. Important compound of Transition elements Oxides and Oxoanions of Metals Generally, transition metal oxides are formed by the reaction of transition metals with molecular oxygen at high temperatures. Except the first member of 3d series, Scandium, all other transition elements form ionic metal oxides. The oxidation number of metal in metal oxides ranges from + to + .
As the oxidation number of a metal increases, ionic character decreases, for example, Mn O is covalent. Mostly higher oxides are acidic in nature, Mn O dissolves in water to give permanganic acid (HMnO ) , similarly CrO gives chromic acid (H CrO ) and dichromic acid (H Cr O ). Generally lower oxides may be amphoteric or basic, for example, Chromium (III) oxide - Cr O , is amphoteric and Chromium(II) oxide, CrO, is basic in nature. Potassium dichromate K Cr O Preparation: Potassium dichromate is prepared from chromite ore.
The ore is concentrated by gravity separation. It is then mixed with excess sodium carbonate and lime and roasted in a reverbratory furnace. FeCr O + Na CO + O Na CrO - C → + Fe O + CO ↑ The roasted mass is treated with water to separate soluble sodium chromate from insoluble iron oxide. The yellow solution of sodium chromate is treated with concentrated sulphuric acid which converts sodium chromate into sodium dichromate.
2Na CrO + H SO Na Cr O sodium chromate (yellow) sodiu m dichromate (orange red) + Na SO + H O The above solution is concentrated to remove less soluble sodium sulphate. The resulting solution is filtered and further concentrated. It is cooled to get the crystals of Na SO .2H O. The saturated solution of sodium dichromate in water is mixed with KCl and then concentrated to get crystals of NaCl.
It is filtered while hot and the filtrate is cooled to obtain K Cr O crystals. XII U4-D-Block-Jerald