(i) Many copper (I) compounds are unstable in aqueous solution and undergo disproportionation reaction. The stability of Cu2+ rather than Cu+ is due to the much more negative hydration enthalpy of Cu2+ than Cu+, which more than compensates for the second ionization enthalpy of Cu.
2Cu+ → Cu2+ + Cu
(ii) The transition metals form a large number of complex compounds. This is due to
• Comparatively smaller sizes of the metal ions,
• their high ionic charges, and
• The availability of d orbitals to accommodate lone pair of electrons donated by ligands.
(b) Cr2O72- + 8H+ + 3NO2- → 2Cr3+ + 3NO3- + 4H2O
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