Q. 25

(a) A blackish br

Answer :

(a)


• (A) must be manfanese dioxide MnO2 which is blackish brown in colour and when fused with alkali metal hydroxide and an oxidizing agent like KNO3, it produces (B) K2MnO4 which is a dark green compound which on disproportionation in neutral and acidic solution gives (C) which is KMnO4 , the purple coloured compound.



• Compound (C) on heated to 513 K decomposes back to potassium managanate (B) and and manganese dioxide (A).



b) (i)


• The two strongest oxidizing agents in the aqueous solution are Mn3+ and Co3+


• Because they have the most positive E0 values which indicates higher potential to be reduced and oxidizing other substances. Rest of the elements are good reducing agents.


(b) (ii)


• Stability of an ion in aqueous solution highly depends upon hydration energy of the ion which is the capacity to attrat water molecules; which is more negative in the case for Cu(II) than Cu(I) which more than compemsates the second ionization enthalpy of copper.


• In aqueous solution Cu+ undergoes oxidation and reduction simultaneously


• And the relative stability of Cu and Cu2+ ions can be seen by their standard reduction potentials.


• Cu2+(aq) + e- Cu(s), E0red =0.52


• Cu2+ + 2e- Cu(s), E0red = 0.47


Having higher electrode potential makes Cu(I) very unstable in aq soln and therefore it readily disproportionates.


(b) (iii)


• Oxygen is possesses a very high eletrotronegativity and has a considerably small size which allows it to be a very good oxidizing agent (probably the best alongside F ) .


• Due to these properties O can oxidize metals to their highest oxidation states and that is why The highest oxidation state of a metal is exhibited in its oxide.


OR


(a) (i)


• When Cu2+ is treated with KI solution CuI (white) is precipitated alongside production of I2 solution.



(ii)


• K2Cr2O7 is a good oxidizing agent therdfore when it is introduced in Fe(II) solution it readily oxidizes it to Fe(III) solution and changes its colour from green to yellowish brownand used for identification of Fe(II).


Cr2O72- + 6 Fe2+ + 14H+ 2Cr3+ + 6 Fe3+ + 7H2O [ E° = 0.56 V ]


(b) (i)


• The 3rd series of transition elements with 5d electrons have the highest first ionization enthalpy.


• The speciality of these elements is that they possess 4f electrons which has very weak shielding abilities there failed to shield the interaction between nuclear charge and valence electrons Which results in greater effective nuclear charge acting on valence electrons and hence higher ionization potentials.


(b)(ii)


• The extraordinary properties of lanthanides are they resemble each other in both physical and chemical properties so closely that it is not an easy job to separate them.


(b) (iii)


• In lanthanide series of elements the most cpmmon and stable oxidation state is +3.


• Hence , for these ions Sm2+, Eu2+ and Yb2+ it is easier and sponmtaneous to loose electrons and converted to 3+ statesand gets oxidised and reduce others , whereas for Ce +4 in aq solution gain +3 state by gaining electrons and gets reduced while oxodizing others.


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