The Unreactive Element that is Essential for Life on Earth
About one-fifth of the atmosphere is oxygen. The remaining four-fifths consist almost entirely of nitrogen. In fact, most of the earth’s nitrogen is found in the air, very little is present in the rocks.
The Stability of Nitrogen
The extreme stability of nitrogen is attributed to the fact that the gas is made up of molecules in which the atoms are linked together in pairs, with each pair being joined by three strong chemical bonds. Nitrogen’s triple bond makes it extremely difficult to separate the atoms once they are joined together. Yet the removal of nitrogen from the air is essential to the sustaining of life on earth.
Removal of Nitrogen From the Air
A few simple, nitrogen-fixing bacteria, found in the soil, or concentrated in nodules on the roots of plants like clover, are able to remove nitrogen from the air.
The only other natural process by which atmospheric nitrogen can enter the soil in a useful form, occurs during thunderstorms. The energy released in a lightning flash is sufficient to split nitrogen and oxygen molecules into single atoms. These may combine to form nitrogen oxides, which dissolve in the rain water to make a dilute solution of nitric acid, which then reacts with minerals in soil to produce nitrates, which can be absorbed by plants.
Importance of Nitrogen in Plants
Without nitrogen, plants would be unable to synthesise the amino acids that are essential for the production of proteins. Animals would be unable to survive, as they would lose their primary source of protein.\
The nitrates removed from soil by plants are replaced largely by the breakdown of proteins and amino acids following death and decay of plants and animals. This is why compost and manure are spread onto fields. However, natural processes alone are insufficient to provide all the protein necessary for a growing world population.
Ammonia and Artificial Fertilizers
The main compound from which artificial fertilizers are made is ammonia, which is manufactured by reacting nitrogen, from air, with hydrogen at high temperatures and pressures. This ammonia is alkaline, and will react with acids to form salts like ammonium sulphate and ammonium nitrate that are excellent fertilizers.
Much manufactured ammonia is reacted with oxygen, in the presence of a platinum catalyst, to form nitrogen oxides. In a process analogous with what happens during a thunderstorm, these are converted into nitric acid.
The Oxides of Nitrogen
Molecules of nitrous oxide contain two nitrogen atoms combined with one of oxygen. For many years, this has been used as an anaesthetic to deaden pain during surgery. Its common name, laughing gas, arose from the hysterical behaviour seen among some patients following operations.
Nitric oxide molecules contain a single atom each of nitrogen and oxygen. It is a colourless gas, and relatively stable so long as it does not contact air. It is, however, a free radical, having an odd number of electrons shared between its atoms.
Immediately on exposure to air, it combines with oxygen to form the brown, acidic and highly toxic gas, nitrogen dioxide. This is also a free radical, and on cooling in ice, its molecules combine in pairs to produce the equally corrosive liquid, dinitrogen tetra-oxide. Both of these latter oxides produce nitric acid on dissolving in water.
Much nitric acid is used to make fertilizers, such as nitrates of potassium, sodium and ammonium. Sodium and potassium nitrates do not themselves burn, but on heating, decompose to release oxygen, which helps other substances to burn. They are sometimes used for this purpose in fireworks.
Explosive Nitrogen Compounds
Because they are generally less stable than the element, nitrogen compounds can decompose explosively. The rapid decomposition of ammonium nitrate is believed to have caused the explosion that wrecked a fertilizer factory in Toulouse, France, in the summer of 2001.The second most important use of nitric acid, after fertilizer manufacture, is in the making of explosives such as trinitrotoluene (TNT) and nitroglycerine.
Nitric Oxide in the Human Body
Nitric oxide molecules occur naturally inside the body where, isolated from air, they remain harmless, and indeed serve the useful purpose of relaxing muscles that surround blood vessels.
Much heart disease is caused by constrictions in the arteries that feed the heart muscle. One treatment for angina involves tablets containing small quantities of nitroglycerine. This releases, into the tissues, nitric oxide, which in turn causes the muscles surrounding the arteries to relax, allowing the artery walls to expand so that the blood can flow more freely.
It is the same mechanism, brought about by the production of nitric oxide, naturally in the body, that brings about the male mammalian response to sexual stimulation - yet another example of the ways in which compounds of this otherwise almost inert element are essential to the continuation of life on the planet.
Nitrogenous Fertilizer
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