Answer the following questions : (i) Name two elements of 3d series for which the third ionisation enthalpies are quite high. (ii) Out of KMnO_4 and K_2MnO_4, which one is paramagnetic and why ? (iii) Write any one consequence of lanthanoid contraction. (iv) How do you prepare potassium manganate from pyrolusite ore ? (v) Why is the ability of oxygen more than fluorine to stabilise higher oxidation states of transition metals ?

ChemistryCBSE Class 12 · Board exam 2025, set 56/7/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2025: question 31(b)

Set in the CBSE Class 12 board exam of 2025, paper 56/7/1, section E, question 31(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

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

Answer the following questions :

(i)
Name two elements of 3d series for which the third ionisation enthalpies are quite high.
1 mark
(ii)
Out of $KMn O_{4}$ and $K_{2} Mn O_{4}$ , which one is paramagnetic and why ?
1 mark
(iii)
Write any one consequence of lanthanoid contraction.
1 mark
(iv)
How do you prepare potassium manganate from pyrolusite ore ?
1 mark
(v)
Why is the ability of oxygen more than fluorine to stabilise higher oxidation states of transition metals ?
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

Look for ions whose third ionisation would disturb a d⁵ or d¹⁰ arrangement.

Hint 2

Find the oxidation state of Mn in each compound and count its d electrons.

Hint 3

Compare how many bonds O and F can form with a metal.

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) Mn and Zn. Removing the third electron would break the stable $d^{5}$ of Mn²⁺ and the stable $d^{10}$ of Zn²⁺. Ni and Cu are also acceptable.
1 mark
(ii) $K_{2} Mn O_{4}$ is paramagnetic. In it, Mn is in the +6 state ( $3 d^{1}$ ) with one unpaired electron. In $KMn O_{4}$ , Mn(+7) is $3 d^{0}$ , so it is diamagnetic.
1 mark
(iii) The 4d and 5d elements of the same group have almost identical atomic radii (e.g. Zr 160 pm, Hf 159 pm), so they have very similar properties. Another consequence is that lanthanoids are difficult to separate.
1 mark
(iv) Pyrolusite ( $Mn O_{2}$ ) is fused with KOH in the presence of air or an oxidising agent such as $KN O_{3}$ :
$2Mn O_{2} + 4KOH + O_{2} \to 2 K_{2} Mn O_{4} + 2 H_{2} O$
1 mark
(v) Oxygen can form multiple bonds (pπ–dπ) with the metal, but fluorine can form only single bonds. For example, Mn forms $M n_{2} O_{7}$ (+7), whereas its highest fluoride is $Mn F_{4}$ (+4).
1 mark

Answer: (i) Mn, Zn; (ii) K₂MnO₄ (Mn⁶⁺, one unpaired electron); (iii) Zr and Hf have similar radii; (iv) fuse MnO₂ with KOH and an oxidant; (v) oxygen forms multiple bonds with metals.

Where the marks are

Any two correct elements (½ + ½ mark)
K₂MnO₄ (½ mark) with reason of one unpaired electron (½ mark)
Any one correct consequence (1 mark)
Fusion of MnO₂ with KOH and an oxidising agent (1 mark)
Ability of oxygen to form multiple bonds (1 mark)

Revise the chapter

Chapter: The d- and f-Block Elements (notes, formulas, where marks are lost and every past question)
Topic: Magnetic Properties and Colour of Transition Metal Ions

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 paper2025 board paper, set 56/7/1: every question, by section.

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CBSE Class 12 Chemistry 2025: question 31(b)

Hints

Step-by-step solution

Revise the chapter

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