Chemical Education Today
NCW 2003: Earth’s Atmosphere and Beyond
Oxygen—Abundant and Essential by Carolyn S. Quinsey
What is colorless, odorless, tasteless, aggressive, necessary for human life, and provides protection from the sun? While it may sound like a revolutionary new product, it describes the element oxygen. Oxygen is the most abundant element in Earth’s crust and makes up approximately twothirds of the human body by mass. This essential element was the focus of Oxygen, a public symposium held at the University of Wisconsin–Madison (1). In conjunction with the symposium, the play Oxygen, written by the renowned chemists Carl Djerassi and Nobel Laureate Roald Hoffmann, was presented by the university’s Department of Theater and Drama. The play centered on the three famous scientists who aided in the discovery of oxygen: Antoine Lavoisier, Joseph Priestley, and Carl Scheele. Rare books, including original works by these three scientists, were also on display in the library. The Oxygen Symposium The symposium included eight presentations on topics related to oxygen, 11 demonstrations recounting various properties and behaviors of oxygen, and music by Mozart— a contemporary of Lavoisier, Priestley, and Scheele (1). These coordinated events aimed to make evident the connections between science and the arts as well as to make the public aware of the wonderful and essential element, oxygen. Bassam Shakhashiri, director of the Wisconsin Initiative for Science Literacy, opened the symposium. His advice to attendees was to “watch the electrons” for a full understanding of the wonders of oxygen. While it is quite impossible to see oxygen’s electrons, his advice was helpful in keeping in mind oxygen’s love for electrons.
Oxygen and the Aging Process. Richard Weindruch Richard Weindruch spoke about metabolic rate theory and oxygen free radicals, natural byproducts of oxygen used to generate energy in cells. While slowing the aging process is of interest now, it was also of interest nearly a century ago. Max Rubner, a German scientist, proposed the metabolic rate theory in 1908 (2). An accepted theory in today’s science, it suggests that mammals with a lower metabolic rate live longer. Metabolic rate correlates with rate of oxygen consumption by the body; an increase in metabolic rate indicates an increased rate of oxygen consumption. But how are oxygen consumption and the aging process linked? The answer lies in the fact that small fractions of oxygen in the cells become partially reduced and form free radicals, which cause oxidative stress to DNA, RNA, proteins, and fats. The main targets of these oxygen free radicals are post mitotic (after mitosis) cells, such as brain, heart, and skeletal muscle cells. These cells are targets because they 1124
rely heavily on mitochondrial oxidative energy metabolism to make ATP (adenosine triphosphate, a molecule that stores energy in cells). The effects of these oxygen free radicals can result in conditions such as Alzheimer’s and Parkinson’s diseases, heart failure, and muscle wasting. Weindruch suggested that perhaps a key factor to slowing the aging process is decreasing metabolic rate and oxygen consumption. Richard Weindruch is a Professor at the University of Wisconsin Medical School. Further information on his work in caloric restriction is available (2).
Where Did Oxygen Come From? Patricia Kiley The air we breathe contains 21% molecular oxygen, 78% molecular nitrogen, and 1% argon and carbon dioxide by volume. Oxygen is necessary for the inhabitants of Earth, and Patricia Kiley addressed its use in the body as well as its origin. The body uses oxygen for power in mitochondria, the tiny intracellular structures that function as the “power plants” of cells. In the mitochondria, molecular oxygen is reduced to water, and energy is stored in the form of ATP. When ATP is broken down, a large quantity of energy is released and can be used by the cell. ATP is made in the matrix, or inner spaces, of the mitochondria. When energy and hydrogen ions create a gradient across the membrane of the matrix, and as the hydrogen ions pass through a channel in the membrane of the matrix, ADP (adenosine diphosphate) combines with another phosphate to yield ATP (adenosine triphosphate). Our bodies are dependent on oxygen—we would be unable to live without it. When we exercise, we breathe heavily to get more oxygen to make extra ATP to power our working muscles. The brain uses approximately one-fifth of all the oxygen consumed by the body: insufficient oxygen to the brain can cause a stroke. The heart also needs oxygen: if a coronary artery (located on the surface of the heart) is blocked, the lack of oxygen causes the person to suffer a heart attack. Oxygen needs to be continuously produced so our bodies don’t run out. Luckily, plants provide oxygen for us. Plants use our exhaled carbon dioxide to produce oxygen, which we, in turn, inhale. Approximately 50% of all oxygen comes from the plants in tropical rainforests. Cutting these rainforests down may limit the necessary oxygen on Earth. Without this partnership between oxygen consumers (such as animals) and oxygen producers (such as plants), the supply of oxygen would be depleted. Oxygen is also made by bacteria and algae called cyanobacteria. These plants and photosynthetic bacteria convert water into molecular oxygen using the energy from sunlight. This process, called photosynthesis, uses chlorophyll inside plant and bacterial cells to generate ATP.
Journal of Chemical Education • Vol. 80 No. 10 October 2003 • JChemEd.chem.wisc.edu
Chemical Education Today
Oxygen hasn’t been around since the beginning of time. It took 600 million years for the oxygen level in the atmosphere to rise from 1% to 21%. Microbes were the only organisms existing before the creation of oxygen. They could function anaerobically, that is, produce energy without consumption of oxygen. These primitive organisms eventually evolved to split water and produce oxygen, and were ancestors to cyanobacteria and chloroplasts. With more oxygen available in the atmosphere, the evolution of aerobic respiration was possible, and higher organisms evolved. The creation of photosynthetic organisms capable of splitting water was a key point in this evolutionary process. Energy production is much more efficient with oxygen, as opposed to processes without oxygen, such as fermentation. Patricia Kiley is a professor of biomolecular chemistry at the University of Wisconsin Medical School. More information on her research in cellular mechanisms for sensing molecular oxygen is available (3).
The Birth of Oxygen: Untangling the Web. Alan Rocke Alan Rocke spoke about the controversy associated with the discovery of oxygen. Oxygen was discovered near-simultaneously and yet relatively independently by three chemists of the late 18th century: Antoine Lavoisier, Carl Wilhelm Scheele, and Joseph Priestley. Joseph Priestley, an Englishman who conducted his research in Calne, Wiltshire, was the first scientist to publish a way to synthesize oxygen in 1774. Because he was the first to publish his discovery, he is often the one credited, even though Carl Wilhelm Scheele had isolated the gas in 1771. Priestley heated red oxide of mercury with a burning lens to produce oxygen that he then used to support combustion. He called the gas, which we now know was oxygen, “dephlogisticated air.” (Phlogiston is a hypothetical substance believed by early scientists to cause combustion by vacating the burning material.) Priestley believed that oxygen was the absence of phlogiston, that oxygen could draw phlogiston out of other substances, thus intensifying combustion. Even after Antoine Lavoisier disproved the phlogiston theory in 1783, Priestley continued to believe it until his death in 1804. In addition to his discovery of oxygen, Priestley studied the gases produced by fermentation and discovered the properties of carbon dioxide. One of Priestley’s discoveries is a common product in today’s world. He found that carbon dioxide produces a very refreshing drink when dissolved in water, making him the father of the soft drink industry. Carl Scheele, a Swede who did research in Uppsala, was the first to document the creation of oxygen. While he may have been the first to record it in his lab notebook, he did not publish his discovery right away, and because of this he did not receive credit for his discovery. He believed that the gas he found, which he called “fire air,” is attracted to and chemically reacts with phlogiston in combustion reactions.
Like Priestley, Scheele clung to the phlogiston theory, which prevented both scientists from really understanding what their discovery of oxygen meant. Scheele did receive credit for discovering several acids and some poisonous gases. He had meticulous notes on the properties of his discoveries, including the taste of hydrogen cyanide, which is a poisonous gas. The Frenchman Antoine Lavoisier did his research in Paris and studied reactions that released oxygen. He noticed a weight increase occurred in the products versus the reactants in combustion, while knowing that matter could neither be created nor destroyed. He concluded that there was an element unaccounted for. When he burned phosphorus and sulfur, he showed that the total products weighed more than the original substances, and that the substances combined with “something” in the air; he called that “something” oxygen. Lavoisier’s principle of identifying all reactants and all products using a precise mass balance, a difficult thing to do with gases, was the key to his success. His discovery debunked the phlogiston theory, and unlike the scientists who had prepared oxygen earlier, Lavoisier truly understood what the discovery meant. Because of his interpretation of contemporary discoveries, he is considered to be the father of modern chemistry. Alan Rocke is Henry Eldridge Bourne Professor of History at Case Western Reserve University, Cleveland, OH. Information about the discovery of oxygen has appeared in JCE (4).
Biological Control of the Reactivity of Molecular Oxygen. Brian Fox Brian Fox spoke about oxygen’s role in central metabolic pathways of living organisms as well as the biological cleanup of hydrocarbons. Living organisms obtain energy from the flow of electrons from food sources to an electron acceptor. Oxygen is the most important electron acceptor during respiration in aerobes since oxygen readily accepts electrons and also provides a large amount of recoverable energy in the process. Events such as the 1989 Exxon Valdez accident in Alaska and other large spills of petroleum or other hydrocarbons are very detrimental to the environment. The Exxon Valdez spilled 11 million gallons of petroleum. Because of the large quantity of energy that can be obtained, oxygen plays a key role in the breakdown of hydrocarbons by bacteria. To accomplish this, many bacteria contain genetic elements, called plasmids. These plasmids contain group of genes, called operons, that code for specific proteins capable of breaking down different hydrocarbons. Operons can be turned on or off to produce proteins, and one particular operon is turned on by the alkanes present in petroleum. When this happens, the pollutant can be used by the bacteria as a food source, thus helping to clean the environment. Brian Fox is a Professor of Biochemistry at the University of Wisconsin–Madison. More information is available about both the Exxon Valdez (5) and Brian Fox’s research (6).
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NCW 2003: Earth’s Atmosphere and Beyond Photo by Charles Harrington
Mme. Lavoisier’s nécessaire, a travel chest. Now in the Cornell University Library, it figures importantly in Djerassi and Hoffmann’s play, Oxygen.
Madame Lavoisier. Roald Hoffmann Roald Hoffmann addressed Madame Lavoisier’s life and her involvement in the research of her husband, Antoine Lavoisier. Marie Anne Pierrette Paulze Lavoisier studied English so that she could be his translator. She learned to paint and draw as well, and did the illustrations for Elementary Treatise of Chemistry written by her husband and published during the first year of the French Revolution. She even painted a portrait for Benjamin Franklin. In the laboratory Madame Lavoisier recorded numbers and protocols (plan of experiment), which meant that she had the important job of keeping all the records of what was done. Hers was not an isolated case of one woman’s involvement in science; it happened to be a good time for women everywhere. Women could do science and talk to scientists, within limits; for example, Madame Lavoisier did some science but did not receive recognition for her contributions. Roald Hoffmann is in the Department of Chemistry and Chemical Biology at Cornell University where he is the Frank H. T. Rhodes Professor of Humane Letters. He is a recipient of the Nobel Prize in Chemistry and the National Medal of Science. More information about Madame Lavoisier and her involvement in the discovery of oxygen may be found by reading the play, Oxygen (7), by Carl Djerassi and Roald Hoffmann.
Toward an Environmentally Friendly Chemical Industry: Selective Chemical Oxidation with Molecular Oxygen. Shannon Stahl Can there be chemistry without environmental cost? Shannon Stahl discussed this question as he spoke about current research and options available to the chemical industry that cause less damage to the environment. The chemical industry provides us with important products such as pharmaceuticals, agrochemicals, and polymeric materials, all of which are derived from petroleum feedstocks. Petroleum feedstocks consist mainly of hydrocarbons, with carbon in a relatively low oxidation state, but many desirable com1126
pounds, such as alcohols or carboxylic acids, involve selective partial oxidation of hydrocarbons. Although it is easy to oxidize (burn) hydrocarbons to CO2 and water, selective oxidation is more difficult. To date selective oxidation has involved chlorine, dichromates, and other reagents. These are effective but they produce detrimental effects on the environment, such as the release of polychlorinated biphenyls (PCBs) into lakes and rivers and metal contamination of ground water. To alleviate such problems, oxygen itself might serve as the oxidizing agent. Oxygen has the thermodynamic capability to carry out selective oxidations, as does hydrogen peroxide, but the necessary reactions are slow. With appropriately designed catalysts, it should be possible to speed up the desired reactions and create new, environmentally friendly routes to industrial chemicals. Stahl is currently investigating catalytic cycles patterned on palladium oxidase chemistry as one means for synthesizing industrial chemicals without hazardous waste products. This research may one day lead to a more environmentally friendly chemical industry. Shannon Stahl is in the Department of Chemistry, University of Wisconsin–Madison. More information about his research in new catalysts for selective oxidation chemistry with molecular oxygen is available at his Web site (8).
The Unmaking and the Making of Chemical Elements: The Chemistry of Salts in the 18th Century. Thomas Broman Thomas Broman spoke about scientists’ understanding of the rules of combination as well as different acids and bases in the production of “middle salts.” This idea of acid– base reactions was used by Lavoisier in his identification and naming of oxygen. In ancient times scientists labeled only four “elements”: earth, water, air, and fire. Their chemical properties were simple: earth was cold and dry, water was cold and wet, air was hot and wet, and fire was hot and dry. In the 16th century, scientists were not able to determine the identity of the product of a combination reaction. If you mix an acid with a base, what do you get? This is a simple combination, but is the product an acid, a base, or something else? A demonstration by Rudolph Glauber and Robert Boyle combined nitric acid and potassium carbonate to produce potassium nitrate, illustrating the idea of combination. Since this process could be reversed, it was clear that the reactants were “in” the products (because they could be extracted), but with none of their own qualities. By the 18th century chemists began to grapple with these issues and to develop a comprehensive understanding of the rules of combination of acids and bases. This understanding was decisive in Lavoisier’s identifying oxygen as an “acidifying principle.” Tom Broman is an Associate Professor of History of Science and History of Medicine at the University of Wisconsin– Madison.
Journal of Chemical Education • Vol. 80 No. 10 October 2003 • JChemEd.chem.wisc.edu
Chemical Education Today
photos by J. J. Jacobsen and J. Maynard
Singlet oxygen
Sulfur burning in oxygen
Iron (steel wool) burning in oxygen
How To Smuggle Science into the Public. Carl Djerassi Carl Djerassi has written several novels and plays, including Oxygen, that entertain and teach science at the same time. He believes that “theater can not only educate one on a subject, but can make you never forget it”. He discussed the border between knowledge and belief, a fuzzy area that is nowhere fuzzier than in the public’s acceptance of science. His attempts at lowering this barrier have been through the medium of fiction, specifically five “science-in-fiction” novels, where all the science is correct or at least plausible. He has thought the approach sufficiently successful to experiment with it in the theater. Carl Djerassi is Professor of Chemistry at Stanford University. He has received the National Medal of Science and the National Medal of Technology. More information about Carl Djerassi’s novels and plays may be found at his Web site (9). Illustrative Demonstrations
photo by J. J. Jacobsen and J. Maynard
In addition to informative lectures about oxygen, there were several interesting, and sometimes explosive, demonstrations during the symposium. These demonstrations illustrated oxygen’s unique and fascinating properties. The concentration of oxygen greatly affects the rate of combustion. In one demonstration, a tiny spoonful of sulfur was ignited in air, and then lowered into a flask containing pure oxygen. The higher concentration of oxygen immediately caused the flame to become more violent, burning an even brighter blue than before. Oxygen combines with other elements more quickly than normal when in higher concentrations, as it did when oxidizing sulfur to sulfur dioxide. The impact of pure oxygen on combustion was also illustrated through the rapid oxidation of iron, as when steel wool was burned in pure oxygen. After the iron burst into flames, two or three forms of iron oxides were produced. The iron residue left at the end of the demonstration weighs more than the original steel wool, an important observation and of the same kind Antoine Lavoisier used to develop theories involving oxygen.
At room temperature most metals react slowly with oxygen from the atmosphere. Some react quite rapidly. The coinage metals—Cu, Ag, and Au—are relatively difficult to oxidize, which explains their use in coins. When heated in air, copper is oxidized much faster, producing black copper(II) oxide. Oxygen has magnetic properties as well. While molecules of most elements have an even number of electrons arranged in pairs that have opposite spin, oxygen has an even number of electrons but two are not paired. These unpaired electrons make oxygen paramagnetic, meaning it is attracted to a magnetic field. Liquid oxygen was poured between two poles of a magnet. It lingered between the poles before evaporating instead of flowing through as would a dimagnetic liquid such as liquid nitrogen. There are also forms of oxygen that are not paramagnetic. One is singlet oxygen, an unusual and unstable form of oxygen, which readily converts back to the ordinary, paramagnetic form. Singlet oxygen can be detected by the dull red glow produced by the release of energy when it is converted to ordinary molecular oxygen, which is produced rapidly, in an intense reaction. This chemiluminescent process was demonstrated by bubbling gaseous chlorine through alkaline 30% hydrogen peroxide (10).
Paramagnetism of liquid oxygen
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Chemical Education Today
NCW 2003: Earth’s Atmosphere and Beyond
photos by J. J. Jacobsen and J. Maynard
Oxidation of copper when heated in air
The symposium had an exciting and dramatic finish: the thermite reaction. This extremely exothermic reaction involves oxidation of a metal by a metal oxide—no oxygen from the air is needed. Aluminum powder and iron(III) oxide powder would react with one another only very, very slowly at room temperature, so the reaction was initiated by heating the mixture of these reactants with a sparkler. The oxygen in iron(III) oxide is then transferred to the aluminum, the aluminum is oxidized, and iron(III) oxide is reduced in such an exothermic process that iron, one of the products, is in the molten state! This demonstration was just the right dramatic finish to a very educational and entertaining symposium.
7. Djerassi, Carl; Hoffmann, Roald. Oxygen. A Play in Two Acts; Wiley-VCH: Weinheim, 2001. 8. Shannon Stahl’s research is described on his Web site at http:// www.chem.wisc.edu/main/people/faculty/stahl-resdes.html (accessed Jul 2003). 9. Descriptions of Carl Djerassi’s novels and plays may be found at http://www.Djerassi.com (accessed Jul 2003). 10. Shakhashiri, B. Z.; Williams, L. G. J. Chem. Educ. 1976, 53, 358–361.
Carolyn S. Quinsey, an undergraduate pre-med student at the University of Wisconsin–Madison, is an intern with the Journal;
[email protected].
Literature Cited
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Thermite reaction
Journal of Chemical Education • Vol. 80 No. 10 October 2003 • JChemEd.chem.wisc.edu
photo by J. J. Jacobsen and J. Maynard
1. The Oxygen symposium was held March 29, 2003, as an activity of the Wisconsin Initiative for Science Literacy, directed by Bassam Z. Shakhashiri. The symposium was sponsored by the Wisconsin Initiative for Science Literacy, the UW–Madison Department of Chemistry, the UW–Madison College of Letters and Science, the University of Wisconsin– Madison, the Wisconsin Academy of Sciences, Arts, and Letters, and the Wisconsin Section of the American Chemical Society. 2. Weindruch, Richard. FASEB Journal 2000, 14, 78–86; FASEB Journal 2000, 14, 1825–1836; Experimental Gerontology 2000, 35, 1131–1149. 3. More information may be found on Patricia Kiley’s Web page at http://www.medsch.wisc.edu/bmolchem/kiley/kiley.html (accessed Jul 2003). 4. Cassebaum, H.; Schufle, J. A. J. Chem. Educ. 1975, 52, 442– 444; Neville, R. G. J. Chem. Educ. 1974, 51, 428–431; Partington, J. R. J. Chem. Educ. 1962, 39, 123–125. 5. Information about the Exxon Valdez can be found at the Web site of the U.S. Environmental Protection Agency at http:// www.epa.gov/oilspill/exxon.htm (accessed Jul 2003). 6. Brian Fox’s research is described on his Web site at http:// www.biochem.wisc.edu/fox (accessed Jul 2003).
