ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 5 marks · Long answer

CBSE Class 12 Chemistry sample paper 2024-25: question 31(a)

From the official CBSE sample question paper for 2024-25, section E, question 31(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(a) · 5 marks · Long answerThe d- and f-Block Elements

a. A purple colour compound A, which is a strong oxidising agent and used for bleaching of wool, cotton, silk and other textile fibres was added to each of the three test tubes along with $H_{2} S O_{4}$ . It was followed by strong heating.

In which of the above test tubes; A,B or C:
(i) Violet vapours will be formed (1)
(ii) The bubbles of gas evolved will extinguish a burning matchstick. Write an equation for each of the above observations. (1)

b. A metal ion $M^{n +}$ of the first transition series having $d^{5}$ configuration combines with three didentate ligands. Assuming $Δ_{0} < P$ :
(i) Draw the crystal field energy level diagram for the 3d orbital of this complex. (1)
(ii) What is the hybridisation of $M^{n +}$ in this complex and why? (1)
(iii) Name the type of isomerism exhibited by this complex. (1)

Figure in the paper: Three test tubes labelled A, B, C containing: A - oxalic acid, B - PbS, C - KI (solid). See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

Open one hint at a time and try again before the next.

Hint 1

Identify the purple oxidising agent first, then think which reagent gives violet vapours and which gives a non-supporter of combustion.

Hint 2

For the complex, three didentate ligands give six donor atoms. What geometry results?

Hint 3

If Δ₀ < P, do the electrons pair in t₂g or occupy e_g? What does that leave for hybridisation?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

A is $K M n O_{4}$ (purple, strong oxidant, used to bleach textiles).
a(i) Violet vapours ( $I_{2}$ ) form in test tube C (KI).
½ mark
$10 I^{-} + 2 M n O_{4}^{-} + 16 H^{+} \to 5 I_{2} + 2 M n^{2 +} + 8 H_{2} O$
½ mark
a(ii) The gas that puts out a burning matchstick is $C O_{2}$ , formed in test tube A (oxalic acid).
½ mark
$5 C_{2} O_{4}^{2 -} + 2 M n O_{4}^{-} + 16 H^{+} \to 10 C O_{2} + 2 M n^{2 +} + 8 H_{2} O$
½ mark
b(i) The complex is octahedral $[M (AA)_{3}]$ and high spin, since $Δ_{0} < P$ . The diagram shows the five degenerate 3d orbitals of the free ion split into: a lower $t_{2 g}$ set ( $d_{x y}, d_{y z}, d_{x z}$ ) at $- 0.4 Δ_{0}$ with one electron each, and an upper $e_{g}$ set ( $d_{x^{2} - y^{2}}, d_{z^{2}}$ ) at $+ 0.6 Δ_{0}$ with one electron each. The gap between them is $Δ_{0}$ . Configuration: $t_{2 g}^{3} e_{g}^{2}$ , 5 unpaired electrons.
1 mark
b(ii) Hybridisation: ** $s p^{3} d^{2}$ ** (outer-orbital complex). Because $Δ_{0} < P$ , the five 3d electrons do not pair and occupy all five 3d orbitals. No inner 3d orbitals are free, so the metal uses $4 s, 4 p$ and $4 d$ orbitals for the six donor atoms.
1 mark
b(iii) Optical isomerism. $[M (AA)_{3}]$ exists as two non-superimposable mirror images (d and l forms).
1 mark

Answer: a(i) Test tube C: 10I⁻ + 2MnO₄⁻ + 16H⁺ → 5I₂ + 2Mn²⁺ + 8H₂O; a(ii) Test tube A: 5C₂O₄²⁻ + 2MnO₄⁻ + 16H⁺ → 10CO₂ + 2Mn²⁺ + 8H₂O; b(i) t₂g³e_g² high-spin splitting; b(ii) sp³d² outer orbital; b(iii) optical isomerism

Where the marks are

Test tube C with balanced equation for I₂ formation (1 mark)
Test tube A with balanced equation for CO₂ formation (1 mark)
Octahedral splitting diagram with t₂g³e_g² (1 mark)
sp³d² with reason: no pairing since Δ₀ < P (1 mark)
Optical isomerism (1 mark)

Revise the chapter

Chapter: The d- and f-Block Elements (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from The d- and f-Block Elements, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

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CBSE Class 12 Chemistry sample paper 2024-25: question 31(a)

Hints

Step-by-step solution

Revise the chapter

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