ü  Dioxygen (O2):  

Ø  Preparation:

(i) By heating chlorates, nitrates and permanganates.

2 KClO3 ¾® 2 KCl + 3O2 (laboratory method)

    2 KMnO4 ¾® K2MnO4 + MnO2 + O2

(ii)    By the thermal decomposition of the oxides of metals low in the electrochemical series and higher oxides of some metals.

2Ag2O(s) → 4Ag(s) + O2(g)    

2Pb3O4(s) → 6PbO(s) + O2(g)

2HgO(s) → 2Hg(l) + O2(g)          

2PbO2(s) → 2PbO(s) + O2(g)

(iii)   By the decomposition of Hydrogen peroxide (H2O2) in presence of manganese dioxide.

      2H2O2 ¾® 2H2O + O2

(iv)  On large scale it can be prepared from water or air. Electrolysis of water leads to the release of hydrogen at the cathode and oxygen at the anode. It is also obtained by the fractional distillation of air.

 

Ø Properties:

·   Colourless, odourless and tasteless gas.

·   It is paramagnetic and exhibits allotropy.

·   Three isotopes of oxygen are 168O, 178O and 188O.

·   Oxygen does not burn but is a strong supporter of combustion.

·   Dioxygen directly reacts with metals and non-metals (except with some metals like Au, Pt etc and with some noble gases). e.g.

2Ca + O2 → 2CaO 

P4 + O2 → P4O10 4

Al + 3O2 → 2Al2O3           

C + O2 → CO2

 

Ø  Uses:

·     Oxygen is used in oxyacetylene welding, in the manufacture of many metals, particularly steel.

·     Oxygen cylinders are widely used in hospitals, high altitude flying and in mountaineering.

·     Liquid O2 is used in rocket fuels.