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NCERT Class XII Chemistry
Chapter - The d- and f-Block Elements
Questions with Solutions

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Question : 21
Total: 49
21.How would you account for the following?
(i) Of the d‌4 species, Cr‌2+ is strongly reducing while manganese(III) is strongly oxidising.
(ii) Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.
(iii) The d‌1 configuration is very unstable in ions.
Solution:  
(i) Both Cr‌2+ and Mn‌3+ have d‌4configuration, Cr‌2+ is reducingsince its configuration is converted to d‌3from d‌4. d‌3has half filled t2gconfiguration with higher stability. Mn‌3+ is oxidising since in changingfrom d‌4to d‌5the configuration becomes half filled which has extra stability.(ii) Co(II) gets oxidised to Co(III) in presence of complexing agent becauseCo(III) is more stable than Co(II). Most of the strong field ligands cause pairing of electrons forming diamagnetic octahedral complexes which arevery stable due to very large crystal field stabilization energy.
(iii) d‌1configuration is very unstable in ions because after losing one moreelectron it will become more stable due to vacant d-orbital. All elementswith d‌1configuration are either reduced or undergo disproportionation.e.g.,
+6‌‌‌‌‌‌‌+7‌‌‌‌+4
3MnO42–+4H+→2MnO4–+MnO2+2H2O
3d1‌‌‌‌‌‌‌‌3d0
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