Question

(a) How would you account for the following?
(i) Actinoid contraction is greater than lanthanoid contraction?
(ii) Transition metals form coloured compounds.
(b) Complete the following equation:
$$2{\mathrm{M}\mathrm{n}\mathrm{O}}_4^{-}+6{\mathrm{H}}^{+}{+5\mathrm{N}\mathrm{O}}_2^{-}\to$$

Answer 1

Hint: (a)(i) Consider difference in the shielding effect of 4f and 5f orbitals. 5f orbitals have a poorer shielding effect than 4f orbitals.
(ii) Consider d-d transitions. Energy is required to excite an electron from lower energy level to higher energy level within non degenerate d orbitals.
(b) Permanganate ion is an oxidizing agent and oxidizes nitrite ion to nitrate ion. Permanganate ion itself is reduced to Mn(II) ions.

Complete step by step answer:
(a)(i) In lanthanoid, 4f orbitals are involved, whereas in actinoids, 5f orbitals are involved. The shielding effect of 5f orbitals is poorer than the shielding effect of 4f orbitals. Due to this, the valence shell electrons of actinide experience greater effective nuclear charge than that experienced by lanthanides. Hence, actinoid contraction is greater than lanthanoid contraction.
ii) In the absence of ligands, the five d orbitals of transition metals are degenerate, having the same energy. But in presence of ligand, these five degenerate d orbitals split into two energy levels having different energies. Electron transition occurs from lower energy level to higher energy level. This energy requires less amount of energy. This energy is in the visible region of the spectrum. Transition metal ions absorb the radiation of a particular wavelength and reflect the remaining radiations. This gives colour to the transition metal solutions.
(b) Write the incomplete equation:

$$2{\mathrm{M}\mathrm{n}\mathrm{O}}_4^{-}+6{\mathrm{H}}^{+}{+5\mathrm{N}\mathrm{O}}_2^{-}\to$$

In the above equation, the protons suggest that the medium is acidic. Protons come from acids such as sulfuric acid.
Permanganate ion is an oxidising agent and oxidizes nitrite ion to nitrate ion. Permanganate ions itself are reduced to Mn(II) ions. Protons combine with oxygen to form the water molecules. Write the complete, balanced, chemical reaction.

$$2{\mathrm{M}\mathrm{n}\mathrm{O}}_4^{-}+6{\mathrm{H}}^{+}{+5\mathrm{N}\mathrm{O}}_2^{-}\to 2\mathrm{M}{\mathrm{n}}^{2+}{+5\mathrm{N}\mathrm{O}}_3^{-}+3{\mathrm{H}}_2\mathrm{O}$$

Note: (a)(I) Lanthanide contraction and actinoid contraction are gradual decrease in the atomic size, with increase in the atomic number.
(ii) Colour absorbed by transition metal ions is complementary to the color transmitted.
(b) In a redox reaction, when one substance acts as an oxidizing agent, it oxidizes another substance and itself gets reduced.