Edited by RALPHK. BIRDWHISTELL University of West Florida Pensamla. FL 32504
textbook forum Nitric Oxide Leftovers. Fred M. Homack University of North Carolina at Wilmington, Wilmington, NC 28403
The production ofnitrates by the oxidation of ammonia is an important commercial process. Ammonia, supplied by the Haher process, is mixed with air and forms nitric oxide in a converter containing a platinum catalyst. The nitric oxide is then further oxidized and hydrated to yield nitric acid.' Chemistry students learn the following equations:
reacts with 1/6 rnol Oz giving 1/3 mol of NO;, which reacts with 1/9 rnol of water. The resulting 1/9 rnol of NO reacts similarly, and, continuing on and on, we obtain two infinite geometric series: Oxygen: 112 + 116 + 1/18 + 1/54 + . Water: 113 + 119 + 1/27 + 1/81 + . . .
The limit of the sum of each series shows that 1rnol of NO requires 3/4 rnol of Oz and 112 mol of H20. Classroom Reply: Rewrite the equations as follows: The reactions have the unusual feature that a reactant, nitric oxide (NO), is also a product. I t appears that some NO, a particularly noxious air pollutant, will remain unreacted. Chemical engineers, however, take pains to insure that all of the NO is converted to HN03 by reoxidation and hydration in the absorber. The product gas can be taken to contain only Nz and excess O2 from the air. Question: What quantities of oxygen and water are needed to completely transform the NO coming from the converter into nitric acid? Use 1rnol of NO as a basis. Scholarly Reply: We need to utilize the stoichiometric coefficients in the two equations involving the NO. One mole of NO reacts with 1/2 molOz and gives 1rnol of NO,, which reacts with 1/3 rnol of water. The resulting 1/3 rnol of NO
' The threestep conversion of ammonia to nitric acid is known as the Ostwald process. The Haber and Ostwaid processes are indexed in many introductory chemistry texts such as: Brown. T. T.: LeMay, H. E. Chemlstiy: The CenfraIScience, 4th ed.; Prentice-Hall: Englewood Cliffs, NJ. 1988.
496
Journal of Chemical Education
Add the equations, deduct a mole of NO and 3 rnol of NOz from both sides and divide by 2. The resulting equation, NO 3/402 1/2Hz0 HNOa verifies the amounts given above. Nuts-and-Bolts Reply: If 1 rnol of NO is going to react with Oz and Hz0 t o form 1rnol of HN03, all we need to do is complete the balancing of the following equation:
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This example shows that a stoichiometric problem can be solved in a number of different but equally valid ways. The nuts-and-boltsreply seems t o be the least abstract, while the other methods give the required result by manipulations that will seem unreal to a student. This provides an opportunity to point out that the practice of chemistry often requires hypothetical quantities and reactions.
