Chemical Education Today
From Past Issues
Nitrogen: It Always Needs a Fix by Kathryn R. Williams
In honor of Earth Day 2005, this month’s look at the past features air—but only 78.1% of it. The October 2003 From Past Issues revisited some papers on oxygen and its discoverer, Joseph Priestley (1). Now it’s time for nitrogen to be the center of the action. What “action”? The word is hardly an appropriate partner for the nitrogen molecule, whose triple bond defies the “if it ain’t broke, don’t fix it” adage. Nitrogen fixation, the term commonly used for the combining of N2(g) with other elements, has been a concern of chemists since the early days of our science. For example, Priestley in 1772 and Henry Cavendish in 1774 independently studied the arc method for reacting atmospheric N2 and O2. Despite these early experiments, nonbiological sources of nitrogen compounds did not become viable until the early 20th century. With two exceptions (2, 3), past accounts of nitrogen fixation in the Journal of Chemical Education appeared primarily in early issues (4–8), a reflection of the growing national interest in nitrogen compounds in the 1920s. World War I led to increased awareness of the U.S. dependence on outside sources of nitrates for explosives and agricultural fertilizers. The latter need, although millennia old, was becoming acute in the United States at the time due to poor farming practices on our once plentiful land. In 1921 the Fixed Nitrogen Research Laboratory, originally under the War Department, was transferred to the Department of Agriculture (9). Authors Hetherington and Allison (4, 5) both worked for the laboratory when they wrote their articles for the Journal. The remainder of this story relies primarily on the Hetherington article (5), with some amplification from Curtis’s 1942 retelling (2). One of the simplest methods of nitrogen fixation, the arc process studied by Cavendish and Priestley, involves passing air through a powerful electric arc. Nitrogen and oxygen combine to produce nitrogen oxides, which are absorbed in water to produce nitric acid. However, the process consumes large quantities of power, and it was feasible only in regions with cheap sources of electricity, especially produced by hydroelectric generation. The site of the first arc facility was Niagara Falls (1902), but the business was not profitable. Commercially successful operations were established a few years later in Norway, where hydroelectric energy was quite cheap.1 The cyanamide process consumes much less power and was somewhat more successful, especially during World War I for production of explosives. In this process gaseous nitro-
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Fritz Haber, winner of the 1918 Nobel Prize for Chemistry and inventor of the process for synthesizing ammonia.
gen reacts with calcium carbide, CaC2, to form calcium cyanamide: CaC2(s) + N2(g) → CaCN2(s) + C(s) The calcium carbide is produced by heating limestone to form CaO, which is subsequently heated with coke in an electric furnace. The limestone must be chosen carefully, because impurities can lead to problems. According to Hetherington (5), the cyanamide process was sufficiently cumbersome that it would never have been adopted were it not for the wartime emergency. Hetherington also discussed the Haber process, which continues to be the most important method of fixing atmospheric nitrogen. As described in many general chemistry texts, thermodynamically the direct synthesis of ammonia from N2 and O2 is favored by low temperature and high pressure, but high pressure techniques were largely undeveloped in the 19th century, and the reaction is slow unless heated. Although several 19th century chemists, most notably Nernst, investigated the reaction, Fritz Haber and his research students succeeded in designing a catalytic synthesis of ammonia with high yields. Further high-pressure studies by Carl Bosch made the process commercially viable.
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Chemical Education Today
Methods of nitrogen fixation provide a relevant way to introduce students to industrial processes. Comparison of the arc, cyanamide, and Haber processes shows the importance of such factors as energy costs and purity of starting materials, as well as fundamental principles of thermodynamics and kinetics.
4. 5. 6. 7. 8. 9.
Note 1. Considering the limited usage of the arc process in the United States, I question why the topic was so prevalent in the Journal of Chemical Education. I found six articles (10–15) describing apparatus for demonstrating the arc process, all of them pretty much the same. I guess that referees/editors of that era did not screen papers for originality to the extent that they do today.
Literature Cited 1. Williams, Kathryn R. J. Chem. Educ. 2003, 80, 1129–1131. 2. Curtis, Harry A. J. Chem. Educ. 1942, 19, 161–165. 3. Schneller, Stewart. W. J. Chem. Educ. 1972, 49, 786–789.
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10. 11. 12. 13. 14. 15.
Allison, F. E. J. Chem. Educ. 1926, 3, 50–58. Hetherington, H. C. J. Chem. Educ. 1926, 3, 170–176. Lipman, J. G. J. Chem. Educ. 1927, 4, 845–860. Reinmuth, Otto. J. Chem. Educ. 1928, 5, 1464–1472. Spears, Ralph D. J. Chem. Educ. 1929, 6, 1691–1696. Records of the Fixed Nitrogen Research Laboratory, National Archives and Records Administration; Section 54.5.2. http:// a r c h i v e s . g o v / r e s e a r c h _ r o o m / f e d e ra l _ r e c o rd s _ g u i d e / bureau_of_plant_industry_soils_and_agricultural_rg054.html#54.5.2 (accessed Nov 2004). Simer, Dorr M.; Brock, Mary G. J. Chem. Educ. 1930, 7, 2169. Kiley, M. Marcus. J. Chem. Educ. 1930, 7, 2167–2168. Doane, Harry Clifford. J. Chem. Educ. 1932, 9, 1113. Olsson, Harry L. J. Chem. Educ. 1932, 9, 1829. Thiessen, Garrett W. J. Chem. Educ. 1933, 10, 498. Zuffanti, Saverio J. Chem. Educ. 1937, 14, 73.
Kathryn R. Williams is in the Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, FL 326117200;
[email protected].
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