The d-And-f-Block Elements

The d-And-f-Block Elements

Question

(a) Complete the following equations :
(i) Cr2O72- + 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+.

Answer

(a) The balanced chemical equations are the following:
(i) Cr2O2-7 + 2OH- ---> 2CrO2-4 + 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+.

More Chapters from The d-And-f-Block Elements