OXYGEN (O2) PROPERTIES AND USES
Oxygen (O2) is an active, life-sustaining component of the atmosphere; making up 20% by volume of the air we breathe. It is colorless, odorless and tasteless. Oxygen is the most widely occurring element on earth. Because it forms compounds with virtually all chemical elements except the noble gases, most oxygen is bound with other elements in compounds such as silicates, oxides, and water.
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It is also dissolved in rivers, lakes, and oceans. Molecular oxygen occurs almost entirely in the atmosphere. Oxygen is highly oxidizing (a general chemical term applying to any substance, like oxygen, that accepts electrons from another substance during reaction). Oxygen reacts vigorously with combustible materials, especially in its pure state, generating heat in the reaction process.
Ozone (O3) is an allotropic form of oxygen that is more reactive than ordinary oxygen. Ozone is formed in nature by electrical discharges or by irradiation with ultraviolet light. Oxygen has a low boiling/ condensing point: -297.3°F (-183°C).
The gas is approximately 1.1 times heavier than air and is slightly soluble in water and alcohol. Below its boiling point, oxygen is a pale blue liquid slightly heavier than water.
Preparation of oxygen
Oxygen is prepared in the lab by catalytic decomposition of hydrogen peroxide using manganese (IV) oxideas shown. The gas is then collected over water.
Oxygen from the 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 -1960C, and liquid oxygen, boiling point -1830C.
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.
The process of fractional distillation involves essentially two stages.
- a) First the air must be cooled until it turns into a liquid.
- b) Then the liquid is allowed to warm up again. The various gases boil off at different temperatures.
Uses of oxygen
- Oxygen is used with fuel gases in gas welding and gas cutting
- The largest user of oxygen is the steel industry.
- Chemicals, Pharmaceuticals and Petroleum
- Oxygen is increasingly important as a bleaching chemical
- In medicine, oxygen is used during surgery, intensive care treatment, inhalation therapy, etc. High standards of purity and handling must be maintained.
- Ozone is used for drinking water treatment, in particular when alternatives, such as chlorine, are undesirable.
- Oxygen has many uses in breathing apparatus, such as those for underwater work and refinery and chemical plant self contained breathing apparatus.
- Aquaculture, the cultivation of fish in ponds uses oxygenated water to increase yields.
- Liquid oxygen is used in liquid-fueled rockets as the oxidizer for liquid hydrogen and liquid methane.
The Reactivity Series
Reaction with Air (Oxygen).
Potassium, sodium, lithium, calcium and magnesium react with oxygen and burn in air. Metals in the reactivity series from aluminium to copper react with oxygen in the air to form the metal oxide.
Aluminium is the fastest and copper is the slowest of the six.
Aluminium reacts quickly to form a surface layer of aluminium oxide.
aluminium + oxygen aluminium oxide.
4Als + 3O2g 2Al2O3s/sub>
Zinc reacts fairly quickly to form zinc oxide.
zinc + oxygen zinc oxide.
2Zns + O2g 2ZnOs
Iron reacts slowly at room temperature but quickly if it is heated.
iron + oxygen iron(III) oxide – see rusting.
4Fes + 3O2g 2Fe2O3s/sub>
Metals below iron react with oxygen when they are heated in air.
tin + oxygen tin(II) oxide.
2Sn(s) + O2(g) 2SnO(g)
lead + oxygen lead(II) oxide.
2Pb(s) + O2(g) 2PbO(s)
copper + oxygen copper(II) oxide.
2Cu(s) + O2(g/sub>) 2CuO(s)
Silver, gold and platinum do not react with oxygen in the air.
Oxygen summary Physical properties
- Makes up 20% of the gases in the air.
- Needed by the majority of living organisms for respiration
- Is produced by green plants as a by-product of photosynthesis.