CBSE Class 12 Chemistry 2024 — Previous-Year Question Paper

The compound which undergoes SN1 reaction most rapidly is : (A) 3-Bromocyclohex-1-ene (C₆H₉Br — cyclohexene ring with Br on the allylic carbon) (B) (Bromomethyl)cyclohexane (C₆H₁₁CH₂Br — cyclohexane ring-CH₂-Br) (C) Bromocyclohexane (C₆H₁₁Br) (D) Bromobenzene (C₆H₅Br)

Acetic acid reacts with PCl₅ to give: (A) Cl-CH₂-COCl (B) Cl-CH₂-COOH (C) CH₃-COCl (D) CCl₃-COOH

The formation of cyanohydrin from an aldehyde is an example of: (A) nucleophilic addition (B) electrophilic addition (C) nucleophilic substitution (D) electrophilic substitution

In the Arrhenius equation, when log k is plotted against 1/T, a straight line is obtained whose: (A) slope is (A)/(R) and intercept is Ea. (B) slope is A and intercept is (-Ea)/(R). (C) slope is (-Ea)/(RT) and intercept is log A. (D) slope is (-Ea)/(2 · 303 R) and intercept is log A.

The reaction of an alkyl halide with sodium alkoxide forming ether is known as: (A) Wurtz reaction (B) Reimer-Tiemann reaction (C) Williamson synthesis (D) Kolbe reaction

The correct order of the ease of dehydration of the following alcohols by the action of conc. H₂SO₄ is: (A) (CH₃)₃C-OH > (CH₃)₂CH-OH > CH₃CH₂-OH (B) (CH₃)₂CH-OH > CH₃CH₂-OH > (CH₃)₃C-OH (C) CH₃CH₂-OH > (CH₃)₂CH-OH > (CH₃)₃C-OH (D) (CH₃)₂CH-OH > (CH₃)₃C-OH > CH₃CH₂-OH

Which functional groups of glucose interact to form cyclic hemiacetal leading to pyranose structure? (A) Aldehyde group and hydroxyl group at C-4 (B) Aldehyde group and hydroxyl group at C-5 (C) Ketone group and hydroxyl group at C-4 (D) Ketone group and hydroxyl group at C-5

Assertion (A): When NaCl is added to water a depression in freezing point is observed. Reason (R): NaCl undergoes dissociation in water. [Select the correct code: (A) Both A and R are true and R is the correct explanation of A; (B) Both A and R are true but R is not the correct explanation of A; (C) A is true but R is false; (D) A is false but R is true.]

Assertion (A): Separation of Zr and Hf is difficult. Reason (R): Zr and Hf have similar radii due to lanthanoid contraction. [Codes (A)-(D) as in the Assertion-Reason instruction.]

Assertion (A): The pKa of ethanoic acid is lower than that of Cl-CH₂-COOH. Reason (R): Chlorine shows electron withdrawing (-I) effect which increases the acidic character of Cl-CH₂-COOH. [Codes (A)-(D) as in the Assertion-Reason instruction.]

Assertion (A): Aniline is a stronger base than ammonia. Reason (R): The unshared electron pair on nitrogen atom in aniline becomes less available for protonation due to resonance. [Codes (A)-(D) as in the Assertion-Reason instruction.]

Calculate the potential of Iron electrode in which the concentration of Fe²⁺ ion is 0.01 M. (EoFe²⁺/Fe = -0.45 V at 298 K) [Given: log 10 = 1]

Define molecularity of the reaction. State any one condition in which a bimolecular reaction may be kinetically of first order.

What happens when D-glucose is treated with the following reagents? (1+1=2) (a) HI (b) Conc. HNO₃

(a) Draw the structures of major monohalo products in each of the following reactions: (i) Styrene [C₆H₅CH=CH₂] + HBr longrightarrow (ii) Cyclohexene [C₆H₁₀] + Br₂ xrightarrowUVlight OR (b) Give reasons for the following : (i) Grignard reagent should be prepared under anhydrous conditions. (ii) Alkyl halides give alcohol with aqueous KOH whereas in the presence of alcoholic KOH, alkenes are formed.

Write the chemical equation when: (1+1=2) (a) Butan-2-one is treated with Zn(Hg) and conc. HCl. (b) Two molecules of benzaldehyde are treated with conc. NaOH.

When a certain conductivity cell was filled with 0.05 M KCl solution, it has a resistance of 100 ohm at 25°C. When the same cell was filled with 0.02 M AgNO₃ solution, the resistance was 90 ohm. Calculate the conductivity and molar conductivity of AgNO₃ solution. (Given: Conductivity of 0.05 M KCl solution = 1.35 × 10⁻²ohm⁻¹cm⁻¹)

The following initial rate data were obtained for the reaction: 2NO(g) + Br₂(g) arrow 2NOBr(g) | Expt. No. | [NO]/mol L⁻¹ | [Br₂]/mol L⁻¹ | Initial Rate (mol L⁻¹ s⁻¹) | |---|---|---|---| | 1 | 0.05 | 0.05 | 1.0×10⁻³ | | 2 | 0.05 | 0.15 | 3.0×10⁻³ | | 3 | 0.15 | 0.05 | 9.0×10⁻³ | (a) What is the order with respect to NO and Br₂ in the reaction? (b) Calculate the rate constant (k). (c) Determine the rate of reaction when concentration of NO and Br₂ are 0.4 M and 0.2 M, respectively. (1+1+1=3)

(a) Write the formula for the following coordination compound: Potassium tetrahydroxidozincate(II) (b) Arrange the following complexes in the increasing order of conductivity of their solution: [Cr(NH₃)₅Cl]Cl₂, [Cr(NH₃)₃Cl₃], [Cr(NH₃)₆]Cl₃ (c) Identify the type of isomerism exhibited by the following complexes: (i) [Co(NH₃)₅NO₂]²⁺ (ii) [Co(en)₃]Cl₃ (1+1+(½+½)=3)

(a) Which of the following is an allylic halide? (i) CH₃-CH=CH-Br (ii) CH₂=CH-CH(Br)-CH₃ (b) Out of chlorobenzene and 2,4,6-trinitrochlorobenzene, which is more reactive towards nucleophilic substitution and why? (c) Which isomer of C₄H₉Cl has the lowest boiling point? (3×1=3)

(a) Write the mechanism of the following reaction: Ethanol CH₃CH₂OH xrightarrow[443K]H^+ CH₂=CH₂ Ethene (b) Write the main product(s) in the following reactions : (i) But-1-ene CH₃ - CH₂ - CH = CH₂ xrightarrowa) B₂H₆ b) 3H₂O₂/OH^- (ii) Phenol C₆H₅OH xrightarrowa) aq. NaOH b) CO₂, H^+

Answer the following: (any three) (3×1=3) (a) What is peptide linkage? (b) What type of bonds hold a DNA double helix together? (c) Which one of the following is a polysaccharide? Sucrose, Glucose, Starch, Fructose (d) Give one example each for water-soluble vitamins and fat-soluble vitamins.

Compound (A) (C₆H₁₂O₂) on reduction with LiAlH₄ gives two compounds (B) and (C). The compound (B) on oxidation with PCC gives compound (D) which upon treatment with dilute NaOH and subsequent heating gives compound (E). Compound (E) on catalytic hydrogenation gives compound (C). The compound (D) is oxidized further to give compound (F) which is found to be a monobasic acid (Molecular weight = 60). Identify the compounds (A), (B), (C), (D), (E) and (F). (½×6=3)

Case study: Batteries and fuel cells are very useful forms of galvanic cell. Any battery or cell that we use as a source of electrical energy is basically a galvanic cell. For a battery to be of practical use it should be reasonably light, compact and its voltage should not vary appreciably during its use. There are mainly two types of batteries — primary batteries and secondary batteries. In primary batteries the reaction occurs only once and after use the battery becomes dead and cannot be reused, whereas secondary batteries are rechargeable. Production of electricity by thermal plants is inefficient and a major source of pollution; to solve this, galvanic cells are designed so that energy of combustion of fuels is directly converted into electrical energy — these are known as fuel cells. One such fuel cell was used in the Apollo space programme. Answer the following questions: (a) How do primary batteries differ from secondary batteries? (1) (b) The cell potential of Mercury cell is 1.35 V, and remains constant during its life. Give reason. (1) (c) Write the reactions involved in the recharging of the lead storage battery. (2) OR (c) Write two advantages of fuel cells over other galvanic cells. (2)

Case study: The Valence Bond Theory (VBT) explains the formation, magnetic behaviour and geometrical shapes of coordination compounds whereas Crystal Field Theory for coordination compounds is based on the effect of different crystal fields (provided by ligands taken as point charges) on the degeneracy of d-orbital energies of the central metal atom/ion. The splitting of the d-orbitals provides different electronic arrangements in strong and weak crystal fields. The crystal field theory attributes the colour of the coordination compounds to d-d transition of the electron. Coordination compounds find extensive applications in metallurgical processes, analytical and medicinal chemistry. Answer the following questions: (a) What is crystal field splitting energy? (1) (b) Give reason for the violet colour of the complex [Ti(H₂O)₆]³⁺ on the basis of crystal field theory. (1) (c) [Cr(NH₃)₆]³⁺ is paramagnetic while [Ni(CN)₄]²⁻ is diamagnetic. Explain why. [Atomic No.: Cr = 24, Ni = 28] (2) OR (c) Explain why [Fe(CN)₆]³⁻ is an inner orbital complex, whereas [Fe(H₂O)₆]³⁺ is an outer orbital complex. [Atomic No.: Fe = 26] (2)

(a)(i) At the same temperature, CO₂ gas is more soluble in water than O₂ gas. Which one of them will have higher value of KH and why? (ii) How does the size of blood cells change when placed in an aqueous solution containing more than 0.9% (mass/volume) sodium chloride? (iii) 1 molal aqueous solution of an electrolyte A₂B₃ is 60% ionized. Calculate the boiling point of the solution. (Given: Kb for H₂O = 0.52 K kg mol⁻¹) (1+1+3=5) OR (b)(i) The vapour pressures of A and B at 25°C are 75 mm Hg and 25 mm Hg, respectively. If A and B are mixed such that the mole fraction of A in the mixture is 0.4, then calculate the mole fraction of B in the vapour phase. (ii) Define colligative property. Which colligative property is preferred for the molar mass determination of macromolecules? (iii) Why are equimolar solutions of sodium chloride and glucose not isotonic? (2+2+1=5)

Answer any five questions of the following: (5×1=5) (a) N,N-diethyl-benzenesulphonamide is insoluble in alkali. Give reason. (b) Aniline does not undergo Friedel-Crafts reaction. Why? (c) Write a simple chemical test to distinguish between methylamine and aniline. (d) Write the chemical reaction involved in Gabriel phthalimide synthesis. (e) How will you convert aniline to p-bromoaniline? (f) Complete the following reaction : C₆H₅-N₂⁺Cl⁻ xrightarrow[(ii) NaNO₂/Cu,Δ](i) HBF₄ (g) Write the structures of A and B in the following reaction : Benzoic acid C₆H₅COOH xrightarrow[Δ]NH₃ A xrightarrowBr₂+NaOH B

(a)(i) Account for the following: (1) The melting and boiling points of Zn, Cd and Hg are low. (2) Of the d⁴ species, Cr²⁺ is strongly reducing while Mn³⁺ is strongly oxidizing. (3) Eo value of Cu²⁺/Cu is +0.34 V. (ii) Complete and balance the following chemical equations: (1) KMnO₄ xrightarrowheat (2) Cr₂O₇²⁻ + 6I^- + 14H^+ arrow (3+2=5) OR (b)(i) Out of Cu₂Cl₂ and CuCl₂, which is more stable in aqueous solution and why? (ii) Write the general electronic configuration of f-block elements. (iii) Predict which of the following will be coloured in aqueous solution and why? Sc³⁺, Fe³⁺, Zn²⁺ [Atomic number: Sc = 21, Fe = 26, Zn = 30] (iv) How can you obtain potassium dichromate from sodium chromate? (v) Why do transition metals and their compounds show catalytic activities? (5×1=5)