For M²⁺/M and M³⁺/M²⁺ systems the E° values for some metals are as follows:
Cr²⁺/Cr – 0.9 V Cr³⁺/Cr²⁺ – 0.4 V
Mn²⁺/Mn – 1.2 V Mn³⁺/Mn²⁺ + 1.5 V
Fe²⁺/Fe – 0.4 V Fe³⁺/Fe²⁺ + 0.8 V
Use this data to comment upon
(a)    The stability of Fe³⁺ in acid solution as compared to that of Cr³⁺ or Mn³⁺ and
(b)    In case with which iron can be oxidised as compared to the similar process for either chromium or manganese metal

Predict which of the following will be coloured in aqueous solution? Ti³⁺, V³⁺, Cu⁺, Sc³⁺, Mn²⁺, Fe³⁺ and Co²⁺. Give reason for each.

Compare the chemistry of actinides with that of the lanthanoids with special reference to:
(i) electronic configuration, (ii) atomic and ionic sizes, (iii) oxidation state (iv) chemical reactivity.

How would you account for the following:
(a) Of the d⁴ species, Cr²⁺ is strongly reducing while manganese(III) is strongly oxidising.

(b) Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidized.

(c) The d¹ configuration is very unstable in ions.

149. Give reasons for the following:
Among the lanthenoids Ce(III) is easily oxidised to Ce(IV).