(i) Account for the following : (1) The melting and boiling points of Zn, Cd and Hg are low. (2) Of the d^4 species, Cr²+ is strongly reducing while Mn³+ is strongly oxidizing. (3) E° value of Cu²+/Cu is + 0·34 V. (ii) Complete and balance the following chemical equations : (1) KMnO_4 heat (2) Cr_2O_7²- + 6 I^- + 14 H^+

ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2024: question 33(a)

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section E, question 33(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(a) · 5 marks · Long answerThe d- and f-Block Elements

(i) Account for the following :
(1) The melting and boiling points of Zn, Cd and Hg are low.
(2) Of the $d^{4}$ species, $C r^{2 +}$ is strongly reducing while $M n^{3 +}$ is strongly oxidizing.
(3) $E^{\circ}$ value of $C u^{2 +} /Cu$ is + 0·34 V.
(ii) Complete and balance the following chemical equations :
(1) $KMn O_{4} heat$
(2) $C r_{2} O_{7}^{2 -} + 6 I^{-} + 14 H^{+} ⟶$

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/1/1.

Hints

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

Hint 1

Write the outer electronic configuration of each metal or ion and look for half-filled or fully filled sets.

Hint 2

For E°, think about the energy cycle for Cu(s) → Cu²⁺(aq): atomisation, ionisation and hydration.

Hint 3

For the equations, KMnO₄ gives manganate and MnO₂ on heating; dichromate is reduced to Cr³⁺ while I⁻ is oxidised to I₂.

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)(1) Zn, Cd and Hg have a completely filled $(n - 1) d^{10}$ configuration. Their d-electrons are not available for metallic bonding, so the metallic bonding between their atoms is weak. This gives them low melting and boiling points.
1 mark
(i)(2) $C r^{2 +}$ ( $3 d^{4}$ ) readily loses an electron to become $C r^{3 +}$ ( $3 d^{3}$ ). That ion has the stable half-filled $t_{2 g}^{3}$ configuration, so $C r^{2 +}$ is a strong reducing agent.
$M n^{3 +}$ ( $3 d^{4}$ ) readily gains an electron to become $M n^{2 +}$ ( $3 d^{5}$ ). That ion has the extra-stable half-filled $d^{5}$ configuration, so $M n^{3 +}$ is a strong oxidising agent.
1 mark
(i)(3) Converting $Cu (s)$ to $C u^{2 +} (aq)$ requires a high enthalpy of atomisation ( $Δ_{a} H^{\circ}$ ) and a large sum of first and second ionisation enthalpies.
The hydration enthalpy ( $Δ_{h y d} H^{\circ}$ ) of $C u^{2 +}$ is not large enough to compensate for this. Hence $E^{\circ} (C u^{2 +} /Cu)$ is positive (+0.34 V), and Cu does not liberate $H_{2}$ from acids.
1 mark
(ii)(1) On heating, potassium permanganate decomposes (at about 513 K):
$2 KMn O_{4} Δ K_{2} Mn O_{4} + Mn O_{2} + O_{2}$
1 mark
(ii)(2) In acid solution, dichromate oxidises iodide to iodine and is itself reduced to $C r^{3 +}$ :
$C r_{2} O_{7}^{2 -} + 6 I^{-} + 14 H^{+} ⟶ 2 C r^{3 +} + 3 I_{2} + 7 H_{2} O$
1 mark

Answer: (i)(1) Fully filled d¹⁰, so weak metallic bonding; (2) Cr²⁺→Cr³⁺ (t2g³ stable), Mn³⁺→Mn²⁺ (d⁵ stable); (3) high ΔaH° + IE not compensated by ΔhydH°. (ii)(1) 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂; (2) Cr₂O₇²⁻ + 6I⁻ + 14H⁺ → 2Cr³⁺ + 3I₂ + 7H₂O

Where the marks are

Links the fully filled d¹⁰ configuration (no unpaired d-electrons) to weak metallic bonding (1 mark)
Explains Cr²⁺ → Cr³⁺ (stable t2g³) and Mn³⁺ → Mn²⁺ (stable d⁵) (1 mark)
States that high enthalpy of atomisation and ionisation enthalpy are not balanced by hydration enthalpy (1 mark)
Gives the balanced decomposition of KMnO₄ (1 mark)
Gives the balanced ionic equation forming Cr³⁺, I₂ and H₂O (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 paper2024 board paper, set 56/1/1: every question, by section.

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CBSE Class 12 Chemistry 2024: question 33(a)

Hints

Step-by-step solution

Revise the chapter

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