(I) Complete and balance the following reaction SO_3²- + MnO_4^- + H^+ → (1) (II) Why does titanium melt at a lower temperature than chromium? (1)

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

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

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

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

(I) Complete and balance the following reaction
$S O_{3}^{2 -} + M n O_{4}^{-} + H^{+} \to$ (1)

(II) Why does titanium melt at a lower temperature than chromium? (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

Find the oxidation-state changes: Mn in $M n O_{4}^{-}$ and S in $S O_{3}^{2 -}$ .

Hint 2

In acid, permanganate becomes $M n^{2 +}$ . Balance the electrons first, then O with water and H with $H^{+}$ .

Hint 3

For (II), compare the number of unpaired 3d electrons in Ti and Cr.

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) In acidic medium $M n O_{4}^{-}$ (Mn +7) is reduced to $M n^{2 +}$ (gain of 5e⁻). $S O_{3}^{2 -}$ (S +4) is oxidised to $S O_{4}^{2 -}$ (loss of 2e⁻). Equalising electrons and balancing O and H gives:
$5 S O_{3}^{2 -} + 2 M n O_{4}^{-} + 6 H^{+} \to 5 S O_{4}^{2 -} + 2 M n^{2 +} + 3 H_{2} O$
1 mark
(II) Ti ( $3 d^{2} 4 s^{2}$ ) has few unpaired d electrons. Cr ( $3 d^{5} 4 s^{1}$ ) has six unpaired electrons. More unpaired electrons take part in metallic bonding, so Cr has stronger interatomic attraction. More energy is needed to break the Cr lattice, so titanium melts at a lower temperature.
1 mark

Answer: (I) $5 S O_{3}^{2 -} + 2 M n O_{4}^{-} + 6 H^{+} \to 5 S O_{4}^{2 -} + 2 M n^{2 +} + 3 H_{2} O$ ; (II) Cr has more unpaired d electrons, so its metallic bonding is stronger than in Ti.

Where the marks are

Correct products $S O_{4}^{2 -}$ , $M n^{2 +}$ , $H_{2} O$ with balanced coefficients (1 mark)
Relates the lower melting point of Ti to fewer unpaired electrons and weaker metallic bonding than Cr (1 mark)

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

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

Hints

Step-by-step solution

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