Board Paper of Class 12-Science 2023 Chemistry Delhi(Set 1) - Solutions

General Instructions:

Read the following instructions very carefully and follow them :

(i) This question paper contains 35 questions. All questions All questions are compulsory.

(ii) Question paper is divided into FIVE sections – Section A, B, C, D and E.

(iii) In section - A: Question Numbers 1 to 18 are Multiple Choice (MCQ) type Questions carrying 1 mark each.

(iv) In section - B: Question Numbers 19 to 25 are Very Short Answer (VSA) type questions carrying 2 marks each.

(v) In section - C: Question Numbers 26 to 30 are Short Answer (SA) type questions carrying 3 marks each.

(vi) In section - D: Question Numbers 31 and 32 are case based questions carrying 4 marks each.

(vii) In section - E: Question Numbers 33 to 35 are Long Answer (LA) type questions carrying 5 marks each.

(viii) There is no overall choice. However, an internal choice has been provided in 2 questions in Section - B, 2 questions in Section - C, 2 questions in Section - D and 2 questions in Section E.

(ix) Use of calculators is NOT allowed.

Question 1
Which of the following molecules has a chiral centre correctly labelled with an asterisk (*)?
(a) CH₃C*HBrCH₃
(b) CH₃C*HC/CH₂Br
(c) HOCH₂C*H(OH)CH₂OH
(d) CH₃C*Br₂CH₃ VIEW SOLUTION

Question 2
Which of the following alcohols will not undergo oxidation?
(a) Butanol
(b) Butan-2-ol
(c) 2-Methylbutan-2-ol
(d) 3-Methylbutan-2-ol VIEW SOLUTION

Question 3
A voltaic cell is made by connecting two half cells represented by half equations below:

${Sn}^{2 +}_{(aq)} + 2 e^{-} \to {Sn}_{(s)} E ° = - 0.14 V {Fe}^{3 +}_{(aq)} + e^{-} \to {Fe}^{2 +}_{(aq)} E ° = + 0.77 V$

Which statement is correct about this voltaic cell?
(a) Fe²⁺ is oxidised and the voltage of the cell is −0.91 V
(b) Sn is oxidised and the voltage of the cell is 0.91 V
(c) Fe²⁺ is oxidised and the voltage of the cell is 0.91 V
(d) Sn is oxidised and the voltage of the cell is 0.63 V VIEW SOLUTION

Question 4
Four half reactions I to IV are shown below :
I. $2 C l^{-} \to C l_{2} + 2 e^{-}$
II. $4 {OH}^{-} \to O_{2} + 2 H_{2} O + 2 e^{-}$
III. ${Na}^{+} + e^{-} \to Na$
IV. $2 H^{+} + 2 e^{-} \to H_{2}$
Which two of these reactions are most likely to occur when concentrated brine is electrolysed?
(a) I and III
(b) I and IV
(c) II and III
(d) II and IV VIEW SOLUTION

Question 5
Which property of transition metals enables them to behave as catalysts?
(a) High melting point
(b) High ionisation enthalpy
(c) Alloy formation
(d) Variable oxidation states VIEW SOLUTION

Question 6
In the two tetrahedral structures of dichromate
(a) 4 Cr − O bonds are equivalent in length.
(b) 6 Cr − O bonds are equivalent in length.
(c) All Cr − O bonds are equivalent in length.
(d) All Cr − O bonds are non-equivalent. VIEW SOLUTION

Question 7
1 mole of liquid A and 2 moles of liquid B make a solution having a total vapour pressure 40 torr. The vapour pressure of pure A and pure B are 45 torr and 30 torr respectively. The above solution (a) is an ideal solution.
(b) shows positive deviation.
(c) shows negative deviation.
(d) is a maximum boiling azeotrope. VIEW SOLUTION

Question 8
Which of the following would not be a good choice for reducing nitrobenzene to aniline?
(a) LiAlH₄
(b) H₂/Ni
(c) Fe and HCl
(d) Sn and HCl VIEW SOLUTION

Question 9
If molality of a dilute solution is doubled, the value of the molal elevation constant (K_b) will be
(a) halved
(b) doubled
(c) tripled
(d) unchanged VIEW SOLUTION

Question 10
Hydrolysis of sucrose is called
(a) inversion
(b) hydration
(c) esterification
(d) saponification VIEW SOLUTION

Question 11
Which one of the following has lowest pK_a value?
(a)
(b)
(c)
(d) HCOOH VIEW SOLUTION

Question 12
Which of the following cell was used in Apollo space programme?
(a) Mercury cell
(b) Daniel cell
(c)
(d) Dry cell VIEW SOLUTION

Question 13

The following experimental rate data were obtained for a reaction carried out at 25^∘C :

A_{(g)} + B_{(g)} + C_{(g)} + D_{(g)}

Initial [A_(g)]/mol dm⁻³	Initial [B_(g₎]/mol dm⁻³	Initial rate/mol dm⁻³s⁻¹
3.0 $\times$ 10⁻²	2.0 $\times$ 10⁻²	1.89 $\times$ 10⁻⁴
3.0 $\times$ 10⁻²	4.0 $\times$ 10⁻²	1.89 $\times$ 10⁻⁴
6.0 $\times$ 10⁻²	4.0 $\times$ 10⁻²	7.56 $\times$ 10⁻⁴

What are the orders with respect to A_(g)and B_(g)?

	Order with respect to A_(g)	Order with respect to B_(g)
(a)	Zero	Second
(b)	First	Zero
(c)	Second	Zero
(d)	Second	First

VIEW SOLUTION

Question 14
The magnetic moment of [NiCl₄]²⁻
(a) 1.82 BM
(b) 2.82 BM
(c) 4.42 BM
(d) 5.46 BM
[Atomic number: Ni = 28] VIEW SOLUTION

Question 15
Assertion (A) : Proteins are polymers of α-amino acids connected by a peptide bond.
Reason (R) : A tetrapeptide contains 4 amino acids linked by 4 peptide bonds.
(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. VIEW SOLUTION

Question 16
Assertion (A) : For a zero order reaction the unit of rate constant and rate of reaction are same.
Reason (R) : Rate of reaction for zero order reaction is independent of concentration of reactant.
(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. VIEW SOLUTION

Question 17
Assertion (A) : Acetic acid but not formic acid can be halogenated in presence of red P and Cl₂.
Reason (R) : Acetic acid is a weaker acid than formic acid.
(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. VIEW SOLUTION

Question 18
Assertion (A): Trans ${[{CrCl}_{2} {(ox)}_{2}]}^{3 -}$ shows optical isomerism.
Reason (R): Optical isomerism is common in octahedral complexes involving didentate ligands.
(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. VIEW SOLUTION

Question 19
(i) What should be the signs (positive/negative) for $E_{cell}^{o}$ and $∆ G °$ for a spontaneous redox reaction occurring under standard conditions?
(ii) State Faraday's first law of electrolysis.
OR

Calculate the emf of the following cell at 298 K:
${Fe}_{(s)} | {Fe}^{2 +} (0.01 M) | | H_{(1 M)}^{+} | H_{2 (g)} (1 bar), {Pt}_{(s)} Given E_{cell}^{o} = 0.44 V .$ VIEW SOLUTION

Question 20
What happens to the rate constant k and activation energy E_a as the temperature of a chemical reaction is increased? Justify. VIEW SOLUTION

Question 21
(a) Which of the following species cannot act as a ligand? Give reason.
${OH}^{-}, {NH}_{4}^{+}, {CH}_{3} {NH}_{2}, H_{2} O$
(b) The complex $[Co {({NH}_{3})}_{5} ({NO}_{2})] {Cl}_{2}$ is red in colour. Give IUPAC name of its linkage isomer. VIEW SOLUTION

Question 22
Why is boiling point of o-dichlorobenzene higher than p-dichlorobenzene but melting point of para isomer is higher than ortho isomer? VIEW SOLUTION

Question 23
For the pair phenol and cyclohexanol, answer the following:
(a) Why is phenol more acidic than cyclohexanol?
(b) Give one chemical test to distinguish between the two. VIEW SOLUTION

Question 24
(i) Draw the zwitter ion structure for sulphanilic acid.
(ii) How can the activating effect of −NH₂group in aniline be controlled?
OR

(i) Complete the reaction with the main product formed:

(ii) Convert Bromoethane to Propanamine. VIEW SOLUTION

Question 25
Give the reaction of glucose with hydrogen cyanide. Presence of which group is confirmed by this reaction? VIEW SOLUTION

Question 26
(a) For the reaction
2N₂O_5(g)→4NO_2(g) +O_2(g) at 318 K
calculate the rate of reaction if rate of disappearance of N₂O_5(g)is 1.4 × 10^–3 m s^–1.

(b) For a first order reaction derive the relationship t_99% = 2t_90% VIEW SOLUTION

Question 27
(a) On the basis of crystal field theory write the electronic configuration for d⁵ion with a strong field ligand for which ∆₀> P.

(b) [Ni(CO)₄] has tetrahedral geometry while [Ni(CN)₄]^2– has square planar yet both exhibit dimagnetism. Explain.
[Atomic number: Ni = 28] VIEW SOLUTION

Question 28
(a) Illustrate Sandmeyer's reaction with an equation.

(b) Explain, why (CH₃)₂NH is more basic than (CH₃)₃N in aqueous solution. VIEW SOLUTION

Question 29
Give reasons for any 3 of the following observations:
(a) Penta-acetate of glucose does not react with hydroxylamine.
(b) Amino acids behave like salts.
(c) Water soluble vitamins must be taken regularly in diet.
(d) The two strands in DNA are complimentary to each other. VIEW SOLUTION

Question 30
(i) Why is the C–O bond length in phenols less than that in methanol?

(ii) Arrange the following in order of increasing boiling point:
Ethoxyethane, Butanal, Butanol, n-butane

(iii) How can phenol be prepared from anisole? Give reaction.
OR

(i) Give mechanism of the following reaction:

(ii) Illustrate hydroboration–oxidation reaction with an example. VIEW SOLUTION

Question 31
The following question is case - based question. Read the passage carefully and answer the question that follow:

Nucleophilic Substitution
Nucleophilic Substitution reaction of haloalkane can be conducted according to both S_Nl and S_N2 mechanisms. S_N1 is a two step reaction while S_N2 is a single step reaction. For any haloalkane which mechanism is followed depends on factors such as structure of haloalkane, properties of leaving group, nucleophilic reagent and solvent.

Influences of solvent polarity: In S_Nl reaction, the polarity of the system increases from the reactant to the transition state, because a polar solvent has a greater effect on the transition state than the reactant, thereby reducing activation energy and accelerating the reaction. In S_N2 reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_N1) of tertiary chlorobutane at 25°C in water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_N2) of 2-Bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. Hence the level of solvent polarity has influence on both S_Nl and S_N2 reaction, but with different results. Generally speaking weak polar solvent is favourable for S_N2 reaction, while strong polar solvent is favourable for S_Nl. Generally speaking the substitution reaction of tertiary haloalkane is based on S_Nl mechanism in solvents with a strong
polarity (for example ethanol containing water).

Answer the following questions
(a) Why racemisation occurs in S_Nl?
(b) Why is ethanol less polar than water ?
(c) Which one of the following in each pair is more reactive towards S_N2 reaction ?

(i)
(ii)

OR
(c) Arrange the following in the increasing order of their reactivity towards S_Nl reactions :
(i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane
(ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3- methylbutane
VIEW SOLUTION

Question 32

Rahul set-up an experiment to find resistance of aqueous KCl solution for different concentrations at 298 K using a conductivity cell connected to a Wheatstone bridge. He fed the Wheatstone bridge with a.c. power in the audio frequency range 550 to 5000 cycles per second. Once the resistance was calculated from null point, he also calculated the conductivity, k and molar conductivity ∧_m and recorded his readings in tabular form.

S. No.	Conc. (M)	k S cm^–1	∧_mS cm²mol^–1
1.	1.00	111.3 × 10^–3	111.3
2.	0.10	12.9 × 10^–3	129.0
3.	0.01	1.41 × 10^–3	141.0

Answer the following questions:
(a) Why does conductivity decrease with dilution ?
(b) If

\land_{m}^{o}

of KCl is 150.0 S cm² mol^–1, calculate the degree of dissociation of 0.01 M KCl.
(c) If Rahul had used HCl instead to KCl then would you expect the ∧_mvalues to be more or less than those per KCl for a given concentration. Justify.

(c) Amit, a classmate of Rahul repeated the same experiment with CH₃COOH solution instead of KCl solution. Give one point that would be similar and one that would be different in his observations as compared to Rahul. VIEW SOLUTION

Question 33
(i) Why is boiling point of 1M NaCl solution more than that of 1M glucose solution?

(ii) A non-volatile solute 'X' (molar mass = 50 g mol⁻¹) when dissolved in 78g of benzene reduced its vapour pressure to 90%. Calculate the mass of X dissolved in the solution.

(iii) Calculate the boiling point elevation for a solution prepared by adding 10g of MgCl₂ to 200g of water assuming MgCl₂ is completely dissociated.
(K_b for Water = 0.512 K kg mol⁻¹, Molar mass MgCl₂, = 95g mol⁻¹)
OR

(i) Why is the value of Van't Hoff factor for ethanoic acid in benzene close to 0.5?

(ii) Determine the osmotic pressure of a solution prepared by dissolving 2.32 $\times$ 10⁻²g of K₂SO₄ in 2L of solution at 25 °C, assuming that K₂SO₄ is completely dissociated.
(R = 0.082 L atm K^−l mol^−l, Molar mass K₂SO₄ = 174g mol⁻¹)

(iii) When 25.6g of Sulphur was dissolved in 1000g of benzene, the freezing point lowered by 0.512 K. Calculate the formula of Sulphur (S_x).
(K_f for benzene = 5.12 K kg mol⁻¹, Atomic mass of Sulphur = 32g mol^−l) VIEW SOLUTION

Question 34
(i) Write the reaction involved in Cannizaro's reaction.

(ii) Why are the boiling point of aldehydes and ketones lower than that of corresponding carboxylic acids ?

(iii) An organic compound 'A' with molecular formula C₅H₈O₂ is reduced to n-pentane with hydrazine followed by heating with NaOH and Glycol.
'A' forms a dioxime with hydroxylamine and gives a positive Iodoform and Tollen's test. Identify 'A' and give its reaction for Iodoform and Tollen's test.

OR

(i) Give a chemical test to distinguish between ethanal and ethanoic acid.

(ii) Why is the α-hydrogens of aldehydes and ketones are acidic in nature?

(iii) An organic compound 'A' with molecular formula C₄H₈O₂ undergoes acid hydrolysis to form two compounds 'B' and 'C'.
Oxidation of 'C' with acidified potassium permanganate also produces 'B'. Sodium salt of 'B' on heating with soda lime gives methane.
(1) Identify 'A', 'B' and C'.
(2) Out of 'B' and 'C', which will have higher boiling point ? Give reason.
VIEW SOLUTION

Question 35
(a) Why is chemistry of actinoids complicated as compared to lanthanoids ?

(b) Complete the following reaction and justify that it is a disproportionation reaction:
$3 {MnO}_{4}^{2 -} + 4 H^{+} \to____+____+ 2 H_{2} O$ .

(c) The given graph shows the trends in melting points of transition metals:

Explain the reason why Cr has highest melting point and manganese (Mn) a lower melting point.
VIEW SOLUTION

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