CBSE Class 12 Chemistry 2026: question 32(a)

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1. (i)(I) $E_{M^{2+}/M}^{\circ }$ depends on the enthalpy of atomisation (sublimation), the sum of the first two ionisation enthalpies (${\Delta }_i{H}_1+{\Delta }_i{H}_2$), and the hydration enthalpy of ${\mathrm{M}}^{2+}$. Ionisation and sublimation enthalpies vary irregularly across the series, so the $E^{\circ }$ values show an irregular trend.

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2. (i)(II) Cu has a high enthalpy of atomisation and a high ${\Delta }_i{H}_1+{\Delta }_i{H}_2$. The relatively low hydration enthalpy of $\mathrm{C}{\mathrm{u}}^{2+}$ does not compensate for the energy needed to form $\mathrm{C}{\mathrm{u}}^{2+}(\mathrm{aq})$ from Cu(s), so $E^{\circ }$ is positive (+0.34 V).

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3. (i)(III) $\mathrm{M}{\mathrm{n}}^{2+}$ has the extra-stable half-filled $3d^5$ configuration (and Mn has a low atomisation enthalpy). Mn is therefore easily oxidised to $\mathrm{M}{\mathrm{n}}^{2+}$, and $E^{\circ }$ is highly negative.

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4. (ii) Acidic solution (I⁻ is oxidised to I₂):
   $2\mathrm{Mn}{\mathrm{O}}_4^{-}+10{\mathrm{I}}^{-}+16{\mathrm{H}}^{+}\to 2\mathrm{M}{\mathrm{n}}^{2+}+5{\mathrm{I}}_2+8{\mathrm{H}}_2\mathrm{O}$

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5. Alkaline (or neutral) solution (I⁻ is oxidised to iodate):
   $2\mathrm{Mn}{\mathrm{O}}_4^{-}+{\mathrm{I}}^{-}+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{Mn}{\mathrm{O}}_2+\mathrm{I}{\mathrm{O}}_3^{-}+2\mathrm{O}{\mathrm{H}}^{-}$

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Answer: (i) (I) Irregular variation of ionisation and sublimation enthalpies; (II) high ΔₐH and low hydration enthalpy of Cu²⁺; (III) stable half-filled d⁵ Mn²⁺. (ii) Acidic: 2MnO₄⁻ + 10I⁻ + 16H⁺ → 2Mn²⁺ + 5I₂ + 8H₂O; alkaline: 2MnO₄⁻ + I⁻ + H₂O → 2MnO₂ + IO₃⁻ + 2OH⁻

Revise the chapter

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