Oxygen is the most abundant element in the earth crust (46.5%). It was discovered by Karl Scheele and Joseph Priestley. It occurs in three isotopic forms :
8O16 8O17 8O18
(Abudance: 99.76%)
(Abudance: 0.037%) (Abudance: 0.204%)
Out of the three isotopes, 8O8
is radioactive.
Occurrence : In free state, it occurs in air and constitutes 21% by volume of air.
Preparation of Dioxygen : Oxygen is prepared by the following methods.
(1)By the decomposition of oxygen rich compounds: e.g.
2KNO3 ————> 2KNO2 + O2; 2KClO3 ————> 2KCl + 3O2
Pot. Nitrate Pot. Chlorate MnO2
2) By heating dioxides, Peroxides and higher oxides : e.g.
2Ag2O ————> 4Ag + O2 ; 3MnO2 ————> Mn3O4 + O2
Silver oxide
Magnesium dioxide
Heat
2BaO2 ————> 2BaO + O2
Barium peroxide Barium dioxide
(3) Laboratory Method:In the laboratory,
O2 is prepared by thermal decomposition of potassium chlorate.
420 K
2KClO3 ————> 2KCl + 3O2
MnO2
In the absence of
MnO2 catalyst, the decomposition takes place at 670-720 K. Therefore,
MnO2 acts as a catalyst and also lowers the temperature for the decomposition of KClO3.
(4)
O2 can also be prepared by the action of water on sodium peroxide as, 2Na2O2 + 2H2O —> 4NaOH + O2
.
(5)Industrial preparation : The main sources for the industrial preparation of dioxygen are air and water.
(i) From air :
O2 is prepared by fractional distillation of air. During this process, N2
with less boiling point (78 K) distills as vapour while O2
with higher boiling point (90 K) remains in the liquid state and can be separated.
(ii) From water :
O2 can also be obtained by the electrolysis of water containing a small amount of acid or alkali,
2H2O ———> 2H2(g) + O2(g). |
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