UPSC Combined Geo-Scientist, Mains · 2022 · Mains Paper I

Q1(j). Oxidising power across the series

Question

Arrange the following metal ions in the increasing order of their oxidising power. Give reasons. Sc³⁺, Ti⁴⁺, V⁵⁺, Cr⁶⁺ and Mn⁷⁺

Conventional acidic-aqueous trend: Sc(III) < Ti(IV) < V(V) < Cr(VI) < Mn(VII).

V(V), Cr(VI) and Mn(VII) occur as oxo species, not bare highly charged aqueous ions.
Across Sc to Mn, very high oxidation states become increasingly oxidising relative to stable lower states. Sc(III) is exceptionally stable and difficult to reduce.
Acidic reduction potentials increase: TiO²⁺/Ti³⁺ about +0.10 V; VO₂⁺/VO²⁺ about +1.00 V; Cr₂O₇²⁻/Cr³⁺ about +1.33 V; MnO₄⁻/Mn²⁺ about +1.51 V.
A larger reduction potential means a stronger tendency to accept electrons; pH changes oxo-ion potentials greatly.

The full model answer

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Coversoxidising powergroup oxidation state3d transition seriesreduction potentialpermanganate dichromate