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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

We know that higher the reduction potential of a specie, there is larger tendency for its reduction to take place. Let us represent the reduction potentials i.e., E° values of three M³⁺/M²⁺ systems on number line.

Cr³⁺ /Cr²⁺ =-0.4V
Fe³⁺/Fe²⁺ =0.8V
Mn³⁺/Mn²⁺ =1.5V

Fom values of reduction potentials, it is clear that the stability of Fe³⁺ in acid solution is more than that Cr³⁺ but less than that of Mn³⁺.

b) Mn²⁺/Mn =-1.2V
Cr²⁺/Cr =-0.9V
Fe²⁺/Fe =-0.4V

The lower the reduction potential of a species, its oxidation will take place more readily.

∴ Mn is oxidised most readily to Mn²⁺ and Fe is oxidised least readily among given metals Mn, Cr and Fe.

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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.

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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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