Board Paper of Class 12-Science 2023 Chemistry Delhi(Set 3) - Solutions

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

(a) Which of the following species cannot act as a ligand? Give reason.
${\mathrm{OH}}^{-}, {\mathrm{NH}}_4^{+}, {\mathrm{CH}}_3{\mathrm{NH}}_2, {\mathrm{H}}_2\mathrm{O}$
(b) The complex $\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5\left({\mathrm{NO}}_2\right)\right]{\mathrm{Cl}}_2$ is red in colour. Give IUPAC name of its linkage isomer.  VIEW SOLUTION

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

Why haloarenes are not reactive towards nucleophilic substitution reaction? Give two reasons. VIEW SOLUTION

(i) What should be the signs (positive/negative) for ${\mathrm{E}}_{\mathrm{cell}}^{\mathrm{o}}$ and $∆\mathrm{G}°$ 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:
$$\begin{gathered} {\mathrm{Fe}}_{\left(\mathrm{s}\right)} | {\mathrm{Fe}}^{2+}(0.01\mathrm{M}) || {\mathrm{H}}_{\left(1\mathrm{M}\right)}^{+} | {\mathrm{H}}_{2\left(\mathrm{g}\right)} (1 \mathrm{bar}), {\mathrm{Pt}}_{\left(\mathrm{s}\right)} \\ \mathrm{Given} {\mathrm{E}}_{\mathrm{cell}}^{\mathrm{o}} = 0.44 \mathrm{V}. \end{gathered}$$ VIEW SOLUTION

(i) Draw the zwitter ion structure for sulphanilic acid.
(ii) How can the activating effect of −NH₂ group in aniline be controlled?

OR

(i) Complete the reaction with the main product formed:
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(ii) Convert Bromoethane to Propanamine. VIEW SOLUTION

Give the reaction of heating glucose with hydroxylamine. Presence of which group is confirmed by this reaction? VIEW SOLUTION

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

(a) On the basis of crystal field theory, write the electronic configuration for d⁴ with a strong field ligand for which Δ₀ > P.
(b) A solution of Ni(H₂O)₆]²⁺ is green but a solution of [Ni(CO)₄] is colourless. Explain.
[Atomic number: Ni = 28] VIEW SOLUTION

(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

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

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

(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

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

OR

(c) Arrange the following in the increasing order of their reactivity towards S_Nl reactions : VIEW SOLUTION

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	∧m S cm2 mol–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 ${\wedge }_{\mathrm{m}}^{\mathrm{o}}$ 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 ∧_m values to be more or less than those per KCl for a given concentration. Justify.

OR

(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

(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.

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'. VIEW SOLUTION

(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

(a) Write the number of unpaired electrons in Cr³⁺.
(Atomic number of Cr = 24)
(b) Complete the reaction mentioning all the products formed :
Cr₂O₇^2– + 3H₂S + 8H⁺ →
(c) Account for the following:
(i) Mn²⁺ is more stable than Fe²⁺ towards oxidation to +3 state.
(ii) Copper has exceptionally positive ${{\mathrm{E}}_{{\mathrm{M}}^{2+}}^{\mathrm{O}}}_{/{\mathrm{M}}^{}}^{}$ value.
(iii) Eu²⁺ with electronic configuration [Xe] 4f⁷6s² is a strong reducing agent. VIEW SOLUTION