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)^(+)

what happens when : (i) Manganate ions (MnO_(4)^(2)) undergoes dispro

1.	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)^(+)
Answer» SOLUTION : (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 .