ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 2 marks · Very short answer

CBSE Class 12 Chemistry sample paper 2026-27: question 17(B)

From the official CBSE sample question paper for 2026-27, section B, question 17(B), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q17(B) · 2 marks · Very short answerThe d- and f-Block Elements

(I) Why do titanium and scandium forms complex compounds, whereas potassium and calcium do not, although they belong to same period? (1)

(II) Complete and balance the following reaction
$F e^{2 +} + C r_{2} O_{7}^{2 -} + H^{+} \to$ (1)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Think about what a metal ion needs in order to accept lone pairs from ligands.

Hint 2

Compare the d-orbital availability of Ti/Sc with that of K/Ca.

Hint 3

For (II), each Cr goes from +6 to +3, and Fe goes from +2 to +3.

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.

(I) Ti and Sc ions are small and carry a high charge, and they have vacant d orbitals of suitable energy to accept lone pairs from ligands. K and Ca have no vacant d orbitals in use, and their ions are larger with low charge. So Ti and Sc form complexes, but K and Ca do not.
1 mark
(II) $C r_{2} O_{7}^{2 -}$ (Cr +6) gains 6e⁻ to give $2 C r^{3 +}$ . Each $F e^{2 +}$ loses 1e⁻. So:
$C r_{2} O_{7}^{2 -} + 14 H^{+} + 6 F e^{2 +} \to 2 C r^{3 +} + 6 F e^{3 +} + 7 H_{2} O$
1 mark

Answer: (I) Small size, high charge and available vacant d orbitals of Ti/Sc ions; (II) $C r_{2} O_{7}^{2 -} + 14 H^{+} + 6 F e^{2 +} \to 2 C r^{3 +} + 6 F e^{3 +} + 7 H_{2} O$

Where the marks are

Gives small ionic size, high charge and vacant d orbitals as the reason (1 mark)
Correctly balanced dichromate–Fe²⁺ equation (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 2026-27: every question, by section.

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CBSE Class 12 Chemistry sample paper 2026-27: question 17(B)

Hints

Step-by-step solution

Revise the chapter

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