Example 4.9

Example . . . Oxides and Oxoanions of Metals These oxides are generally formed by the reaction of metals with oxygen at high temperatures. All the metals except scandium form MO oxides which are ionic. The highest oxidation number in the oxides, coincides with the group number and is attained in Sc O to Mn O . Beyond group , no higher oxides of iron above Fe O are known. Besides the oxides, the oxocations stabilise V V as VO + , V IV as VO + and Ti IV as TiO + . As the oxidation number of a metal increases, ionic character decreases. In the case of Mn, Mn O is a covalent green oil. Even CrO and V O have low melting points. In these higher oxides, the acidic character is predominant. Thus, Mn O gives HMnO and CrO gives H CrO and H Cr O . V O is, however, amphoteric though mainly acidic and it gives VO – as well as VO + salts. In vanadium there is gradual change from the basic V O to less basic V O and to amphoteric V O . V O dissolves in acids to give VO + salts. Similarly, V O reacts with alkalies as well as acids to give VO  and VO  respectively. The well characterised CrO is basic but Cr O is amphoteric. Potassium dichromate K Cr O Potassium dichromate is a very important chemical used in leather industry and as an oxidant for preparation of many azo compounds. Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr O ) with sodium or potassium carbonate in free access of air. The reaction with sodium carbonate occurs as follows: FeCr O + Na CO + O