Attempt any five of the following: (a) Which of the following ions will have a magnetic moment value of 1.73 BM. Sc³+, Ti³+, Ti²+, Cu²+, Zn²+ (b) In order to protect iron from corrosion, which one will you prefer as a sacrificial electrode, Ni or Zn? Why? (Given standard electrode potentials of Ni, Fe and Zn are -0.25 V, -0.44 V and -0.76 V respectively.) (c) The second ionization enthalpies of chromium and manganese are 1592 and 1509 kJ/mol respectively. Explain the lower value of Mn. (d) Give two similarities in the properties of Sc and Zn. (e) What is actinoid contraction? What causes actinoid contraction? (f) What is the oxidation state of chromium in chromate ion and dichromate ion? (g) Write the ionic equation for reaction of KI with acidified KMnO_4.

ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 5 marks · Long answer

CBSE Class 12 Chemistry sample paper 2023-24: question 31

From the official CBSE sample question paper for 2023-24, section E, question 31, for 5 marks. Try it first; the hints and our step-by-step solution are below.

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

Attempt any five of the following:

(a)
Which of the following ions will have a magnetic moment value of 1.73 BM.
$S c^{3 +}, T i^{3 +}, T i^{2 +}, C u^{2 +}, Z n^{2 +}$
0 marks
(b)
In order to protect iron from corrosion, which one will you prefer as a sacrificial electrode, Ni or Zn? Why? (Given standard electrode potentials of Ni, Fe and Zn are -0.25 V, -0.44 V and -0.76 V respectively.)
0 marks
(c)
The second ionization enthalpies of chromium and manganese are 1592 and 1509 kJ/mol respectively. Explain the lower value of Mn.
0 marks
(d)
Give two similarities in the properties of Sc and Zn.
0 marks
(e)
What is actinoid contraction? What causes actinoid contraction?
0 marks
(f)
What is the oxidation state of chromium in chromate ion and dichromate ion?
0 marks
(g)
Write the ionic equation for reaction of KI with acidified $KMn O_{4}$ .
0 marks

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

For (a), write the d-electron configuration of each ion and count unpaired electrons.

Hint 2

For (b) and (c), compare electrode potentials, then the configurations of Cr⁺ and Mn⁺.

Hint 3

For (e) to (g), think of 5f shielding, oxygen = –2, and MnO₄⁻ reducing to Mn²⁺ in acid.

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) Ti³⁺ (3d¹) and Cu²⁺ (3d⁹) each have one unpaired electron, so $μ = n (n + 2) = 3 = 1.73$ BM. Sc³⁺ (3d⁰) and Zn²⁺ (3d¹⁰) have none, and Ti²⁺ (3d²) gives 2.83 BM.
1 mark
(b) Zn. Its E° (–0.76 V) is more negative than that of Fe (–0.44 V), so Zn is oxidised in preference and protects iron. Ni (–0.25 V) is less negative than Fe, so iron would corrode instead.
1 mark
(c) Cr⁺ is 3d⁵, a stable half-filled configuration, so removing a second electron from it is difficult. Mn⁺ is 3d⁵4s¹, and losing the 4s electron leaves a stable 3d⁵ ion. Hence the second ionisation enthalpy of Mn is lower.
1 mark
(d) Both show only one oxidation state (Sc +3, Zn +2). Their ions (Sc³⁺ 3d⁰, Zn²⁺ 3d¹⁰) have no unpaired electrons, so their compounds are colourless and diamagnetic.
1 mark
(e) Actinoid contraction is the steady decrease in atomic and ionic (M³⁺) radii from Th to Lr as atomic number increases. It is caused by the poor shielding of the nuclear charge by 5f electrons, so the increasing nuclear charge pulls the electrons closer.
1 mark
(f) Cr is in the +6 oxidation state in both chromate ( $Cr O_{4}^{2 -}$ ) and dichromate ( $C r_{2} O_{7}^{2 -}$ ). (Alternative part, 1 mark if chosen)
(g) $2Mn O_{4}^{-} + 10 I^{-} + 16 H^{+} \to 2M n^{2 +} + 5 I_{2} + 8 H_{2} O$ (Alternative part, 1 mark if chosen)

Answer: (a) Ti³⁺ and Cu²⁺; (b) Zn; (c) Cr⁺ is stable 3d⁵, Mn⁺ loses its 4s¹ electron easily; (d) single oxidation state; colourless, diamagnetic compounds; (e) poor 5f shielding; (f) +6 in both; (g) 2MnO₄⁻ + 10I⁻ + 16H⁺ → 2Mn²⁺ + 5I₂ + 8H₂O

Where the marks are

(a) Ti³⁺ and Cu²⁺, one unpaired electron each (1 mark)
(b) Zn, more negative electrode potential so corrodes in place of iron (1 mark)
(c) Cr⁺ 3d⁵ half-filled stability vs Mn⁺ 3d⁵4s¹ (1 mark)
(d) Two similarities, e.g. colourless compounds and diamagnetic (1 mark)
(e) Definition and cause: poor shielding by 5f electrons (1 mark)
(f) +6 in both ions (1 mark)
(g) Balanced ionic equation (1 mark); any five parts for 5 marks

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 2023-24: every question, by section.

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CBSE Class 12 Chemistry sample paper 2023-24: question 31

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Step-by-step solution

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