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OXYGEN

*Occurrence : oxygen occurs free in the atmosphere of which it forms 20.8 % by volume ,combined with hydrogen in water it constitutes 86% of the ocean , oxygen combined with other elements is present in many minerals  nearly 50% of the earths crust is oxygen.

*Laboratory preparation : oxygen is prepared on a small scale by heating a mixture of 5 parts of potassium chlorate with 1 part of manganese dioxide. The potassium chlorate alone decomposed , the manganese dioxide acts catalytically:

2KClO₃ = 2KCl + 3O₂

The oxygen may collected over water in which it is only slightly soluble. If required dry it is passed over calcium chloride or phosphorus pentoxide

Very pure Oxygen is prepared by heating silver oxide made by the action of potassium hydroxide or silver nitrate solution:

2KOH + 2AgNO₃= 2KNO₃ + Ag₂O +H₂O

Or by the thermal decomposition of pure potassium permanganate followed by liquefaction and fractionation.

*Industrial preparation : oxygen is prepared on the  Industrial scale by :

 1- the fractional distillation of liquid air

2- electrolysis of sodium hydroxide solution.

On a small scale oxygen is prepared in the gaseous state and stored in steel cylinders, on a large scale it is collected and transported in  the liquid form .

Other reactions in which oxygen is librated: oxygen is librated by*

1- The thermal decomposition of the following substances:

a- Oxides of electropositive metals, such as Ag₂O, HgO, oxides of copper, zinc, and more basic metals are not decomposed by heat.

b- dioxides and peroxides of metals, such as BaO₂ , PbO₂ , and the higher oxides of non- metals such as H₂O₂, N₂O₅

c- salts containing anions rich in oxygen , such as NaNO₃ , KMnO₄ , Na₂S₂O₈.

2- The action of water on sodium peroxide

2Na₂O₂ +2H₂O= 4NaOH +O₂

3- The action of warm dilute sulphuric acid on potassium permanganate.

4KMnO4 + 6H2SO₄ = 2K2SO4 + 4MnSO4 + 6H2O + 5O₂

4- The action of potassium permanganate on an acetified solution of hydrogen peroxide.

2KMnO₄ + 5H₂O₂ + 3H₂SO₄ = K₂SO₄ + 2MnSO₄ + 8H₂O + 5O₂

Properties of oxygen : *

Oxygen is colorless, tasteless and odourless gas . It condenses to a blue liquid which freezes to a blue solid if cooled in a tube immersed in liquid hydrogen. Liquid and solid oxygen are strongly magnetic.

Oxygen combined with many elements and compounds , both metallic and non metallic, slowly at low temperature , and vigorously with the evolution of light and heat if the temperature is raised. Magnesium , sulphur, phosphorus, and carbon burn in oxygen with extreme brilliance to form their respective oxides.

Oxygen combines  fairly rapidly in the cold with the following reagents:

1- With nitric oxide oxygen gives brown fumes of nitrogen dioxide (this reaction is used as a test for oxygen):

2NO + O₂ = 2NO₂

2- Moist white phosphorus pentoxide( this reaction is used to removed traces of oxygen from inert gases such as nitrogen).

3- A solution of potassium pyrogallate in potassium hydroxide absorbs oxygen( this reaction is used in gas analysis) The absorption of oxygen by a solution of sodium hyposulphite is also in gas analysis.     

2Na₂S₂O₄ + O₂ = 2Na₂S₂O₅

*Uses of oxygen : oxygen is used on a large scale manufacture, in oxy- acetylene burners, in the organic chemical industry, and in medicine.