(i) Out of Cu_2Cl_2 and CuCl_2, which is more stable in aqueous solution and why ? (ii) Write the general electronic configuration of f-block elements. (iii) Predict which of the following will be coloured in aqueous solution and why ? Sc³+, Fe³+, Zn²+ [Atomic number : Sc = 21, Fe = 26, Zn = 30] (iv) How can you obtain potassium dichromate from sodium chromate ? (v) Why do transition metals and their compounds show catalytic activities ?

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

CBSE Class 12 Chemistry 2024: question 33(b)

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

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

(i) Out of $C u_{2} C l_{2}$ and $CuC l_{2}$ , which is more stable in aqueous solution and why ?
(ii) Write the general electronic configuration of f-block elements.
(iii) Predict which of the following will be coloured in aqueous solution and why ? $S c^{3 +}$ , $F e^{3 +}$ , $Z n^{2 +}$ [Atomic number : Sc = 21, Fe = 26, Zn = 30]
(iv) How can you obtain potassium dichromate from sodium chromate ?
(v) Why do transition metals and their compounds show catalytic activities ?

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

Compare the hydration enthalpies of Cu⁺ and Cu²⁺, and recall what happens to Cu⁺ in water.

Hint 2

For colour, write the 3d configuration of each ion and check for unpaired electrons.

Hint 3

For K₂Cr₂O₇, chromate first changes to dichromate in acid; then think of a double decomposition with a potassium salt.

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) $CuC l_{2}$ is more stable in aqueous solution.
The hydration enthalpy of $C u^{2 +}$ is much more negative than that of $C u^{+}$ , and this more than compensates for the second ionisation enthalpy. Therefore $C u^{+}$ disproportionates in water:
$2 C u^{+} (aq) \to C u^{2 +} (aq) + Cu (s)$
1 mark
(ii) The general electronic configuration of f-block elements is:
$(n - 2) f^{1 - 14} (n - 1) d^{0 - 1} n s^{2}$
1 mark
(iii) $F e^{3 +}$ is coloured in aqueous solution.
- $F e^{3 +}$ is $3 d^{5}$ . It has unpaired electrons, so d–d transitions occur by absorbing visible light.
- $S c^{3 +}$ ( $3 d^{0}$ ) and $Z n^{2 +}$ ( $3 d^{10}$ ) have no unpaired d-electrons, so no d–d transition is possible and they are colourless.
1 mark
(iv) Step 1: acidify sodium chromate with sulphuric acid to convert it to sodium dichromate:
$2 N a_{2} Cr O_{4} + 2 H^{+} \to N a_{2} C r_{2} O_{7} + 2 N a^{+} + H_{2} O$
Step 2: treat the sodium dichromate solution with KCl. The less soluble potassium dichromate crystallises out:
$N a_{2} C r_{2} O_{7} + 2 KCl \to K_{2} C r_{2} O_{7} + 2 NaCl$
1 mark
(v) Transition metals and their compounds act as catalysts for these reasons:
- They can show variable oxidation states.
- They can form complexes or unstable intermediates with the reactants.
- They provide a large surface area for the reactants to be adsorbed on.
1 mark

Answer: (i) CuCl₂ (higher hydration enthalpy of Cu²⁺; Cu⁺ disproportionates); (ii) (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns²; (iii) Fe³⁺ (3d⁵, unpaired electrons, d–d transition); (iv) Acidify Na₂CrO₄ to Na₂Cr₂O₇, then add KCl to get K₂Cr₂O₇; (v) Variable oxidation states, complex formation, large surface area

Where the marks are

Identifies CuCl₂ and gives the reason: high hydration enthalpy of Cu²⁺ / disproportionation of Cu⁺ (1 mark)
Writes the correct general configuration (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns² (1 mark)
Identifies Fe³⁺ (½ mark) and gives the reason: unpaired electrons allow d–d transition (½ mark)
Acidifies Na₂CrO₄ to give Na₂Cr₂O₇ (½ mark) and then treats it with KCl to obtain K₂Cr₂O₇ (½ mark)
Gives variable oxidation states, complex formation or large surface area as the reason for catalytic activity (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(b)

Hints

Step-by-step solution

Revise the chapter

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