(a) What happens when
(i) Manganate ions `(MnO_(4)^(2-))` undergoes disproportionation reaction in acidic medium ?
(ii) Lanthanum is heated with Sulphur?
(b) Explain the following trends in the properties of the members of the First series of transition elements:
(i) ` E^(@) (M^(2+)//M)` value for copper is positive `(+ 0.34 V)` in contrast to the other members of the series.
(ii) `Cr^(2+)` is reducing while `Mn^(3+)` is oxidising, though both have `d^(4)` configuration.
(iii) The oxidising power in the series increases in the order `VO_(2)^(+) lt Cr_(2)O_(7)^(2-) lt MnO_94)^(-)`.

Explanation

(a)
(i) `MnO_(4)^(2-)` ions diSQProportionate in acidic medium to give Permanganate ion and Manganese (IV) oxide.
`3MnO_(4)^(2-)+4H^(+)toMnO_(4)^(-)+MnO_(2)+2H_(2)O`
(ii) Lanthanum sulphide if formed.
`2La+3Soverset("heat")toLa_(2)S_(3)`
(Dedult overall `(1)/(2)` mark if equation not balanced/statements not written
(b)
(i) Copper has high energy to transform `Cu(s)" to "Cu^(2+)(aq)` is not balance by hydration enthalpy, therefore `E^(0)(M^(2+)//M)` value for copper is positive (+0.34V).
(ii) `Cr^(2+)` is reducing as its configuration changes from `d^(4)" to "d^(3)`, the having more stable half filled `t_(2g)` level. On the other hand, the change from `Mn^(3+)` to `Mn^(2+)` results in extra stable `d^(5)` configuration.
(iii) This is due to the increasing stability of the SQPecies of lower oxidation state to which they are reduced.