CBSE Class 12 Chemistry 2017 Outside Delhi Set 1 Solved Paper

Question : 24

Total: 26

(a) Account for the following:
(i) Transition metals show variable oxidation states.
(ii) Zn,Cd and Hg are soft metals.
(iii) E∘ value for the Mn3+∕Mn2+ couple is highly positive (+1.57V) as compared to Cr3+∕Cr2+.
(b) Write one similarity and one difference between the chemistry of lanthanoid and actinoid elements.
OR
(a) Following are the transition metal ions of 3d series:
Ti4+,V2+,Mn3+,Cr3+
(Atomic numbers: Ti=22,V=23,Mn=25, Cr=24 )
Answer the following :
(i) Which ion is most stable in an aqueous solution and why?
(ii) Which ion is a strong oxidising agent and why?
(iii) Which ion is colourless and why?
(b) Complete the following equations:
(i) 2MnO4−+16H++5S2−→
(ii) KMnO4

‌ heat ‌

→

Solution:

(a) (i) Transition metals show variable oxidation states because of the small difference in energy of the ns and (n−1)d orbitals. Thus, addition to ns electrons, (n−1)d electrons also participate in bonding.
(ii) Zn,Cd and Hg are not transition elements but are regarded as soft metals as they easily oxidize to +2 oxidation state with low melting point.
(iii) Because Mn2+ has d5 as a stable configuration whereas Cr3+ is more stable due to stable t2g3.
(b) Similarity-both are stable in +3 oxidation state/ both show contraction/ irregular electronic configuration (or any other suitable similarity)
Difference-actinoids are radioactive and lanthanoids are not / actinoids show wide range of oxidation states but lanthanoids don't (or any other correct difference).
OR
(a) (i) Cr3+ is the most stable in aqueous solution as the tripositive ions with increasing atomic number.
(ii) Mn3+ as they have the highest E∘ (M3+∕M2+).
(iii) Ti4+ is colourless.
Electronic configuration : 1S22S22p63S23p6 4S23d∘
It has completely empty d-orbital and there are no electrons for the d−d transition.
(b) (i) 2MnO4−+16H++5S2−→5S +2Mn2++ 8H2O
(ii) 2KMnO4→K2MnO4+MnO2+O2

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