Explain why : K, Rb, and Cs, on being heated in the presence of excess supply of air form superoxides into oxides and peroxides.

Explain why : Lithium on being heated in air mainly forms the monoxide and not peroxide.

Explain why is roasting of zinc blende carried out in controlled supply of air.

Explain why lithium, sodium and potassium forms monoxide, peroxide and superoxide respectively.

Lithium on being heated in air mainly forms the monoxide and not the peroxide. Why?

Name the alkali metals which form superoxides when heated with excess oxygen.

When metals A and B are heated in air, A only forms oxide but B forms both oxide and nitride. A and B are

Explain why-(a) A green layeris formed on the surface of copper which is left exposed in moist air for a long period. (b) blue colour of the copper sulphate solution is discharged slowly when an iron rod is dipped in it. (c) Galvanised irono is not used as a fondstuff container.

An organic compound (A) with molecular formula C_(8)H_(8)O forms an orange-red precipitate with 2,4-DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens or Fehlings reagent, nor does it decolourise bromine water or Baeyerâs reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C_(7)H_(6)O_(2) . Identify the compounds (A) and (B) and explain the reactions involved.

Explain why the peroxides and superoxides of the alkali metals act as strong oxidising agents.

Alkali metals on combustion in excess of air form different type of oxides depending on the size of metal cations e.g . lithium forms mainly the oxide Li_2O , sodium forms peroxide and some super oxides NaO_2 and potassium, Rubidium and caesium form the superoxides, MO_2 . The stability of peroxide and superoxide increases, as the size of the metal cation increases because of the stabilisation of larger anion by larger cations through lattice energy effect. These oxides are easily hydrolysed by water. The oxides and the peroxides are colourless when pure, but the superoxides are yellow or orange in colour. Peroxide and superoxide of sodium and potassium are widely used as oxidising agent and as air purifier in space capsules, submarines and breathing mask. When sodium reacts with excess of oxygen at 350^@ C , the oxidation number of oxygen per atom changes from: