TN Board 12th Solved Model Paper 2026

Tamil Nadu Board Plus Two Public Exam 2026
Chemistry Solved Sampel Paper

Part-I
1 Mark Questions Answers

Note: (i) Answer all the questions.
(ii) Choose the most appropriate answer from the given four alternatives and write the option code and corresponding answer.
15 × 1 = 15 Marks

Part-II
2 Marks Questions Answers

Note: Answer any six questions. Question No. 24 is compulsory.
6 × 2 = 12 Marks

16. What is Calcination?

Calcination is the process of heating an ore in the absence or limited supply of air at a temperature below its melting point to bring about thermal decomposition, phase transition, or removal of volatile fractions (like CO₂ or H₂O). It is commonly used for carbonate ores, e.g., conversion of CaCO₃ to CaO.

17. How will you convert boric acid to boron nitride?

Boric acid (H₃BO₃) is first heated strongly to form boric oxide (B₂O₃), which is then heated with ammonia (NH₃) at high temperature to yield boron nitride (BN).
Reactions:
2H₃BO₃ → B₂O₃ + 3H₂O
B₂O₃ + 2NH₃ → 2BN + 3H₂O

18. Sulphuric acid is a dehydrating agent. Justify with an example.

Concentrated sulphuric acid has a strong affinity for water and can remove water molecules from other compounds, acting as a dehydrating agent.
Example: It dehydrates sucrose (C₁₂H₂₂O₁₁) to carbon.
C₁₂H₂₂O₁₁ + conc. H₂SO₄ → 12C + 11H₂O (absorbed by H₂SO₄)

19. Explain common ion effect with an example.

The common ion effect is the suppression of the dissociation of a weak electrolyte by the addition of a strong electrolyte having a common ion.
Example: In a solution of acetic acid (CH₃COOH, weak acid), adding sodium acetate (CH₃COONa) provides common acetate ions (CH₃COO^-), which shifts the equilibrium CH₃COOH ⇌ CH₃COO^- + H⁺ to the left, decreasing the dissociation of acetic acid.

20. Can Fe³⁺ oxidise bromide to bromine under Standard Conditions?
Given: E^o_{Fe³⁺|Fe²⁺} = 0.771 V
E^o_Br2|Br^- = 1.09 V.

No, Fe³⁺ cannot oxidise Br^- to Br₂ under standard conditions.
The proposed reaction: 2Fe³⁺ + 2Br^- → 2Fe²⁺ + Br₂
E^o_cell = E^o_cathode (Fe³⁺/Fe²⁺) - E^o_anode (Br₂/Br^-) = 0.771 V - 1.09 V = -0.319 V
Since E^o_cell is negative, the reaction is not spontaneous.

21. Write Kolbe's reaction.

Kolbe’s reaction (Kolbe-Schmitt reaction): Sodium phenoxide is treated with CO₂ under pressure at 125°C, followed by acidification to give salicylic acid.

22. Write the structure of the following:
α−D-glucopyranose and β−D-glucopyranose

The structures are shown in Haworth projection:

In α-D-glucopyranose, the -OH at anomeric carbon (C-1) is below the plane (trans to CH₂OH).
In β-D-glucopyranose, the -OH at C-1 is above the plane (cis to CH₂OH).

23. What are antibiotics?

Antibiotics are chemical substances produced by microorganisms (or synthetically) that have the ability to kill or inhibit the growth of other microorganisms (bacteria) at low concentrations.
Examples: Penicillin, Streptomycin.

24. What is an order of a reaction?

The order of a reaction is the sum of the powers of the concentration terms in the rate law expression. It is an experimentally determined value and indicates how the rate depends on the concentration of reactants.
Example: For rate = k[A]²[B], order = 3 (second order in A, first in B).

Part-III
3 Marks Questions Answers

Note: Answer any six questions. Question No. 33 is compulsory.
6 × 3 = 18 Marks

25. Give the uses of helium.

Helium is used for the following purposes:

• Filling balloons and airships due to its low density and non-inflammable nature.
• As a coolant in nuclear reactors and cryogenic applications (e.g., MRI machines) because of its low boiling point.
• In gas lasers (e.g., He-Ne laser).
• In diving cylinders (heliox mixture) to prevent nitrogen narcosis.
• As an inert atmosphere in welding and growing crystals for semiconductors.

26. Which is more stable Fe³⁺ or Fe²⁺? Why?

Fe³⁺ is more stable than Fe²⁺.
Reason: Fe³⁺ has electronic configuration [Ar] 3d⁵, which is half-filled d-subshell, providing extra stability due to symmetrical distribution and maximum exchange energy.
Fe²⁺ has [Ar] 3d⁶, which is less stable.
Additionally, the higher charge on Fe³⁺ results in greater lattice energy or hydration energy in compounds.

27. Aluminium crystallizes in a cubic close packed structure. Its metallic radius is 125 pm. Calculate the edge length of unit cell.

Aluminium has a face-centered cubic (FCC) structure (cubic close packed).
In FCC, the relationship between edge length 'a' and atomic radius 'r' is:
4r = a√2
Remember: for BCC: r = √3a/4 Thus, a = 4r / √2 = 2√2 r
Given r = 125 pm,
a = 2 × 1.414 × 125 ≈ 353.5 pm
(Exact: √2 ≈ 1.4142, 4 × 125 = 500, 500 / 1.4142 ≈ 353.55 pm)

28. Write Arrhenius equation and explain the terms involved.

The Arrhenius equation is:
k = A e^-E_a/RT
where,
k = rate constant of the reaction
A = pre-exponential factor (frequency factor), represents the number of collisions with proper orientation
E_a = activation energy, the minimum energy required for reactants to form products
R = gas constant
T = absolute temperature

29. Explain the effect of temperature and pressure on physisorption and chemisorption.

Physisorption: Weak van der Waals forces involved.
- Effect of temperature: Decreases with increase in temperature (exothermic process).
- Effect of pressure: Increases with increase in pressure (multilayer adsorption follows Freundlich isotherm).
Chemisorption: Strong chemical bonds formed.
- Effect of temperature: Initially increases with temperature (requires activation energy), then may decrease at very high temperatures.
- Effect of pressure: Increases with pressure but usually forms monolayer (Langmuir isotherm).

30. Explain Knoevenagel reaction.

The Knoevenagel reaction (or Knoevenagel condensation) is a nucleophilic addition reaction between an aldehyde or ketone (having α-hydrogen) and an active methylene compound (e.g., malonic ester, cyanoacetic ester) in the presence of a base catalyst (like piperidine or ammonia), followed by dehydration to form an α,β-unsaturated compound.

31. Write the reaction of primary amine with Carbon disulphide (CS₂).

Primary aliphatic or aromatic amines react with CS₂ in the presence of HgCl₂ (or base) to form isothiocyanate (Mustard oil reaction).
R-NH₂ + CS₂ → R-NH-CS-SH (dithiocarbamic acid)
Then with HgCl₂: R-NCS (alkyl/aryl isothiocyanate) + H₂S + HgS

32. Write a short note on peptide bond.

A peptide bond is an amide linkage formed between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another amino acid, with the elimination of a water molecule. It is a covalent bond with partial double bond character due to resonance, making it planar and rigid. Peptide bonds link amino acids in proteins and polypeptides.

33. In the complex, [Co(CN)₂Cl₂]Cl, identify the following.
(i) IUPAC name
(ii) Central metal ion
(iii) Co-ordination number

(i) IUPAC name: Dichloridodicyanidocobalt(III) chloride
(or more precisely: Dichloridobis(cyanido-κC)cobaltate(III) chloride, but commonly Dicyanidodichloridocobalt(III) chloride)
(ii) Central metal ion: Co³⁺ (cobalt in +3 oxidation state)
(iii) Co-ordination number: 4 (two CN^- and two Cl^- ligands inside the coordination sphere)

Part-IV
5 Marks Questions Answers

Note: Answer all the questions.
5 × 5 = 25 Marks

34. (a) (i) What are the differences between minerals and ores?

Minerals	Ores
All minerals are naturally occurring compounds containing metals or their derivatives.	Ores are minerals from which metals can be extracted profitably and conveniently.
Not all minerals are ores.	All ores are minerals.
Example: Bauxite (AlOx(OH)3-2x) is a mineral containing aluminium.	Example: Bauxite is an ore of aluminium as Al is extracted economically from it.

(ii) What is the role of silica in the extraction of copper?

In the extraction of copper from copper pyrites (CuFeS₂), silica acts as an acidic flux. It combines with the basic impurity FeO (formed during roasting and smelting) to form ferrous silicate slag, which is easily removable.
FeO + SiO₂ → FeSiO₃ (slag)

OR

(b) (i) Give the uses of Boric acid.

• As an antiseptic in eye washes and for washing wounds.
• In the manufacture of enamels and glazes for pottery and tiles.
• As a mild antiseptic in talcum powders and ointments.
• In the preparation of borax and boron compounds.
• As a food preservative (in small amounts).

(ii) What are silicates?

Silicates are compounds containing silicon and oxygen in the form of anionic silicon-oxygen tetrahedra [SiO₄]^4-, linked in various ways (chains, sheets, 3D networks) with metal cations. They form the major component of rocks, soils, and clays. Examples: Orthosilicates (ZrSiO₄), pyrosilicates, cyclic silicates, chain silicates (asbestos), sheet silicates (mica), framework silicates (quartz, feldspar).

35. (a) What is Lanthanoid Contraction and what are the consequences of Lanthanoid Contraction?

Lanthanoid contraction is the gradual decrease in atomic and ionic radii of the lanthanoids (from La to Lu) due to poor shielding effect of 4f electrons, leading to increased effective nuclear charge across the series.

Consequences:

• Similar atomic/ionic sizes of lanthanoids make their separation difficult.
• Radii of elements of 5d series (Hf) are very close to 4d series (Zr), leading to similar properties (lanthanoid contraction affects the next series).
• Lanthanoids generally show +3 oxidation state predominantly.
• Basic strength of Ln(OH)₃ decreases from La to Lu.

OR

(b) (i) Write short notes on double salts and co-ordination compounds.

Double salts: Compounds that contain more than one cation or anion, exist only in solid state, and dissociate completely into simple ions in solution. Example: Carnallite (KCl·MgCl₂·6H₂O), Mohr's salt (FeSO₄·(NH₄)₂SO₄·6H₂O).

Coordination compounds: Compounds containing a central metal atom/ion surrounded by ligands (molecules/ions) through coordinate bonds, forming a coordination entity. They retain their identity in solution. Example: [Co(NH₃)₆]Cl₃, K₃[Fe(CN)₆].

(ii) Give an example of Coordination Compound used in medicine and a biologically important Coordination Compound.

Used in medicine: Cis-platin [Pt(NH₃)₂Cl₂] (anticancer drug).
Biologically important: Haemoglobin ([Fe-porphyrin complex] with coordinated O₂) or Chlorophyll (Mg-porphyrin complex).

36. (a) Calculate the percentage efficiency of packing in case of simple cubic crystal.

In simple cubic (SC) structure, atoms touch along the edge, so a = 2r.
Number of atoms per unit cell = 1 (corner atoms contribute 1/8 × 8 = 1).
Volume of unit cell = a³ = (2r)³ = 8r³
Volume occupied by atoms = 1 × (4/3)πr³
Packing efficiency = [ (4/3)πr³ / 8r³ ] × 100 = (π/6) × 100 ≈ 52.36%

Read also Packing Efficiency in Simple Cubic Crystal

OR

(b) (i) Derive the integrated rate law for a Zero order reaction, A → product.

For zero order: rate = k [A]⁰ = k
dx/dt = -k (or d[A]/dt = -k)
Integrating: [A] = -kt + [A]₀
[A]₀ - [A] = kt
Integrated rate law: [A] = [A]₀ - kt

Read also Kinetics of Zero Order Reaction

(ii) Define buffer Index.

Buffer index (or buffer capacity, β) is the amount of acid or base (in moles) that can be added per litre of buffer solution to change its pH by one unit.
β = dB / dpH, where dB is moles of base added per litre.

37. (a) (i) Explain about Galvanic cell notation.

Galvanic cell is represented by cell notation where anode is written on left, cathode on right, separated by || (salt bridge), and | separates phases.
Example: Zn | Zn²⁺(aq) || Cu²⁺(aq) | Cu
Anode half: Zn → Zn²⁺ + 2e^-
Cathode half: Cu²⁺ + 2e^- → Cu

(ii) Define gold number.

Gold number is the minimum amount of lyophilic colloid (in milligrams) required to prevent the coagulation of 10 mL standard gold sol when 1 mL of 10% NaCl solution is added. Lower gold number indicates higher protective power.

OR

(b) Write notes on Lucas Test.

Lucas test distinguishes primary, secondary, and tertiary alcohols using Lucas reagent (anhydrous ZnCl₂ + conc. HCl).
- Tertiary alcohols: Immediate turbidity (cloudiness) due to formation of alkyl chloride (insoluble).
- Secondary alcohols: Turbidity in 5-10 minutes.
- Primary alcohols: No reaction at room temperature (heating required).
Reason: Formation of carbocation intermediate; tertiary > secondary > primary in stability.

38. (a) (i) How acetic acid is prepared from Grignard reagent?

Methyl magnesium bromide (Grignard reagent) reacts with CO₂ (dry ice) to form magnesium salt of acetic acid, which on acidification gives acetic acid.
CH₃MgBr + CO₂ → CH₃COOMgBr
CH₃COOMgBr + H₂O/H⁺ → CH₃COOH + Mg(OH)Br

(ii) What are bio-degradable polymers? Give an example.

Biodegradable polymers are polymers that can be decomposed by microorganisms (bacteria, fungi) into natural products like CO₂, H₂O, and biomass.
Example: PHBV (Poly-β-hydroxybutyrate-co-β-hydroxyvalerate), Nylon-2-Nylon-6, PGA (polyglycolic acid).

OR

(b) An organic Compound (A) of molecular formula C₂H₄O reacts with Zn-Hg/Conc. HCl to give Compound (B) which reacts with HNO₃ forming Compound (C) (as major product) and Compound (D). Compound (C) reacts with conc. HCl to give Compound (E) (Table vinegar) and hydroxylamine. Identify A, B, C, D and E with suitable reactions.

A: CH₃CHO (acetaldehyde)
B: CH₃CH₃ (ethane) [Clemmensen reduction]
C: CH₃NO₂ (nitroethane) [major product of vapour phase nitration]
D: CH₃CH₂NO₂ (lesser nitroethane isomer? but mainly nitroethane)
E: CH₃COOH (acetic acid) [Nef reaction: nitroalkane + conc. HCl → carboxylic acid + NH₂OH]

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