Give reasons for the following:(i)    Transition elements and their compounds act as catalysts.(ii)     $E^{\circ}$ value for $\left ( Mn^{2+}/Mn \right )$ is negative whereas for $\left ( Cu^{2+}/Cu \right )$ is positive.(iii)     Actinoids shows irregularities in their electronic confriguration.

(i) The different reasons as to why transition elements act as catalysts are given as follows:

(a) Due to the presence of unpaired electrons in their incomplete d - orbitals and hence possess the capacity to absorbs and re - emit wide range of energies. This makes the required energy of activation available.

(b) In number of cases the transition metal provides a suitable large surface area with free valencies on which the reactants are absorbed.

(ii) $E^{\circ}$ value for $Mn^{2+}/Mn$ is negative due to extra stability of half filled 3d- orbitals of $Mn^{2+}(3d^{5})$ whereas $E^{\circ}$ value for $Cu^{2+}/Cu$ is positive because the high energy to transform $\left [ Cu\rightarrow Cu^{2+} \right ]$ is not balanced by low hydration energy.

(iii) The irregularities in the electronic configuration of the actinoids is due to the extra stability of the $f^{\circ},f^{5}and f^{10}$ in 5-f orbitals.

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