The elements of 3d transition series are given as : (1 + 1 + 1 = 3) Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn Answer the following : (a) Copper has exceptionally positive E°M²+/M value, why ? (b) Which element is a strong reducing agent in +2 oxidation state and why ? (c) Zn²+ salts are colourless. Why ?

ChemistryCBSE Class 12 · Board exam 2025, set 56/1/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2025: question 25

Set in the CBSE Class 12 board exam of 2025, paper 56/1/1, section C, question 25, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q25 · 3 marks · Short answerThe d- and f-Block Elements

The elements of 3d transition series are given as : (1 + 1 + 1 = 3)
Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn
Answer the following :

(a)
Copper has exceptionally positive $E_{M^{2 +} /M}^{\circ}$ value, why ?
1 mark
(b)
Which element is a strong reducing agent in +2 oxidation state and why ?
1 mark
(c)
$Z n^{2 +}$ salts are colourless. Why ?
1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/1/1.

Hints

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

Hint 1

E° depends on the balance between atomisation, ionisation and hydration energies.

Hint 2

Which M²⁺ ion becomes more stable when it loses one more electron?

Hint 3

Colour in transition metal ions comes from d–d transitions. What is the d-configuration of Zn²⁺?

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) Converting Cu(s) to $C u^{2 +}$ (aq) needs a high enthalpy of atomisation plus a high sum of ionisation enthalpies. Its hydration enthalpy is relatively low and does not compensate for this energy, so $E_{C u^{2 +} / C u}^{\circ}$ is positive (+0.34 V).
1 mark
(b) Chromium: $C r^{2 +}$ is a strong reducing agent.
½ mark
$C r^{2 +}$ ( $d^{4}$ ) readily loses an electron to become $C r^{3 +}$ ( $d^{3}$ ). $d^{3}$ is a stable configuration with a half-filled $t_{2 g}$ level.
½ mark
(c) $Z n^{2 +}$ has the configuration $3 d^{10}$ , with completely filled d-orbitals and no unpaired electrons. No d–d transition is possible, so no visible light is absorbed and its salts are colourless.
1 mark

Answer: (a) High ΔaH and ionisation enthalpy not balanced by hydration enthalpy; (b) Cr, since Cr²⁺ (d⁴) → Cr³⁺ (d³, stable half-filled t₂g); (c) 3d¹⁰, so no d–d transition

Where the marks are

High enthalpy of atomisation/ionisation with low hydration enthalpy (1 mark)
Identifies Cr (½ mark)
Reason: d⁴ → d³ with half-filled t₂g (½ mark)
Zn²⁺ is d¹⁰, so there is no d–d transition (1 mark)

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Chapter: The d- and f-Block Elements (notes, formulas, where marks are lost and every past question)

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CBSE Class 12 Chemistry 2025: question 25

Hints

Step-by-step solution

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