (c) (1/16)g
   (Calculation: 560 days = 4 half-lives. Amount remaining = 1g × (1/2)⁴ = 1/16g)
10. (d) If the standard electrode potential (E°) of a metal is large and negative, the metal is a powerful oxidising agent.
    (Correction: It would be a powerful reducing agent, not an oxidising agent.)
11. (b) antagonists
12. (b) C₆H₅OH
    (Explanation: A is Aniline, B is Benzene diazonium chloride, C is Phenol.)
13. (a) Al < Ga < In < Tl
    (Explanation: Due to the inert pair effect down the group.)
14. (b) Both magnitude and sign of the charge on the ion.
15. (a) XeO₃

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PART - II (2 Mark Questions)

• 16. Calcination:
  Calcination is the process of heating an ore strongly in the absence of air or with a limited supply of air, below its melting point.
  Example: ZnCO₃ → ZnO + CO₂↑
• 17. Hybridisation:
  • (a) BrF₅: sp³d² hybridisation (Square pyramidal)
  • (b) BrF₃: sp³d hybridisation (T-shaped)
• 18. Double Salts vs. Coordination Compounds:
  Double Salts: Dissociate completely into simple ions when dissolved in water (e.g., Mohr's salt).
  Coordination Compounds: Do not dissociate completely into simple ions in solution; the complex ion retains its identity.
• 19. Rate Law:
  The rate law is written as: Rate = k[X]^1/2[Y]^3/2[Z]⁰ (or simply k[X]^1/2[Y]^3/2)
• 20. Colloidal Preparation:
  • Ink: Prepared by Peptization or Mechanical Dispersion.
  • Graphite (Aquadag/Oildag): Prepared by Mechanical Dispersion.
• 21. Phenol to Phenolphthalein:
  Heating phenol with phthalic anhydride in the presence of a dehydrating agent like concentrated H₂SO₄ yields phenolphthalein.
• 22. Tests for Carboxylic Acid Group:
  1. Bicarbonate Test: Produces brisk effervescence of CO₂ gas when treated with NaHCO₃.
  2. Esterification Test: Heating with alcohol and conc. H₂SO₄ produces a fruity-smelling ester.
• 23. Antiseptics vs. Disinfectants:
  Antiseptics: Applied to living tissues (e.g., skin, wounds) to stop microbial growth.
  Disinfectants: Applied to inanimate objects (e.g., floors, instruments) to kill microorganisms.
• 24. pH Calculation (Compulsory):
  Ba(OH)₂ → Ba²⁺ + 2OH^-
  [OH^-] = 2 × 1.5 × 10^-3 = 3 × 10^-3 M
  pOH = -log(3 × 10^-3) = 3 - log 3 = 3 - 0.4771 = 2.5229
  pH = 14 - 2.5229 = 11.477

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PART - III (3 Mark Questions)

• 25. Basic Requirements for Vapour Phase Refining:
  1. The metal should form a volatile compound with a suitable reagent.
  2. The volatile compound should be easily decomposable at higher temperatures to yield the pure metal.
• 26. Electronic Configuration:
  Ce⁴⁺: [Xe] 4f⁰ (or [Xe] 4f⁰ 5d⁰ 6s⁰)
  Co²⁺: [Ar] 3d⁷ (or [Ar] 3d⁷ 4s⁰)
• 27. Types of Isomerism:
  • (a) [Co(NH₃)₄Br₂]Cl and [Co(NH₃)₄ClBr]Br exhibit Ionisation Isomerism.
  • (b) [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺ exhibit Linkage Isomerism.
  • (c) [Pt(NH₃)₄][PdCl₄] and [Pd(NH₃)₄][PtCl₄] exhibit Coordination Isomerism.
• 28. Packing Efficiency of BCC:
  Radius r = (√3a)/4. Volume of 2 spheres = 2 × (4/3)πr³. Divide this by the total volume of the cube (a³), and multiply by 100 to get approximately 68%.
• 29. First Order Reaction Proof:
  t = (2.303/k) log(a / (a-x))
  For 99.9%, x = 0.999a ⇒ t_99.9 = (2.303/k) log(100 / 0.1) = (2.303 × 3) / k
  t_1/2 = 0.693 / k = (2.303 × 0.3010) / k
  Ratio t_99.9 / t_1/2 ≈ 10. Hence proved.
• 30. Characteristics of Catalysts (Any 3):
  1. Remains unchanged in mass and chemical composition at the end of the reaction.
  2. A small quantity is often sufficient to bring about a large change in reaction rate.
  3. Catalysts are highly specific in their action.
• 31. Hormones:
  Hormones are chemical messengers secreted by ductless (endocrine) glands directly into the blood, regulating various biological processes.
  Examples: Insulin, Adrenaline, Estrogen, Testosterone.
• 32. Preparation of Terylene:
  Terylene (Dacron) is prepared by the condensation polymerization of ethylene glycol and terephthalic acid at 420-460 K in the presence of a zinc acetate-antimony trioxide catalyst.
• 33. Organic Compound Identification (Compulsory):
  • (A) Ethylamine (CH₃CH₂NH₂)
  • (B) Ethyl diazonium chloride ([CH₃CH₂N₂⁺]Cl^-) - Unstable intermediate
  • (C) Ethanol (CH₃CH₂OH) - Used as a substitute for petrol (power alcohol)

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PART - IV (5 Mark Questions)

• 34. (a)
  • (i) Gravity Separation Process: Based on the difference in specific gravities of the ore and gangue particles. The crushed ore is washed with a rapidly flowing stream of water. Lighter gangue particles are washed away, and heavier ore particles settle down.
  • (ii) Sulphuric acid as a dehydrating agent: Concentrated H₂SO₄ has a strong affinity for water. Example: Dehydration of sucrose (cane sugar) to carbon.
    C₁₂H₂₂O₁₁ + conc. H₂SO₄ → 12C + 11H₂O
  OR
  34. (b)
  • (i) Uses of Silicones (Any 3): Used as sealants, greases, electrical insulators, and for water-proofing fabrics.
  • (ii) Fischer-Tropsch Synthesis: The reaction of carbon monoxide with hydrogen at high pressure and temperature in the presence of a catalyst to produce liquid hydrocarbons.
• 35. (a) Lanthanoid Contraction:
  The steady decrease in atomic and ionic radii of lanthanoid elements with increasing atomic number.
  Causes: Imperfect shielding of 4f electrons.
  Effects: Similarities in properties of 2nd and 3rd transition series, difficulty in separation of lanthanoids.
  
  OR
  35. (b) VBT for Ni Complexes:
  • [Ni(CO)₄]: Ni is d¹⁰, sp³ hybridisation → Tetrahedral geometry.
  • [Ni(CN)₄]^2-: Ni²⁺ is d⁸, CN^- is a strong field ligand forcing pairing, dsp² hybridisation → Square planar geometry.
• 36. (a)
  • (i) Crystalline vs Amorphous: Crystalline solids have a regular arrangement, sharp melting points, and are anisotropic. Amorphous solids have an irregular arrangement, melt over a range of temperatures, and are isotropic.
  • (ii) Rate Determining Step: The slowest step in a complex, multi-step reaction dictates the overall reaction rate. Example: In the decomposition of H₂O₂ catalyzed by I^-, the first step forming IO^- is the slowest and determines the rate.
  OR
  36. (b) Ostwald's Dilution Law:
  For a weak binary electrolyte AB ⇌ A⁺ + B^-, the dissociation constant is K_a = (α²C) / (1-α). When α is very small, 1-α ≈ 1, making K_a = α²C, or α = √(K_a/C).
• 37. (a)
  • (i) Thermodynamics of Cell Reactions: The standard Gibbs free energy change (ΔG°) is related to standard cell potential by ΔG° = -nFE°_cell. It can also be related to the equilibrium constant K_eq by ΔG° = -RT ln K_eq.
  • (ii) Salt Bridge: It maintains electrical neutrality in the half-cells and completes the internal circuit, allowing the continuous flow of current.
  OR
  37. (b) Reactions of Diethyl Ether:
  • (i) With Cl₂/light: Complete substitution yielding perchlorodiethyl ether (C₂Cl₅-O-C₂Cl₅).
  • (ii) With dil H₂SO₄/H₂O: Hydrolysis to form Ethanol (2 CH₃CH₂OH).
  • (iii) With CH₃COCl/anhydrous ZnCl₂: Cleavage forming ethyl chloride (C₂H₅Cl) and ethyl acetate (CH₃COOC₂H₅).
• 38. (a) Cannizzaro Reaction Mechanism:
  Involves aldehydes without α-hydrogen. The three main steps are: 1) Attack of OH^- on the carbonyl carbon. 2) Hydride ion transfer (the rate-determining step). 3) Acid-base proton exchange to form an alcohol and a carboxylate ion.
  
  OR
  38. (b)
  • (i) Benzene diazonium chloride to biphenyl: It reacts with benzene in the presence of NaOH to form biphenyl. The name of the reaction is the Gomberg-Bachmann reaction.
  • (ii) Fructose C-2 Keto Confirmation: Fructose reacts with HCN to form a cyanohydrin, or with hydroxylamine to form an oxime, confirming the carbonyl group. Oxidation with strong oxidizing agents yields a mixture of glycolic and tartaric acids containing fewer carbon atoms, which confirms the keto group is at the C-2 position.