CBSE Class 12 Chemistry 2025 — Previous-Year Question Paper

Out of 2-Bromobutane, 1-Bromobutane, 2-Bromopropane and 1-Bromopropane, the molecule which is chiral in nature is : (A) 2-Bromobutane (B) 1-Bromobutane (C) 2-Bromopropane (D) 1-Bromopropane

In the given reaction sequence, the structure of Y would be : Aniline [C₆H₅NH₂] xrightarrowNaNO₂, HCl, 0-5^° C X xrightarrowC₂H₅OH Y (A) Phenol [C₆H₅OH] (B) Benzene [C₆H₆] (C) Nitrobenzene [C₆H₅NO₂] (D) Benzenediazonium chloride [C₆H₅N₂^+Cl^-]

The product of the oxidation of I⁻ with MnO₄⁻ in alkaline medium is : (A) IO₄⁻ (B) I₂ (C) IO⁻ (D) IO₃⁻

Polyhalogen compounds have wide application in industries and agriculture. DDT is also a very important polyhalogen compound. It is a : (A) greenhouse gas (B) fertilizer (C) biodegradable insecticide (D) non-biodegradable insecticide

What amount of electric charge is required for the reduction of 1 mole of MnO₄⁻ into Mn²⁺ ? (A) 1 F (B) 5 F (C) 4 F (D) 6 F

Alkenes are formed by heating alcohols with conc. H₂SO₄. The first step in the reaction is : (A) formation of carbocation (B) formation of ester (C) protonation of alcohol molecule (D) elimination of water

For the following question, two statements are given — one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) given below. Assertion (A) : Electrolysis of aqueous NaCl gives H₂ at cathode and Cl₂ at anode. Reason (R) : Chlorine has higher oxidation potential than H₂O. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

For the following question, two statements are given — one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) given below. Assertion (A) : Cuprous salts are diamagnetic. Reason (R) : Cuprous ion has completely filled 3d-orbitals. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

For the following question, two statements are given — one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) given below. Assertion (A) : n-Butyl chloride has higher boiling point than n-Butyl bromide. Reason (R) : C – Cl bond is more polar than C – Br bond. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

For the following question, two statements are given — one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) given below. Assertion (A) : Acetanilide is less basic than aniline. Reason (R) : Acetylation of aniline results in decrease of electron density on nitrogen. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

(a) Reactant 'A' underwent a decomposition reaction. The concentration of 'A' was measured periodically and recorded in the table given below : Time/Hours | [A]/M 0 | 0·40 1 | 0·20 2 | 0·10 3 | 0·05 Based on the above data, predict the order of the reaction and write the expression for the rate law. OR (b) The reaction between H₂ (g) and I₂ (g) was carried out in a sealed isothermal container. The rate law for the reaction was found to be : Rate = k[H₂][I₂] If 1 mole of H₂ (g) was added to the reaction chamber and the temperature was kept constant, then predict the change in rate of the reaction and the rate constant.

PtCl₄ · 2KCl doesn't give precipitate of AgCl with AgNO₃ solution. Write the structural formula and IUPAC name of the complex.

Define fuel cell. Give two advantages of fuel cell over ordinary cell.

Write the structures of the main products of the following reactions : (a) Methyl 3-(2-oxocyclohexyl)propanoate (drawn: a cyclohexanone ring bearing, on the carbon next to the C=O, the side chain -CH₂CH₂-oversetOunderset\|C-O-CH₃) xrightarrowNaBH₄ (b) 6(CH₃)₃C-OH + 2Al longrightarrow

What is meant by essential amino acids ? Why are amino acids amphoteric in nature ?

(a) Account for the following : (i) Allyl chloride is hydrolysed more readily than n-propyl chloride. (ii) Isocyanides are formed when alkyl halides are treated with silver cyanide. (iii) Methyl chloride reacts faster with ⁻OH ion in SN2 reaction than t-butyl chloride. OR (b) Complete the following reactions by writing the structural formulae of 'A' and 'B' : (i) CH₃CH=CH₂ xrightarrow[Peroxide]HBr 'A' xrightarrowaq. KOH 'B' (ii) CH₃CH₂undersettextstyle |undersettextstyle ClCHCH₃ xrightarrowalc. KOH 'A' xrightarrowHBr 'B' (iii) 'A' xrightarrowMg CH₃CH₂MgCl xrightarrow[H^+]H₂O 'B' (Main product)

Calculate the cell voltage of the voltaic cell which is set up by joining following half-cells at 25°C : Al/Al³⁺ (0·001M) and Ni/Ni²⁺ (0·1M) Given : E°Ni²⁺/Ni = -0·25 V, E°Al³⁺/Al = -1·66 V

Give explanation for each of the following observations : (a) With the same d-orbital configuration (d⁴), Mn³⁺ ion is an oxidising agent whereas Cr²⁺ ion is a reducing agent. (b) Actinoid contraction is greater from element to element than that among lanthanoids. (c) Transition metals form large number of interstitial compounds with H, B, C and N.

An aqueous solution of NaOH was made and its molar mass from the measurement of osmotic pressure at 27°C was found to be 25 g mol⁻¹. Calculate the percentage dissociation of NaOH in this solution. [Atomic mass : Na = 23 u, O = 16 u, H = 1 u]

Arrange the following compounds as asked : (a) in decreasing order of pKb values : C₂H₅NH₂, (C₂H₅)₂NH, C₆H₅NHCH₃, C₆H₅NH₂ (b) increasing order of boiling point : C₂H₅OH, C₂H₅NH₂, (CH₃)₂NH (c) increasing order of solubility in water : C₆H₅NH₂, (C₂H₅)₂NH, C₂H₅NH₂

An aromatic compound 'A' with molecular formula C₈H₈O gives positive 2,4-DNP test. It gives yellow precipitate of compound 'B' on treatment with sodium hypoiodite. Compound 'A' does not react with Tollen's or Fehling's reagent; on drastic oxidation with KMnO₄ it forms a carboxylic acid 'C'. Elucidate the structures of A, B and C. Also give their IUPAC names.

(a) Can sodium ethoxide and t-butyl chloride be used for the preparation of t-butyl ethyl ether ? Give suitable explanation. Justify your answer by suggesting the appropriate starting material required for preparation of t-butyl ethyl ether. (b) Give the IUPAC name of above mentioned ether.

The following question is a case-based question. Read the case carefully and answer the questions that follow. According to the generally accepted definition of the ideal solution there are equal interaction forces acting between molecules belonging to the same or different species. (This is equivalent to the statement that the activity of the components equals the concentration.) Strictly speaking, this condition is fulfilled only in exceptional cases for mixtures (optical isomers, isotopic mixtures of an element, hydrocarbon mixtures). It is still usual to talk about ideal solutions as limiting cases in reality since very dilute solutions behave ideally with respect to the solvent. This view is further supported by the fact that Raoult's law empirically found for describing the behaviour of the solvent in dilute solutions can be deduced thermodynamically via the assumption of ideal behaviour of the solvent. Answer the following questions : (a) Give one example of miscible liquid pair which shows negative deviation from Raoult's law. What is the reason for such deviation ? (b) (i) State Raoult's law for a solution containing volatile components. OR (b) (ii) Raoult's law is a special case of Henry's law. Comment. (c) Write two characteristics of an ideal solution.

The following question is a case-based question. Read the case carefully and answer the questions that follow. Ribose and 2-deoxyribose have an important role in biology. Among the most important derivatives are those with phosphate groups attached at the 5 position. Mono-, di- and tri-phosphate forms are important, as well as 3-5 cyclic monophosphates. Purines and pyrimidines form an important class of compounds with ribose and deoxyribose. When these purine and pyrimidine derivatives are coupled to a ribose sugar, they are called nucleosides. Answer the following questions : (a) What products would be formed when DNA is hydrolysed ? How is DNA different from RNA with reference to a structure ? (b) Differentiate between nucleotide and nucleoside. (c) (i) Mention two important functions of nucleic acid. OR (c) (ii) Name the linkage which joins two nucleotides. Name the base that is found in nucleotide of RNA but not in DNA.

(a) (i) Complete the following reactions by writing the structure of the main products : (I) Cyclohexanone (C₆H₁₀O, drawn ring with C=O) xrightarrowH₂NCONH-NH₂ (II) (CH₃)₂Cd + 2CH₃COCl longrightarrow (III) Benzoyl chloride (C₆H₅COCl, drawn ring bearing -COCl) xrightarrow[Pd-BaSO₄]H₂ (ii) Give simple chemical test to distinguish between the following pairs of compounds : (I) Ethyl benzoate and benzoic acid (II) Propanal and propanone OR (b) (i) Complete each synthesis by giving missing starting material, reagent or products : (I) Ethylbenzene (C₆H₅CH₂CH₃, drawn ring bearing -CH₂CH₃) xrightarrow[(ii) H₃O^+](i) KMnO₄,KOH ? (II) Methylenecyclohexane (drawn: cyclohexane ring with an exocyclic =CH₂) xrightarrow? Cyclohexanecarbaldehyde (drawn: cyclohexane ring bearing -CHO) (III) 4-Oxocyclohexane-1-carbaldehyde (drawn: cyclohexanone ring with a -CHO group on the carbon opposite the C=O) xrightarrow[Ag(NH₃)₂]^+ ? (ii) Carry out the following conversions : (I) Benzaldehyde to Benzophenone (II) Benzaldehyde to 3-phenyl propanol

(a) (i) Give reasons : (I) [Ni(CO)₄] is diamagnetic whereas [NiCl₄]²⁻ is paramagnetic. [Atomic number : Ni = 28] (II) CO is a stronger complexing agent than NH₃. (III) The trans isomer of complex [Co(en)₂Cl₂]⁺ is optically inactive. (ii) Using Crystal Field theory, write the number of unpaired electrons in octahedral complexes of Fe³⁺ in the presence of : (I) Strong field ligand (II) Weak field ligand [Atomic number : Fe = 26] OR (b) (i) Name the type of isomerism exhibited by the following compounds. Also draw their corresponding isomers. (I) [Co(NH₃)₆][Cr(CN)₆] (II) [Co(en)₃]³⁺ (III) [Co(NH₃)₃(NO₂)₃] (ii) Differentiate between weak field and strong field ligands. How does the strength of the ligand influence the spin of the complex ?

(a) (i) The initial concentration of N₂O₅ in the first order reaction : N₂O₅ (g) arrow 2NO₂ (g) + (1)/(2) O₂ (g) was 1·2 × 10⁻² mol L⁻¹. The concentration of N₂O₅ after 60 minutes was 0·2 × 10⁻² mol L⁻¹. Calculate the rate constant of the reaction at 318 K. [log 6 = 0·778] (ii) Account for the following : (I) We cannot determine the order of a reaction by taking into consideration the balanced chemical equation. (II) A bimolecular reaction may become kinetically of first order under a specified condition. OR (b) (i) The rate of the chemical reaction doubles for an increase of 10 K in absolute temperature from 298 K. Calculate activation energy (Ea). [2·303 R = 19·15 JK⁻¹ mol⁻¹, log 2 = 0·3] (ii) For a reaction : 2H₂O₂ xrightarrowI^- 2H₂O + O₂ the proposed mechanism is as given below : (I) H₂O₂ + I^- arrow H₂O + IO^- (slow) (II) H₂O₂ + IO^- arrow H₂O + I^- + O₂ (fast) (1) Write rate law for the reaction. (2) Write the overall order and molecularity of the reaction.