Q. 275.0( 1 Vote )

a. Account for th

Answer :

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 t2g3 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 + O22K2MnO4 + 2H2O
3MnO42- + 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:


Cr2O72- + 14 H+ + 6 Fe2+ 2Cr3++ 6 Fe3++ 7 H2O


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