226 | The History and Use of Our Earth’s Chemical Elements AbundanceandSource
226 | The History and Use of Our Earth’s Chemical Elements AbundanceandSource
Oxygen is the third most abundant element in the universe, making up nearly half the mass of the Earth’s crust and nine-tenths of the total mass of water. Even the mass of our bodies consists of two-thirds oxygen. Oxygen is also the most abundant element in the Earth’s atmosphere at 20.947% by volume.
Oxygen is produced commercially by liquefying air under reduced temperatures and increased pressure. Then oxygen (and other gases) can be collected as the temperature rises in the liquid air, allowing the various gases to boil off at their specific boiling points. This process is known as fractional distillation. Liquid air can be transported in vacuum vessels in the liquid form as long as there is a small vent to allow the escape of some of the gas that boils away as temperatures rise above the boiling point.
Fractional distillation is based on the principle that each element has its own temperature at which it changes from a liquid to a gas. Thus, any gas can be separated from other liquefied components of air and then collected. The same process is used in the petroleum industry to separate various fractions from the crude oil.
There are several methods of producing oxygen gas in the laboratory. Oxygen can be produced by electrolysis of water using a salt as an electrolyte that produces
hydrogen at the opposite electrode. When potassium chlorate (KClO 3 ) is heated in a test tube with a small amount of manganese dioxide (MnO 2 ) as a catalyst, the chemical reaction that releases the oxygen from potassium chlorate will be accelerated. Use of potassium nitrate (KNO 3 ) will also produce small amounts of oxygen.
A recent, and more productive, method is to pass air through fine molecular-size sieves of material that will absorb the nitrogen gas of air, which then allows the oxygen gas to pass through the sieve to be collected.
History At one time in history it was thought that air was an elemental substance. Leonardo da
Vinci (1452–1519) believed that air was composed of at least two gases. He was the first to report that air was a mixture of two gases and that one of them supported life and combus- tion.
Alchemists of the seventeenth century were aware that when metals were heated in air, they gained weight; when heated in the absence of air, metals lost weight. We now know that oxides of metals are heavier than the metals themselves.
In the eighteenth century, several chemists worked with the concept that air consists of more than one gas. Some of them produced oxygen without realizing it. They all knew that air supported life. For example, Joseph Priestley (1733–1804) produced a special gas by heating oxides of mercury and lead. He called it “dephlogisticated air.” In 1772 Carl Wilhelm Scheele (1742–1786) published information explaining how air is composed of two different gases and how one supported fire whereas the other did not. He called one of his gases “fire air” (also “empyreal air”) and the other “vitiated air.” Daniel Rutherford (1749–1819) called the gas he isolated “phlogisticated air.”
Antoine-Laurent Lavoisier (1743–1794) followed up Priestley’s work by making quantitative measurements of the ratio of oxygen to nitrogen in air. At first he named the new gas “highly respirable air” and later, “vital air.” Lavoisier is often considered the “father of modern chemistry”
227 for the experimental procedures he used and for making precise measurements and recording
Guide to the Elements |
his data when conducting investigations. Credit for naming the element was given to Lavoisier. He and other scientists of his time believed that all acids contained oxygen. Because acids smell “sharp,” Lavoisier named the element oxygen for oxys, the Greek word for “sharp.” Later it was discovered that not all acids contained oxygen (e.g., hydrochloric acid [HCl]).
CommonUses Oxygen has many uses due to its high electronegativity with the ability to oxidize many
other substances. Only fluorine has higher electronegativity and is thus a stronger oxidizer. Besides the essential use to support life, oxygen has many other uses.
It is used in the smelting process to free metals from their ores. It is particularly important in the oxygen-converter process in the production of steel from iron ore. Oxygen is used in making several important synthetic gases and in the production of ammonia, methyl alcohol, and so on. It is the oxidizer for liquid rocket fuels, and as a gas, oxygen is used in a mixture with helium to support the breathing of astronauts and divers and to aid patients who have dif- ficulty breathing. It is use to treat (oxidize) sewage and industrial organic wastes.
Oxygen has many uses because of its ability to accept electrons from other elements to form ionic bonds or to share electrons with other elements to form covalent bonds. As previously mentioned, oxygen is the by-product of photosynthesis, wherein carbon dioxide and water are transformed into food by the chlorophyll of green plants, using sunlight as energy. Carbon dioxide is produced by the decay of organic material and the burning of fossil fuels in homes, in industry, and for transportation. There is much concern about the increase of carbon dioxide in our atmosphere. There is about 0.03+% carbon dioxide in the atmosphere, and this amount has increased over the past century. It is claimed that excess carbon dioxide produced by modern society is responsible for global warming, a claim with
which not all scientists agree. At the same time, an atmosphere enriched with CO 2 enhances the growth of plants, which are ultimately the source of all our food. The respiration (breath- ing) of plants and animals also involves the intake of oxygen, followed by the process of oxida- tion, which converts food into energy that is required to sustain all life.
ExamplesofCompounds The 10 most common compounds found in the Earth’s crust are oxides. Silicone dioxide
(SiO 2 ), or common sand, makes up about half of the oxides in the crust. Oxygen, with an –2 oxidation state, reacts vigorously with group 1 (1A) metals, which have
a +1 oxidation state, and is somewhat less reactive with group 2 (2A) metals, which have a +2 oxidation state. A general formula is used for the reactions of oxygen with the group 1 ele- ments (Li, Na, K, Rb, Cs, and Fr). An upper case “M” represents the metallic element in oxi-
dation reactions: 4M 1+ +O 2- 2 → 2M 2 O. Similarly, a common formula for +2 group 2 metals (Be, Mg, Ca, Sr, Ba, and Ra) uses the “M” to represent the metals: 2M 2+ +O 2- 2 → 2MO. One of the most important oxides is dihydrogen oxide, or rather water (H 2 O). There are numerous oxygen compounds on Earth, many of them with more that two elements. They include the silicates, which make up rocks and soil, as well as limestone (calcium carbonate), gypsum (calcium sulfate), bauxite (aluminum oxide), and many iron oxides.