Question

(a) Complete the following chemical equations `:`
(i) `Cr_(2)O_(7)^(2-) (aq) + H_(2)S(g) + H^(+) (aq) rarr`
(ii) `Cr^(2+) (aq) +I^(-) (aq) rarr`
(b) How wouldyou accound for the following ?
(i) The oxidizing power is the order `: VO_(2)^(+) lt Cr_(2)O_(7)^(2-) lt MnO_(4)^(-)`
(ii) The third ionization enthalpy ofmanganese `(Z= 25)` is exceptionally high .
(iii) `Cr^(2+)`is stronger reducting agent than `Fe^(2+)`.
or (a) Complete the following chemical equations `:`
(i) `MnO_(4)^(-)(aq) +S_(2)O_(3)^(2-) (aq) + H_(2)O(l) rarr`
(ii) `Cr_(2)O_(7)^(2-)(aq) + Fe^(2+) (aq) + H^(+)(aq) rarr`
(b) Explain the follwoing observations `:`
(i) `La^(3+) (Z= 57) ` and `Lu^(3+) (Z= 71)` do not show any colour in the solution.
(ii) Among the divalent cations in the first series of transition elements, manganese exhibits the maximum paramagnetism
(iii) `Cu^(+)` ion is not known in aqueous solutions.

Answer 1

(a) (i) `Cr_(2) O_(7)^(2-) (aq) + 3H_(2)S(g) + 8H^(+)(aq) rarr 2Cr^(2+) (aq) +7H_(2)O(l) + 3S(s). ` (ii) `2Cu^(2+)(aq) +4I^(-)(aq) rarr Cu_(2)I_(2)(s) +I_(2)(g)`
(b) (i)This is due to increasing stability of the lower species to which they are reduced.
(ii) Because 3rd electron has to be removed from stable half-filled3d-orbitals `(._(25)Mn= 3d^(5) 4s^(2))`.
(iii)`E^(@) ( Cr^(3+) //Cr^(2+))-ve(0.41V) ,E^(@)(Fe^(3+) //Fe^(2+))= +ve( +0.77V)`
Hence, `Cr^(2+)` is easily oxidized to `Cr^(3+)` but `Fe^(2+)` cannot be easily oxidized to `Fe^(3+)`]
or (a) (i) `8 MnO_(4)^(-)(aq) + 3S_(2)O_(3)^(2-) (aq) + H_(2)O(l) rarr 8 MnO_(2)(s) +6 SO_(4)^(2-) + 2OH^(-) (aq) `
(ii) `Cr_(2) O_(7)^(2-) (aq) +6Fe^(2+)(aq)+ 14H^(+)(aq) rarr 2Cr^(3+)(aq) + 6Fe^(3+) (aq) + 7H_(2)O(l)`
(b) (i) Becaue they have completely filled 4f subshell. (II) `Mn^(2+)` has maximum number of unpaired electrons.
(iii) Because `Cu^(+)` in aqueous solution undergoes disproportionation `: 2 Cu^(+)(aq) rarr Cu^(2+)(aq) +Cu(s)`
(as `E^(@)` value is favourable for this reaction ) ]