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Preparation, Properties, Uses - Oxygen | 12th Chemistry : UNIT 3 : p-Block Elements-II

Chapter: 12th Chemistry : UNIT 3 : p-Block Elements-II


The atmosphere and water contain 23% and 83% by mass of oxygen respectively.



The atmosphere and water contain 23% and 83% by mass of oxygen respectively. Most of the world’s rock contain combined oxygen. Industrially oxygen is obtained by fractional distillation of liquefied air. In the laboratory, oxygen is prepared by one of the following methods.

The decomposition of hydrogen peroxide in the presence of catalyst (MnO2) or by oxidation with potassium permanganate.

2H2 O2 ↔ 2H2O + O2

5H2O2 + 2MnO4 + 6H+ 5O2 + 8H2O + 2Mn2+

The thermal decomposition of certain metallic oxides or oxoanions gives oxygen.


Under ordinary condition oxygen exists as a diatomic gas. Oxygen is paramagnetic. Like nitrogen and fluorine, oxygen form strong hydrogen bonds. Oxygen exists in two allotropic forms namely dioxygen (O2) and ozone or trioxygen (O3). Although negligible amounts of ozone occurs at sea level it is formed in the upper atmosphere by the action of ultraviolet light. In the laboratory ozone is prepared by passing electrical discharge through oxygen. At a potential of 20,000 V about 10% of oxygen is converted into ozone it gives a mixture known as ozonised oxygen. Pure ozone is obtained as a pale blue gas by the fractional distillation of liquefied ozonised oxygen.

The ozone molecule have a bent shape and symmetrical with delocalised bonding between the oxygen atoms.

Chemical properties:

The chemical properties of oxygen and ozone differ vastly. Oxygen combines with many metals and non-metals to form oxides. With some elements such as s-block elements combination of oxygen occurs at room temperature. Some of less reactive metals react when powdered finely and made to react exothermically with oxygen at room temperature but a lump of metal is unaffected under same condition. These finely divided metals are known as pyrophoric and when set the powder on fire, heat is liberated during a reaction.

On the other hand ozone is a powerful oxidising agent and it reacts with many substances under conditions where oxygen will not react. For example, it oxidises potassium iodide to iodine. This reaction is quantitative and can be used for estimation of ozone.

O3 + 2KI + H2O → 2KOH + O2 + I2

Ozone is commonly used for oxidation of organic compounds. In acidic solution ozone exceeds the oxidising power of fluorine and atomic oxygen. The rate of decomposition of ozone drops sharply in alkaline solution.


·           Oxygen is one of the essential component for the survival of living organisms.

·           It is used in welding (oxyacetylene welding)

·           Liquid oxygen is used as fuel in rockets etc...


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