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Assign reason for each of… (Class 12 Chemistry 2023, Q34)

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ChemistryCBSE Class 12 · Board exam 2023, set 56/1/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2023: question 34

Set in the CBSE Class 12 board exam of 2023, paper 56/1/1, section E, question 34, for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34 · 5 marks · Long answerThe d- and f-Block Elements

Assign reason for each of the following :
(i) Manganese exhibits the highest oxidation state of +7 among the 3d series of transition elements.
(ii) Transition metals and their compounds are generally found to be good catalysts in chemical reactions.
(iii) is reducing in nature while with the same d-orbital configuration () is an oxidising agent.
(iv) Zn has lowest enthalpy of atomization.
(v) is unstable in an aqueous solution.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/1/1.

Hints

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

Hint 1

Write the electronic configuration of each species involved.

Hint 2

Ask which configuration the ion reaches after it gains or loses an electron.

Hint 3

For Cu⁺, compare the hydration enthalpies of Cu⁺ and Cu²⁺.

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.

  1. (i) Mn has the configuration [Ar]3d⁵4s², with the maximum number of unpaired d electrons in the 3d series. All seven 3d and 4s electrons can take part in bonding, giving the highest oxidation state of +7 (e.g. in MnO₄⁻).

    1 mark

  2. (ii) Transition metals can show variable oxidation states and form unstable intermediate complexes with reactants. They also provide a large surface area for adsorption, which weakens reactant bonds and lowers the activation energy. For example, Fe is used in the Haber process and V₂O₅ in the contact process.

    1 mark

  3. (iii) Cr²⁺ (d⁴) is oxidised to Cr³⁺, which has a stable half-filled configuration, so Cr²⁺ acts as a reducing agent. Mn³⁺ (d⁴) is reduced to Mn²⁺, which has the extra-stable half-filled d⁵ configuration, so Mn³⁺ acts as an oxidising agent.

    1 mark

  4. (iv) Zn ([Ar]3d¹⁰4s²) has a completely filled d subshell and no unpaired electrons for metallic bonding. Its interatomic (metallic) bonding is therefore weak, so its enthalpy of atomisation is the lowest.

    1 mark

  5. (v) In water, Cu⁺ disproportionates: . The much more negative hydration enthalpy of the smaller, more highly charged Cu²⁺ ion more than compensates for the second ionisation enthalpy of Cu. Hence Cu²⁺(aq) is more stable than Cu⁺(aq).

    1 mark

Answer: (i) all 3d and 4s electrons used; (ii) variable oxidation states/complex formation/large surface area; (iii) Cr³⁺ gets t₂g³, Mn²⁺ gets d⁵; (iv) no unpaired electrons, weak metallic bonding; (v) Cu⁺ disproportionates to Cu²⁺ + Cu

Where the marks are

  • (i) Use of all 3d and 4s electrons / maximum unpaired electrons (1 mark)
  • (ii) Variable oxidation states, large surface area or complex formation (1 mark)
  • (iii) Cr³⁺ attains t₂g³ and Mn²⁺ attains half-filled d⁵ (1 mark)
  • (iv) No unpaired electrons, weak interatomic bonding (1 mark)
  • (v) Disproportionation 2Cu⁺ → Cu²⁺ + Cu (1 mark)

Revise the chapter

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 paper2023 board paper, set 56/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.