OXYGEN
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12. Oxygen (ISBN 1-869860-
54-3) Contents: Introduction; Oxygen in the air; Oxygen in water; Oxygen in minerals and rocks; Ozone; The oxygen cycle; Making oxygen chemically; Making oxygen using electricity; Oxidising agents; Colours; Oxidation/reduction; Oxygen in iron and steelmaking; Refining; Corrosion; Preventing corrosion; Combustion; Spontaneous combustion; Combustion of fuels; Internal combustion; Smog: oxidation of exhaust fumes; Key facts about oxygen; The Periodic Table; Understanding equations; Glossary of technical terms; Index |
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Oxygen is the most common element by volume or mass (weight) on Earth. In each breath you take, one-fifth of the molecules are oxygen; thereare twelve trillion tonnes of oxygen in the air. Yetdespite the fact that it is so common, we are hardly aware of this vital element because oxygen istransparent, it has no colour, taste or smell. Oxygen occurs uncombined (as one of a mixture of gases) in air, but it also readily combines (reacts) with a wide variety of other elements to make compounds. Indeed, oxygen is among the most reactive of all the elements. For example, nearly allthe rocks of the Earth are compounds containing oxygen. Water is also a compound of oxygen along with hydrogen. Oxygen is vital for life. When oxygen reacts with a fuel, the fuel is oxidised and becomes an oxide, sometimes turning into a solid, in other cases forming a gas such as carbon dioxide. Thisreaction called burning releases heat energy. The reaction of oxygen with fuels such as coal, oil and natural gas can also be harnessed to convertthe chemical energy in the fuel into movement energy. This is how we power many ofour machines. All kinds of fuels can "burn" in the sense of being oxidised by oxygen, although not all give out large amounts of heat. The carbohydrates in our food, for example, oxidise to release energy for our bodies. When dead organic material decays, it is actually oxidising. Oxidation also happens to food left exposed to the air and to the cells in our bodies. Oxygen reacts with most metals. Sometimes theeffects cannot be seen, such as when an invisible oxide film forms on the surface of a sheetof aluminium. But other metals react more dramatically. Iron, for example, reacts with oxygen in the presence of water to produce the familiar flakes of rust, while potassium instantly catches fire. The formula for oxygen gas is written as
O2. This means that oxygen gas (like most other gases)
normally occurs as a molecule, made of two atoms, rather than as
a single atom. Most single atoms of oxygen occur high
in the atmosphere. Oxygen can also occur as a
molecule with three atoms, in which case the chemical symbol
is Oxygen in the air Air is a gas made up chiefly of nitrogen and oxygen. Oxygen (O2) makes up 21% of the atmosphere by volume and 23% by mass (weight). The weight of oxygen molecules in the atmosphere helps to create the air pressure on the Earth's surface, on average one kilogram on every square centimetre. Although ozone (O3) is only present in concentrations as little as12 parts per million, it is present both in the upper atmosphere (seepage 12) and close to the ground. The composition of clean air. Oxygen in water In the past, oxygen atoms combined with hydrogen atoms to form water. Oxygen is a much more massive element than hydrogen (the atomic weight of hydrogen is 1, that of oxygen is 16). As a result, oxygen makes up about nine-tenths of the mass of the oceans. The oxygen in the compound water is very strongly bonded to the hydrogen atoms and it will not break up even on heating. This is why when water boils the molecules of water change from liquid water to water vapour, rather than breaking up into hydrogen and oxygen atoms. The modern atmosphere and oceans contain almost no free hydrogen, so oxygen and hydrogen can no
longer combine. Becausethere is a surplus of oxygen
atoms, theatmosphere contains free oxygen. Oxygenalso
occurs as free oxygen molecules dissolved in water.
These molecules do not combine with the water molecules.
In most surface water there may be 45 grams of
oxygen dissolved in every cubic metre of water. These are
the oxygen molecules that are vital for life in the water. Also Oxygen is less common than hydrogen, helium,
and neon in the Universe, but by far the most
common element on the Earth. Itmakes 90% of water (bymass), about 47%
of theEarth's crust, as well as23% of dry air and
65% ofthe human body. Also The chemical symbol for water is normally written H2O, meaning that water molecules contain three atoms: two hydrogen atoms bound to each oxygen atom. The bonds are very strong and rarely break. However, a few water molecules ionise, that is the neutral molecules break up into oppositely charged particles hydrogen (H+(aq)) ionsand hydroxide (OH-(aq)) ions. The ions in a liquid allow it to behave asan electrolyte a carrier of
electric current. Because water ionises so poorly itis a poor
conductor of electricity. Thisiswhy, for example, pure
water cannot be used alone in a vehicle battery. Compounds that readily ionise
(substances like dilute sulphuric acid) have to be
added to improve the conductivity. Ozone Ozone (O3), named for the Greek word for "smell" is a poisonous, colourless and tasteless gas with a distinctive strong smell. Molecules of ozone are probably the source of the smell that can be detected close to working electrical equipment such asmotors and TVs. If a vehicle with a catalytic converter is startedcold, ozone can be detected in the exhaust fumes. The ozone layer Most ozone is found high in the atmosphere, in a region of the stratosphere called the ozone layer. Here ozone performs a vital life-protecting role, absorbing the ultraviolet rays of the Sun that would be harmful to both plants and animal life. The number of ozone molecules at this level is
very small. If brought down to the ground surface they
would EQUATION: Reaction of CFCs with ozone 1 CFC + ultraviolet light = chlorine CFC(g) + UV light = Cl(g) 2 Chlorine atom + ozone = chlorine oxide + oxygen Cl(g) + O3(g) = ClO(g) + O2(g) 3 Chlorine oxide + oxygen atom = chlorine atom + oxygen ClO(g) + O(g)
= Cl(g) + O2(g) When chlorofluorocarbons (CFCs) reach the
stratosphere they are broken down by ultraviolet solar radiation
releasing chlorine atoms. These atoms attack the ozone molecules in
the air, but the chain reaction that occurs releases a
chlorine atom at the end. Thus a single chlorine atom can survive in the
upper atmosphere for four to ten years. During that time it can
destroy countless ozone molecules. The oxygen cycle
There is a continual exchange of oxygen between the It begins with the reservoir of carbon dioxide in
the air. The process of photosynthesis uses carbon dioxide
and water from
the soil to produce cellulose, the material from
which plants are made. This releases oxygen gas into the
air. Photosynthesis ceases when darkness falls and plants
then
burn off some of the cellulose they have made,
returning carbon dioxide to the atmosphere.
Animals use oxygen in the atmosphere for
respiration, oxidising the sugars in their food to give energy
and releasingcarbon dioxide to the atmosphere.
When dead tissue (carbon compounds) decays by
a combination of oxidation and microorganism decay, >
carbon dioxide is released back to the atmosphere.
A slower cycle occurs whenever mineral matter
is oxidised, such as in the formation of rocks. Carbon dioxide + water = glucose + oxygen
6CO2(g) + 6H2O(l)
=
C6H12O6(s) +
6O2(g) Glucose + oxygen = carbon dioxide + water
C6H12O6(s) +
6O2(g) =
6CO2(g) + 6H2O(l) The oxygen cycle.
Preparaing oxygen
Pure oxygen has a large number of uses in the
modern world. It is used, forexample, in medicine, but also
on amuch larger scale for making fuels burn more effectively, such as in aniron-making blast furnace or
in an oxyacetylene welding and cutting lance.
On a large industrial scale, atmospheric air is
cooled. At -183°C oxygen liquefies; it can then be stored
intanks and transported easily.
Laboratory preparation
In the school laboratory oxygen is made byreacting
two chemicals together or bycausing an oxygen-rich compound todecompose.
It is common to use the apparatus shown on this
page for small samples. Here an oxygen-rich
compound, hydrogen peroxide in solution in water is dripped
onto manganese oxide. The manganese oxidecauses
the hydrogen peroxide to decompose and release oxygen
and water. The oxygen is collected in a gas jar over
water using a support called a beehive shelf.
An alternative procedure involves heating a compound (for example potassium chlorate)
which liberates oxygen as it is heated (i.e. as it decomposes). Test for oxygen: rekindles a glowing splint EQUATION : Production of oxygen using hydrogen peroxide
Hydrogen peroxide = oxygen + water
2H2O2(aq) =
O2(g) + 2H2O(l)
catalyst Properties of oxygen
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