d and f Block Elements — CBSE Class 12 · 2010 · All India Board Question with Solution

a) Complete the following chemical equations:
i) Cr₂O₇²⁻(aq) + H₂S(g) + H⁺(aq) →
ii) Cu²⁺(aq) + I⁻(aq) →
b) How would you account for the following?
i) The oxidising power of the oxoanions is in the order VO₂⁺ < Cr₂O₇²⁻ < MnO₄⁻.
ii) The third ionisation enthalpy of manganese (Z = 25) is exceptionally high.
iii) Cr²⁺ is a stronger reducing agent than Fe²⁺.

Solution

(a)(i)

C r_{2} O_{7}^{2 -} + 3 H_{2} S + 8 H^{+} \to 2C r^{3 +} + 3S + 7 H_{2} O

(ii) Copper(II) oxidises iodide and precipitates cuprous iodide:

2C u^{2 +} + 4 I^{-} \to C u_{2} I_{2} ↓ + I_{2}

(b)(i) The central-atom oxidation state rises along V (+5 in VO

_{2}^{+}

) < Cr (+6) < Mn (+7); the higher the state, the greater the drive to be reduced to the stable lower state, and the standard potentials increase in the same order — so oxidising power VO

_{2}^{+}

< Cr

_{2}

_{7}^{2 -}

< MnO

_{4}^{-}

.
(ii) Mn

^{2 +}

is exactly half-filled (3d

^{5}

); the third ionisation must break this maximum-exchange-energy arrangement, so IE

_{3}

of Mn is abnormally high.
(iii) Cr

^{2 +}

^{4}

) oxidises to Cr

^{3 +}

with the very stable t

_{2 g}^{3}

configuration, so it gives up its electron readily — E°(Cr

^{3 +}

/Cr

^{2 +}

) = −0.41 V; Fe

^{2 +}

→ Fe

^{3 +}

(+0.77 V) is far less favourable. Hence Cr

^{2 +}

is the stronger reductant.

CBSE Class 12 Board · 2010 · All India · Subjective

CBSE Class 12 · 2010 · All India · d and f Block ElementsLong Answer

Solution

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