ChemistryCBSE Class 12 · Board exam 2025, set 56/7/1 · 4 marks · Case-based

CBSE Class 12 Chemistry 2025: question 29

Set in the CBSE Class 12 board exam of 2025, paper 56/7/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedCoordination Compounds

The Crystal Field Theory (CFT) of coordination compounds is based on the effect of different crystal fields (provided by the 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. In tetrahedral coordination entity formation, the d-orbital splitting is smaller as compared to the octahedral entity.
Answer the following questions :

(a)
On the basis of CFT, explain why $[Ti (H_{2} O)_{6}] C l_{3}$ complex is coloured ? What happens on heating the complex $[Ti (H_{2} O)_{6}] C l_{3}$ ? Give reason. [Atomic no. : Ti = 22]
2 marks
(b)(i)
What is crystal field splitting energy ?
1 mark
OR
(b)(ii)
On the basis of $Δ_{o}$ and P (pairing energy), how can you differentiate between a strong field ligand and a weak field ligand ?
1 mark
(c)
Why are low spin tetrahedral complexes rarely observed ?
1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/7/1.

Hints

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

Hint 1

Write the d-electron configuration of Ti³⁺ in an octahedral field.

Hint 2

What happens to the ligand field when the water ligands are removed?

Hint 3

Compare $Δ_{t}$ with the pairing energy.

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) Colour: Ti³⁺ has configuration [Ar]3d¹, i.e. $t_{2 g}^{1} e_{g}^{0}$ in the octahedral field of water. The electron absorbs visible light (green-yellow region) and is promoted from $t_{2 g}$ to $e_{g}$ , a d–d transition. The complex therefore appears violet.
1 mark
(a) On heating: the complex loses its water ligands and becomes colourless. Without ligands there is no crystal field splitting, so no d–d transition is possible.
1 mark
(b)(i) Crystal field splitting energy is the energy difference between the two sets of d-orbitals ( $t_{2 g}$ and $e_{g}$ ). These sets form when the degenerate d-orbitals split under the ligand field in a definite geometry ( $Δ_{o}$ for octahedral).
1 mark
(b)(ii) If $Δ_{o} < P$ , the fourth electron enters $e_{g}$ (high spin): this is a weak field ligand. If $Δ_{o} > P$ , the electron pairs in $t_{2 g}$ (low spin): this is a strong field ligand.
(c) In tetrahedral complexes, $Δ_{t} = \frac{4}{9} Δ_{o}$ is small and generally less than the pairing energy. Electrons occupy the higher orbitals singly instead of pairing, so low spin tetrahedral complexes are rarely observed.
1 mark

Answer: (a) d–d transition $t_{2 g}^{1} \to e_{g}^{1}$ gives colour; on heating, water is lost, there is no splitting and it becomes colourless. (b)(i) Energy gap between $t_{2 g}$ and $e_{g}$ . (b)(ii) $Δ_{o} < P$ : weak field; $Δ_{o} > P$ : strong field. (c) $Δ_{t}$ is too small to force pairing.

Where the marks are

Colour due to d–d transition of the single t2g electron (1 mark)
Loses water on heating, so no splitting and colourless (1 mark)
(b)(i) Definition of CFSE, OR (b)(ii) Δo < P weak field (½ mark) and Δo > P strong field (½ mark) (1 mark)
Small Δt, insufficient to force pairing (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 paper2025 board paper, set 56/7/1: every question, by section.

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CBSE Class 12 Chemistry 2025: question 29

Hints

Step-by-step solution

Revise the chapter

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