a. Account for the following :

i. Manganese shows maximum number of oxidation states in 3d series.

ii. E^o value for Mn³+/Mn²+ couple is much more positive than that for Cr³+/Cr²⁺.

iii. Ti⁴⁺ is colourless whereas V⁴⁺ is coloured in an aqueous solution.

b. Write the chemical equations for the preparation of KMnO₄ from MnO₂. Why does purple colour of acidified permanganate solution decolourise when it oxidises Fe²⁺ to Fe³⁺?

OR

a. Write one difference between transition elements and p-block elements with reference to variability of oxidation states.

b. Why do transition metals exhibit higher enthalpies of atomization?

c. Name an element of lanthanoid series which is well known to shown +4 oxidation state. Is it a strong oxidising agent or reducing agent?

d. What is lanthanoid contraction? Write its one consequence.

e. Write the ionic equation showing the oxidation of Fe(II) salt by acidified dichromate solution.

a.

(i)Manganese shows maximum number of oxidation states in 3d series due to the presence of maximum no. of unpaired electrons, i.e., 7

(ii) E⁰ value for Mn³+/Mn²+ couple is much more positive than that for Cr³+/Cr²⁺ because Cr is more stable in +3 oxidation state due to stable t_2g³ configuration whereas Mn is more stable +2 oxidation state due to half filled 3d⁵ configuration.

(iii) Ti⁴⁺ is colourless due to the absence of any unpaired electron as it is in 3d⁰ configuration whereas V⁴⁺ is blue coloured in an aqueous solution due to the presence of unpaired electron as it is in 3d¹ configuration.

b. The chemical equations for the preparation of KMnO₄ from MnO₂ are :

2MnO₂ + 4KOH + O₂→2K₂MnO₄ + 2H₂O
3MnO₄^2- + 4H⁺→ 2 MnO₄^-+ MnO₂ + 2H₂O

The purple colour of acidified permanganate solution decolourise when it oxidises Fe²⁺ to Fe³⁺ due to the formation of Mn²⁺ ion from MnO₄^-

OR

a. Transition elements show variable oxidation states that differ by 1 unit whereas in p-block elements it differs by 2 units.

Heavier transition elements are stable in higher oxidation state whereas p-block elements are stable in lower oxidation state.

b. Transition metals exhibit higher enthalpies of atomization because of strong interatomic interactions / strong metallic bonding between atoms.

c. Cerium is well known to shown +4 oxidation states.

Ce⁺⁴ is anoxidising agent as it readily gains an electron and converts to Ce⁺³.

d. Lanthanoid Contraction: The steady decrease in atomic radii of Lanthanoids with increase in atomic number due to poor shielding effect of 4f orbital electrons is called Lanthanoid Contraction.
Consequence: 5d series have almost same size as 4d series due to Lanthanoid Contraction and due to this they occur simultaneously in the ores and thus their separation becomes difficult

e. The ionic equation showing the oxidation of Fe(II) salt by acidified dichromate solution:

Cr₂O₇^2- + 14 H⁺ + 6 Fe²⁺ → 2Cr³⁺+ 6 Fe³⁺+ 7 H₂O