Oxygen

Oxygen, O2, is a colourless odourless gaseous main group element which belong to Group VIb of the periodic table.

Atomic Number : 8
Atomic Mass : 15.9994
Melting Point : -214 degC
Boiling Point : -183 degC
Density : 1.429

Atmospheric oxygen is of vital importance for all aerobic organisms. For industrial purposes, oxygen is obtained by fractional distillation of liquid air. It is used in metallurgical processes, in high-temperature flames and in breathing apparatus.

Discovery

The discovery of Oxygen was credited to Priestley in 1774 AD. However in a paper looking into Alchemy, by Richard Brzezinski, an expert in the history of science and Zbigniew Szydlo, a chemistry lecturer, published in the authoritative magazine History Today credit the discovery of Oxygen to a Polish alchemist called Michael Sendivogius who found that heated saltpeter produced "the elixir of life" and who, in 1604, described his experiments in a book regarded as so authoritative that it found its way into every major scientific library in Europe. They say that Priestley, would surely have had access to it. Cornelis Drebbel a Dutch inventor employed by the King of England James 1 in 1621 used Sendivogius work which was about 150 years before Priestley was credited with the discovery of Oxygen. Drebbel built a submarine which was manned by 12 oarsmen, made of wood and waterproofed by a coat of greases leather. It successfully traveled along the River Thames from Westminster to Greenwich, at a dept of 15 ft. The trip, and the method used to keep the oarsmen alive, was subsequently verified by Robert Boyle.

Occurrence

Oxygen occurs in the free state as a gas, to the extent of 21 per cent by volume or 23 per cent by weight in the atmosphere.

Combined Oxygen also occurs

in water,
in vegetable and animal tissues,
in nearly all rocks and
in many minerals.

Oxygen occurs to a larger extent in the earth's crust than any other element.

Extraction

Preparation

Because oxygen is a component of air, it has been studies extensively over the centuries and there is a large number of different methods for its preparation.

The most convenient method for preparing oxygen in the laboratory involves either the catalytic decomposition of solid potassium chlorate or the catalytic decomposition of hydrogen peroxide.

Preparation of oxygen Using potassium chlorate

Potassium chlorate decomposes at a low temperature if previously mixed with manganese dioxide which is a catalyst for the decomposition. Only the potassium chlorate is decomposed, and no perchlorate is formed :


		2 KClO3   ==>   2 KCl  +  3 O2

Preparation of oxygen using hydrogen peroxide

The decomposition of hydrogen peroxide using manganese dioxide as a catalyst also results in the production of oxygen gas.


		2 H2O2   ==>   2 H2O   +   O2

Preparation of oxygen by electrolysis of water

The electrolysis of acidified water is carried out in a Hofmann Voltameter. Oxygen is evolved at the positive electrode in the electrolysis.


		2 H2O   ==>   2 H2   +   O2

A solution of barium hydroxide with nickel electrodes may also be used. However, on prolonged electrolysis an explosive mixture of oxygen and hydrogen may be evolved at the positive electrode.

Preparation of oxygen by the chemical decomposition of water

Oxygen is obtained from water by passing a mixture of steam and chlorine through a strongly heated silica tube containing pieces of broken porcelain.


		2 H2O   +   2 Cl2   ==>   4 HCl   +   O2

The hydrogen chloride is removed by a wash-bottle containing sodium hydroxide solution and the Oxygen collected over water.

Preparation of oxygen By decomposition of oxides

Oxygen may be obtained by heating some metallic oxides.

When mercuric oxide is strongly heated in a hard glass tube, it decomposes, globules of mercury collect in the cooler part of the tube and oxygen gas is evolved. It may be collected over mercury in a pure and dry state.
```
		2 HgO   ==>   2 Hg   +   O2	
```
Silver oxide decomposes at a lower temperature than mercuric oxide. When the silver oxide is prepared by precipitation from a solution of silver nitrate by a solution of pure potassium hydroxide in absence of atmospheric carbon dioxide the silver oxide formed gives very pure oxygen.
```
		2 Ag2O   ==>    4 Ag   +   O2
```
Many higher oxides of metals decompose on heating forming the lower oxides and oxygen gas.
Manganese dioxide evolves oxygen more readily when heated with concentrated sulphuric acid.
```
             2 MnO2  +  2 H2SO4   ==>   2 MnSO4  +  2 H2O  +  O2		
```

Preparation of oxygen by the decomposition of salts

Some salts containing oxygen decompose and release oxygen gas on heating.

Potassium nitrate melts on heating and at a slightly high temperature decomposes, giving off bubbles of oxygen and forming potassium nitrite which solidifies on cooling.
```
	2 KNO3   ==>   2 KNO2   +   O2	
```
Potassium chlorate crystals melt when heated in a hard glass tube at 360 degC and then decompose to form potassium chloride and releasing oxygen.
```
		2 KClO3   ==>   2 KCl   +   3 O2	
```
Potassium permanganate which is a purple crystalline solid, decomposed without fusing on heating to 240 degC, forming a black powder consisting of a mixture of potassium manganate and manganese dioxide and releasing oxygen.
```
	2 KMnO4  ==>    K2MnO4   +   MnO2   +   O2	
```
Potassium permanganate explodes violently when heated with concentrated sulphuric acid. However, when a solution of hydrogen peroxide is mixed with a solution of the permanganate and diluted sulphuric acid added, the two compounds decompose together, forming a nearly colourless solution, and oxygen is evolved.
```
      2 KMnO4   +  3 H2SO4  +  5 H2O2  ==>  K2SO4  +  2MnSO4  +  8H2O  +5O2	
```
Chromic trioxide which is a red crystalline solid, melts on heating at about 420 degC, leaving a green residue of chromic oxide and evolves oxygen.
```
		4 CrO3   ==>   2 Cr2O3   +   3 O2	
```
Potassium dichromate which is a bright-red crystalline solid, melts on heating and when strongly heated releases oxygen leaving a mixture of yellow potassium chromate which is soluble in water, and green chromic oxide, which is insoluble in water.
```
	
             4 K2Cr2O7   ==>   4 K2CrO4  +  2 Cr2O3  +  3 O2	
```

Chromium trioxide and potassium dichromate when heated with concentrated sulphuric acid forms chromic sulphate and releases oxygen.


              4 CrO3   +   6 H2SO4  ==>   2 Cr2(SO4)3  +  6 H2O  +3 O2	

        2 K2Cr2O7  +  10H2SO4   ==>  4 KHSO4  +  2 Cr2(SO4)3  +  8 H2O  +3 O2

Preparation of oxygen from air

Oxygen may be obtained from the atmosphere in a chemical process, by heating mercury in a confined volume of air, when the oxygen reacts with the mercury to form mercuric oxide. The mercuric oxide so formed is then heated strongly, when it decomposes and pure oxygen is evolved.

In a similar process, if yellow lead monoxide is carefully heated in an iron dish and freely exposed to air, it takes up oxygen from the air and forms red lead.


		6 PbO   +   O2    ==>   2 Pb3O4	
		Yellow		Red	
		Lead		Lead	
		Monoxide

On heating strongly, the red lead decomposes into lead monoxide and Oxygen gas which is evolved.


		2 Pb3O4   ==>   6 PbO  +  O2

Manufacture

Various methods have been used for the large scale production of oxygen, but at present the two mostly used are the electrolysis of an aqueous solution of dilute sulphuric acid, and the fractional distillation of Liquid Air.

Manufacture from liquefied air

Oxygen may be obtained from the atmosphere by the liquefaction and fractional distillation of air. Liquid air is a mixture of liquid nitrogen, boiling point -196 degC, and liquid oxygen, boiling point -183 degC. The nitrogen is more volatile (i.e. it has a lower boiling point) and boils off first during evaporation. Because some oxygen evaporates with the nitrogen, separation of the two gases is brought about by fractionation (i.e. by letting the evolved gas mixture bubble through liquid air rich in oxygen in a tall rectifying column). The oxygen in the gas mixture condenses and almost pure nitrogen gas leaves the top of the column, leaving almost pure liquid oxygen which is then evaporated to give oxygen gas. The oxygen gas is distributed as a compressed gas in high pressure cylinders.

Properties

Oxygen is

a colourless gas, without smell or taste,
is slightly heavier than air,
is sparingly soluble in water,
is difficult to liquefy, boiling point -183 degC, and the liquid is pale blue in colour and is appreciably magnetic. At still lower temperatures, light-blue solid oxygen is obtained, which has a melting point of -218.4 degC.

Reactions

Uses

Oxygen is essential for life and it takes part in processes of combustion, its biological functions in respiration make it important.

Oxygen is sparingly soluble in water, but the small quantity of dissolved oxygen in is essential to the life of fish.

Oxygen gas is used with hydrogen or coal gas in blowpipes and with acetylene in the oxy-acetylene torch for welding and cutting metals.

Oxygen gas is also used in a number of industrial processes.

Medicinally, oxygen gas is used in the treatment of pneumonia and gas poisoning, and it is used as an anesthetic when mixed with nitrous oxide, ether vapour, etc..
Carbon Dioxide is often mixed with the oxygen as this stimulates breathing, and this mixture is also used in cases of poisoning and collapse for restoring respiration.

Liquid oxygen mixed with powdered charcoal has been used as an explosive.

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