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 `(+0.34V)` in contrast to the other members of the series .
(ii) `Cr^(2+)` is reducing while `Mn^(3+)` is oxidising power in the series increases in the order `VO_(2)^(+)

Explanation

(i) `MNO_(4)^(2-)` ions dispropotionate in acidic medium to give permanganate ions and manganese (IV) oxide .
`3MnO_(4)^(2-) +4H^(+)rightarrow2MnO_(4)^(-)+MnO_(2)+2H_(2)O`
(ii) Lanthanum sulphide is formed
`2La+3Soverset(""Heat")to La_(2)S_(3)`
(ii) lathanum sulphide is firmed .
( `2La+3Soverset(".Heat".)toLa_(2)S_(3)`
(b)(i) Copper has high enthalpy of atomisation and low enthalpy of hydratio . Since the high energy to transform Cu to `Cu^(2+)` (aq) is not balanced by hydration e thalpy, thterfore `E^(@)(M^(2+)//M)` value for copper is positive `(+0.34v)`
(ii) `Cr^(2+)` is reducing as its co nnfiguration changes from the `d^(4)` to `d^(3)`, the latter 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 .
(ii) This due to increasing stabillity of the species of lower oxidation states to which they are reduced .