The electrode potential values E° (M²⁺/M) for some first-row transition elements are given below: | V(23) | Cr(24) | Mn(25) | Fe(26) | Co(27) | Ni(28) | Cu(29) | | −1.18 | −0.91 | −1.18 | −0.44 | −0.28 | −0.25 | +0.34 | Explain the irregularities in these values on the basis of the electronic structures of the atoms.

E° reflects ΔₐH + IE₁+IE₂ − ΔhydH; the general rise V→Cu tracks increasing IEs. Mn is anomalously negative (stable d⁵ Mn²⁺); Cu alone is positive (very high ΔₐH + ΣIE not repaid by hydration).

d and f Block Elements — CBSE Class 12 · 2006 Board Question with Solution

The electrode potential values E° (M²⁺/M) for some first-row transition elements are given below:

V(23)	Cr(24)	Mn(25)	Fe(26)	Co(27)	Ni(28)	Cu(29)
−1.18	−0.91	−1.18	−0.44	−0.28	−0.25	+0.34

Explain the irregularities in these values on the basis of the electronic structures of the atoms.

Solution

E°(M

^{2 +}

/M) is governed by the balance of enthalpy of atomisation, the first two ionisation enthalpies and the hydration enthalpy of M

^{2 +}

.
General trend: E° becomes less negative from V to Cu as the ionisation enthalpies rise with nuclear charge.
Irregularities:
• Mn (−1.18 V, as negative as V): Mn

^{2 +}

has the extra-stable half-filled 3d

^{5}

, so forming it is unusually easy — E° dips below the trend.
• Zn-like stability also makes Ni/Co values close together (small radius → large hydration energy).
• Cu (+0.34 V, the only positive value): copper's high enthalpy of atomisation plus large IE

_{1}

+IE

_{2}

are not compensated by the hydration enthalpy of Cu

^{2 +}

— copper does not displace H

_{2}

from acids.

CBSE Class 12 Board · 2006 · Subjective

CBSE Class 12 · 2006 · d and f Block ElementsShort Answer

Solution

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