ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 4 marks · Case-based

CBSE Class 12 Chemistry 2024: question 30

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedCoordination Compounds

The Valence Bond Theory (VBT) explains the formation, magnetic behaviour and geometrical shapes of coordination compounds whereas 'The 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 mark
(b)
Give reason for the violet colour of the complex $[Ti (H_{2} O)_{6}]^{3 +}$ on the basis of crystal field theory.
1 mark
(c)
$[Cr (N H_{3})_{6}]^{3 +}$ is paramagnetic while $[Ni (CN)_{4}]^{2 -}$ is diamagnetic. Explain why. [Atomic No. : Cr = 24, Ni = 28]
2 marks
OR
(c)
Explain why $[Fe (CN)_{6}]^{3 -}$ is an inner orbital complex, whereas $[Fe (H_{2} O)_{6}]^{3 +}$ is an outer orbital complex. [Atomic No. : Fe = 26]
2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/1/1.

Hints

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

Hint 1

Write the d-electron configuration of each metal ion first.

Hint 2

Ask whether the ligand is strong field (causes pairing) or weak field, and which d orbitals (n–1 or n) are then used.

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) Crystal field splitting energy ( $Δ_{o}$ for octahedral) is the energy difference between the two sets of d-orbitals ( $t_{2 g}$ and $e_{g}$ ) formed when the degenerate d-orbitals split in the presence of ligands arranged in a definite geometry.
1 mark
(b) Ti³⁺ is 3d¹, i.e. $t_{2 g}^{1} e_{g}^{0}$ . The electron absorbs light in the yellow-green region and is promoted from $t_{2 g}$ to $e_{g}$ (d–d transition); the complementary colour, violet, is seen.
1 mark
(c) Cr³⁺ (Z = 24) is 3d³: configuration $t_{2 g}^{3}$ with three unpaired electrons (d²sp³), whatever the ligand strength, so $[Cr (N H_{3})_{6}]^{3 +}$ is paramagnetic.
1 mark
Ni²⁺ (Z = 28) is 3d⁸; CN⁻ is a strong field ligand that pairs up the electrons, leaving one 3d orbital empty for dsp² (square planar) hybridisation; with no unpaired electron, $[Ni (CN)_{4}]^{2 -}$ is diamagnetic.
1 mark
(c) OR: Fe³⁺ (Z = 26) is 3d⁵. CN⁻ is a strong field ligand, pairs the electrons into three 3d orbitals leaving two inner (n–1)d orbitals free → d²sp³ hybridisation, so $[Fe (CN)_{6}]^{3 -}$ is an inner orbital complex. H₂O is a weak field ligand, no pairing occurs, so outer 4d orbitals are used → sp³d² hybridisation, making $[Fe (H_{2} O)_{6}]^{3 +}$ an outer orbital complex.

Answer: (a) Energy gap between t₂g and e_g sets; (b) d–d transition of the t₂g¹ electron; (c) Cr³⁺ t₂g³ has 3 unpaired e⁻ (paramagnetic), Ni²⁺ with strong-field CN⁻ has all electrons paired, dsp² (diamagnetic) / OR CN⁻ strong → d²sp³ inner; H₂O weak → sp³d² outer

Where the marks are

Definition of crystal field splitting energy (1 mark)
d–d transition explanation for violet colour (1 mark)
Cr³⁺ t₂g³ with unpaired electrons → paramagnetic (1 mark)
CN⁻ strong field pairs Ni²⁺ electrons → diamagnetic (1 mark); OR d²sp³ inner orbital vs sp³d² outer orbital with reason (1 + 1 mark)

Revise the chapter

Chapter: Coordination Compounds (notes, formulas, where marks are lost and every past question)
Topic: Crystal Field Theory

Practise this chapterBoard-style questions from Coordination Compounds, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 56/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.

CBSE Class 12 Chemistry 2024: question 30

Hints

Step-by-step solution

Revise the chapter

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