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Account for the… (Class 12 Chemistry SQP 2024-25, Q23)

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ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2024-25: question 23

From the official CBSE sample question paper for 2024-25, section C, question 23, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q23 · 3 marks · Short answerThe d- and f-Block Elements

Account for the following:
a. The lowest oxide of transition metal is basic, and the highest is acidic. (1)
b. Chromium is a hard metal while mercury is a liquid metal (1)
c. The ionisation energy of elements of the 3d series does not vary much with increasing atomic number. (1)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Compare the oxidation state of the metal in the lowest and highest oxides.

Hint 2

Hardness depends on the strength of metallic bonding. What controls that in d-block metals?

Hint 3

Where does the added electron go across the 3d series, and how does it affect the outer 4s electrons?

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. (a) In the lowest oxide (e.g. MnO), the metal is in a low oxidation state and keeps valence electrons not used in bonding. It can donate electrons and the oxide is basic. In the highest oxide (e.g. ), the metal is in a high oxidation state with no spare valence electrons, so it accepts electrons and the oxide is acidic (covalent, acid-forming).

    1 mark

  2. (b) Chromium () has many unpaired d-electrons. These take part in strong metallic bonding, so Cr is hard. Mercury () has completely filled orbitals with no unpaired electrons, so its metallic bonding is very weak and it is a liquid at room temperature.

    1 mark

  3. (c) Across the 3d series, nuclear charge rises by one unit at each step, but the added electron goes into the inner (n–1)d subshell. These d-electrons shield the outer 4s electrons, largely cancelling the rise in nuclear charge. The effective nuclear charge on the 4s electrons rises only slightly, so ionisation enthalpy increases only slowly.

    1 mark

Answer: (a) Low oxidation state metal can donate electrons (basic); high oxidation state accepts electrons (acidic); (b) unpaired d-electrons give strong metallic bonding in Cr, none in Hg; (c) increase in nuclear charge is offset by shielding by (n–1)d electrons

Where the marks are

  • Low oxidation state oxide has electrons to donate (basic); high oxidation state oxide accepts electrons (acidic) (1 mark)
  • Unpaired electrons give strong metallic bonding in Cr; filled d¹⁰s² in Hg gives weak bonding (1 mark)
  • Rising nuclear charge is balanced by shielding of (n–1)d electrons (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 paperSample paper 2024-25: 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.