353.

(a) Complete the following equations :
(i) Cr₂O₇^2- + 2OH^- --->
(ii) MnO₄^- + 4H⁺ + 3e^- --->

(b) Account for the following :
(i) Zn is not considered as a transition element.
(ii) Transition metals form a large number of complexes.
(iii) The E° value for the Mn³⁺/Mn²⁺ couple is much more positive than that for Cr³⁺/Cr²⁺ couple.

OR

(i) With reference to structural variability and chemical reactivity, write the differences between lanthanoids and actinoids.

(ii) Name a member of the lanthanoid series which is well known to exhibit +4 oxidation state.
(iii) Complete the following equation :
MnO₄^- + 8H⁺ + 5e^---->
iv) Mn³⁺ is more paramagnetic than Cr³⁺.

(a) The balanced chemical equations are the following:
(i) Cr₂O^2-₇ + 2OH^- ---> 2CrO^2-₄ + H₂O
(ii) MnO^-₄ + 4H⁺ + 3e^- ---> MnO₂ + 2H₂O

(b)
(i) The electronic configuration of zinc is  1s²2s²2p⁶3s²3p⁶3d¹⁰4s². Zinc has the stable filled valence shell d¹⁰ electronic configuration in its ground state as well as in its most common oxidation state of +2. Hence, it is not considered as a transition element.

(ii) Ions formed by transition metals have small sizes and high ionic charges. Also, they possess vacant d-orbitals to accommodate lone pairs of electrons for bond formation. As a result transition, metals form a large number of complexes.

(iii) The E° value for the Mn³⁺/Mn²⁺ couple is much more positive than that for a Cr³⁺/Cr²⁺ couple. This is because Mn²⁺ ion is particularly stable due to the extra stability of its half filled valence electronic configuration (d⁵). Thus Mn³⁺ ion has a very high tendency to gain an electron and form the much more stable Mn²⁺ ion.

OR

(i) The differences between lanthanoids and actinoids, with reference to structural variability and chemical reactivity, are as follows:

a) Actinoids has far greater irregularities in metallic radii than lanthanoids, as a result actinoids display a variety of structures.

b) The ionisation enthalpies of the early actinoids are lower than those of the early lanthanoids.

c) Actinoids are more reactive and have more complex magnetic properties than lanthanoids.

(ii) Cerium is the lanthanoid element which is well known to exhibit +4 oxidation state.

(iii) The complete equation is as follows:

MnO₄^- + 8H⁺ + 5e^----> Mn²⁺ + 4H₂O

(iv) Mn³⁺ has four unpaired electrons (d⁴) in its valence shell whereas Cr³⁺ (d³) has three unpaired electrons. Thus, Mn³⁺ is more paramagnetic than Cr³⁺.