CBSE Class 12 · 2008 · Foreign · d and f Block ElementsShort Answer

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a) Complete the following chemical equations for reactions in aqueous media:
i) Cr₂O₇²⁻ + H⁺ + Fe²⁺ →
ii) MnO₄⁻ + I⁻ + H⁺ →
b) How many unpaired electrons are present in the Mn²⁺ ion (Z of Mn = 25)? How do they influence the magnetic behaviour of Mn²⁺ ions?

Solution

(a)(i)

C r_{2} O_{7}^{2 -} + 14 H^{+} + 6F e^{2 +} \to 2C r^{3 +} + 6F e^{3 +} + 7 H_{2} O

(ii)

2Mn O_{4}^{-} + 10 I^{-} + 16 H^{+} \to 2M n^{2 +} + 5 I_{2} + 8 H_{2} O

(b) Mn

^{2 +}

: [Ar]3d

^{5}

— five unpaired electrons.
Spin-only moment:

μ = n (n + 2) = 5 \times 7 = 35 \approx 5.92 BM

The unpaired electrons make Mn

^{2 +}

strongly paramagnetic — it is attracted into a magnetic field.

CBSE Class 12 Board · 2008 · Foreign · Subjective

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