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141. Describe the preparation of potassium permanganate. How does the acidified permanganate solution reacts with (a) iron (II) ions (b) SO₂ and (c) oxalic acid ? Write the ionic equations for the reactions.

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142.

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

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143.

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

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Short Answer Type

144. Compare the stability of +2 oxidation state for the elements of the first transition series.

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145.

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.

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Long Answer Type

146.

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.

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Short Answer Type

147. What is meant by disproportionation? Give two examples of disproportionation reaction in aqueous solution.

A reaction in which the same species is simultaneously oxidised as well as reduced is called a disproportionatiion reaction.

Thus we can say taht disproportionation reaction is a special type of redox reaction. In which an element in one oxidation state simutaneously undergoes both oxidation and reduction. For example, Cr(V) and Mn(VI) species undergo disproportionation reaction in acidic medium as follows:

3 {CrO}_{4}^{3 -} + 8 H^{+} \to 2 {CrO}_{4}^{2 -} + {Cr}^{3 +} + 4 H_{2} O (Cr in + 5 o . s) (Cr in + 6 o . s) 3 {MnO}_{4}^{3 -} + 4 H^{+} \to 2 {MnO}_{4}^{-} + {MnO}_{2} + 2 H_{2} O Mn in + 6 o . s + 7 o . s 4 o . s .

Here, we can say that Cr in + 5 oxidation state undergo disproportionation into its +6 and +3 states. Similarly, Mn in +6 oxidation state undergo disproportionation into +7 and +4 oxidation states.

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148. Give reasons for the following:
Transition metals have high enthalpies of atomisation.

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

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150. Give reasons for the following:
Fe³⁺/Fe²⁺ redox couple has less positive electrode potential than Mn^3+r/Mn²⁺ couple.

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