Question

Answer the following questions :

(i) Why is chemistry of actinoids more complicated as compared to lanthanoids ?

(ii) \(E^\circ_{M^{2+}/M}\) values are not regular for first row transition elements (3d series). Why ?

(iii) Identify the following :

(I) Oxoanion of chromium which is stable in acidic medium.

(II) The lanthanoid element that exhibits +4 oxidation state.

(iv) Write the products obtained on heating \(\mathrm{KMnO_4}\).

(v) Predict which of the following ions will be coloured in aqueous solution :

\(\mathrm{Cu^{+}}\), \(\mathrm{Sc^{3+}}\), \(\mathrm{Fe^{2+}}\), \(\mathrm{Ti^{3+}}\), \(\mathrm{Zn^{2+}}\)

Hint:

Use the bonding of 5f electrons, the three energy terms in E0, dichromate and cerium, the products of heating KMnO4, and partly filled d subshells for colour.

Answer 1

Step 1: (i) Actinoids against lanthanoids:
The 5f orbitals of actinoids are less shielded than the 4f orbitals of lanthanoids. The 5f, 6d and 7s energies are very close, so many electrons take part in bonding.
This gives actinoids a wide range of oxidation states (+3 to +7).
Actinoids are also radioactive and many have a very short life, so their study is hard.

Step 2: (ii) Irregular \(E^\circ_{M^{2+}/M}\) values:
The value depends on the sum of three energy terms: enthalpy of atomization, ionisation enthalpy (first plus second) and enthalpy of hydration of \(\mathrm{M^{2+}}\).
Across the 3d series none of these changes in a regular way, so the \(E^\circ\) values are not regular.

Step 3: (iii) Identify:
(I) The dichromate ion \(\mathrm{Cr_2O_7^{2-}}\) is stable in acidic medium. (In basic medium chromate \(\mathrm{CrO_4^{2-}}\) is stable.)
(II) Cerium (Ce) shows the +4 state. Its \(\mathrm{Ce^{4+}}\) ion has the stable noble gas configuration.

Step 4: (iv) Heating KMnO4:
On heating at 513 K, potassium permanganate breaks down into potassium manganate, manganese dioxide and oxygen.
\[ \mathrm{2KMnO_4 \xrightarrow{\Delta} K_2MnO_4 + MnO_2 + O_2} \]

Step 5: (v) Coloured ions:
Colour in water needs an ion with a partly filled d subshell, so that d-d transitions are possible.
\(\mathrm{Cu^+}\) (\(3d^{10}\)), \(\mathrm{Sc^{3+}}\) (\(3d^0\)) and \(\mathrm{Zn^{2+}}\) (\(3d^{10}\)) have empty or full d subshells, so they are colourless.
\(\mathrm{Fe^{2+}}\) (\(3d^6\)) and \(\mathrm{Ti^{3+}}\) (\(3d^1\)) are partly filled, so they are coloured.

Final Answer:
(i) Close 5f, 6d, 7s energies, many oxidation states and radioactivity. (ii) Irregular atomization, ionisation and hydration enthalpies. (iii) \(\mathrm{Cr_2O_7^{2-}}\); Ce. (iv) \(\mathrm{K_2MnO_4}\), \(\mathrm{MnO_2}\), \(\mathrm{O_2}\). (v) Coloured ions: \(\mathrm{Fe^{2+}}\) and \(\mathrm{Ti^{3+}}\).