Chemical Education Today
Letters Winter’s Entertainment
and oxygen is equation 2.
Many, if not most, of your readers eagerly anticipate the December issue because it previously had the “Winter’s Entertainment” section with ingenious puzzles and games. These were always very useful, as well as entertaining, because they taught chemical nomenclature, concepts, and knowledge while disguised as fun. They gave students and teachers alike a chance to practice critical thinking and reasoning. Is that not what we are about in chemical education, now, more than ever? When they were absent from the December 1996 issue, I thought maybe it was an oversight or a sparse year for submissions, but with the absence from the last issue, I must assume that this section has been dropped editorially. Why? Is chemistry such a stuffy, archaic profession that we only teach these skills in mathematical problems? Have the physical chemists taken over the Journal in a covert coup? Have all of the puzzle and mystery writers retired or been forced to sell hamburgers for a living by the conservative politicians? Please explain to your disappointed readers what has happened to one of the most useful sections of our Journal. I think each issue should have at least a couple of these tools included. Then I would anxiously await EVERY issue, not just the December one. Terry L. Helser Department of Chemistry SUNY College at Oneonta Oneonta, NY 13820-4015
[email protected] From the Editor: For all the fans of the “Winter’s Entertainment”, puzzles, games, and humor articles have been moved to the April issue in honor of April Fools’ Day. Enjoy! The first of April is the day we remember what we are the other 364 days of the year. Mark Twain
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Ammonia–Air Mixtures Can Be Explosive A recent article in this Journal nicely highlights the lack of scientific knowledge of newspaper reporters and its usefulness to develop critical thinking in students (1). Nevertheless, one of the points in the article deserves a further comment. It is stated that ammonia does not burn in air because equation 1, despite being exothermic by nearly a megajoule, it is controlled by kinetics, not thermodynamics. 4 NH3 + 5 O2 → 4 NO + 6 H2O
(1)
Therefore, the reaction takes place only in the presence of an efficient catalyst. However, it is important to note that the thermodynamically favored reaction between ammonia 468
4 NH3 + 3 O2 → 2 N2 + 6 H2O
(2)
A very simple and instructive exercise is to calculate ∆H for equations 1 and 2 from bond energy data (2). The calculated ∆H values, –910 kJ mol-1 for equation 1 and –1278 kJ mol-1 for equation 2, help the students to rationalize that, while normal combustion of ammonia yields nitrogen (equation 2), in the presence of a Pt or Pt/Rh catalyst at 750–900 ˚C, the reaction yields the thermodynamically less-favored product NO (equation 1) (3). Equation 1 is the main step in the Ostwald process for the synthesis of nitric acid, and it can be easily demonstrated in the laboratory by introducing a piece of glowing platinum foil into a jar containing gaseous NH3 and O2; the foil will continue to glow because of the heat of reaction, and brown fumes will appear owing to the reaction of NO with the excess of O2 to produce NO2 (4). Nevertheless, in the absence of a suitable catalyst, the reaction of ammonia with oxygen proceeds mainly according to equation 2, and, although ammonia burns in air with difficulty (3, 5), it can indeed burn (3, 5–8) and even explode (6, 7). The flammability and explosive limits of ammonia in air at atmospheric pressure are ca. 16–26% by volume (3, 6–8). Hazard and safety information about ammonia can be found on the Internet at many places (9). Although ammonia does not readily ignite (its flammability hazard is slight), sources of ignition such as smoking and open flames are prohibited where ammonia is used, handled, or stored in a manner that could create a potential fire or explosion hazard. Therefore, The Star Ledger was right that ammonia-air mixtures can be explosive (1), although readings of 16,000 parts of ammonia per million are far from the explosive range. Literature Cited 1. Toby, S. J. Chem. Educ., 1997, 74, 1285. 2. Kildahl, N. K. J. Chem. Educ., 1995, 72, 423. 3. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, Pergamon: Oxford, 1984; p 485. 4. Cotton, F. A. ; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; p 313. 5. Remy, H. Treatise on Inorganic Chemistry, Elsevier: Amsterdam, 1956; Vol. 1, p 610. 6. Jones, K. In Comprehensive Inorganic Chemistry; TrotmanDickenson, A. F., Ed.; Pergamon: Oxford, 1973; Vol. 2, p 214. 7. Powell, P.; Timms, P. L. The Chemistry of the Non-Metals, Chapman and Hall: London, 1974; p 89. 8. King, R. B. Inorganic Chemistry of Main Group Elements, VCH: New York, 1995; p 74. 9. For example, see: gopher://ecosys.drdr.Virginia.EDU:70/00/library/ gen/.toxicsold/Ammonia; http://netbase.net/~na/msds/25/25530/ 25530190.txt; http://www.qrc.com/hhmi/science/ labsafe/lcss/ lcss10.htm (accessed December 1998). David Tudela Departamento de Química Inorgánica Universidad Autónoma de Madrid 28049-Madrid Spain
NOTE: the author’s reply appears in J. Chem. Educ. 1998, 75, 1087. ❖❖❖
Journal of Chemical Education • Vol. 76 No. 4 April 1999 • JChemEd.chem.wisc.edu
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Teaching Statistical Methods Dixon’s Q-test for testing outlying observations is described in this Journal (1). Teaching the statistical methods that relate to problems in the chemical laboratory is essential, and thus an article like this must be useful. However, I would like to point out the problems in this article, in which a single outlier which may be the largest or the smallest value in the sample of 10 observations is tested by a two-tailed test. This test uses the ratio known as r10 parameter as the test statistic, one of the six such ratios proposed by Dixon (2–5). There are three problems here: (i) Dixon’s original critical values are for a one-tailed test, and Dixon never published the critical values for the two-tailed test at 95% level of confidence; (ii) none of the critical values in the article’s Table 1 are found anywhere in Dixon’s original tables; and (iii) Dixon’s published critical values were found to contain numerous typographical errors that include the displacement of a whole row in a table (6). The percentage points that should be used for the example in the article are 0.412 and 0.466 for two-tailed 90% and 95% confidence levels, respectively (6 ). Q-test critical points can be computed by stochastic methods (7 ). A general Fortran program called QTEST (and QTEST2) for computing the percentage points for both normal and lognormal populations and for more than the six ratios originally proposed by Dixon is also available (8). Literature Cited 1. Thomasson, K.; Lofthus-Merschman, S.; Humbert, M.; Kulevsky, N. J. Chem. Educ. 1998, 75, 231–233. 2. Dixon, W. J. Ann. Math. Stat. 1950, 21, 488-506. 3. Dixon, W. J. Ann. Math. Stat. 1951, 22, 68–78. 4. Dean, R. B.; Dixon, W. J. Anal. Chem. 1951, 23, 636–638. 5. Dixon, W. J. Biometrics 1953, 9, 74–89. 6. Rorabacher, D. B. Anal. Chem. 1991, 63, 139–146. 7. Efstathiou, C. E. J. Chem. Educ. 1992, 69, 733–736. 8. Muranaka, K. QCPE Bull. 1998, 18, QCMP 178. Ken Muranaka K’s Garden Nishioji Suite 401, Kasuga Hachijo Sagaru Minami-ku, Kyoto 601-8312, Japan
[email protected] The author replies: Muranaka is correct in saying that Dixon described a one-tailed test where outliers occur only on one side of the curve distribution (i.e., only the high or low end is tested and the experimenter must know which end is questionable). Muranaka is also correct that the Q-test as discussed in physical chemistry laboratory and quantitative analysis texts is really a two-tailed test where the point tested may be on either end of the distribution curve. Thus, the two-tailed test means that the point to be considered as a candidate for possible rejection is either the largest or the smallest measurement. As pointed out by Rorabacher (1), there has been some confusion in texts concerning the confidence levels for the two-tailed test, and most books avoided the problem by either publishing Q-test criteria for only the 90% confidence level or at the 90%, 96%, and the 99% confidence levels (1). For this reason, the critical values in Table I of my original article (2) came from Table A-14
of the National Bureau of Standards Experimental Statistics Handbook 91 (3), which I cited in the text of my original article. These values originated in Dixon’s 1953 Biometrics paper (4), which was a condensation with recommendations for use of Dixon’s earlier work on outliers and contained the same problems as his previous articles (the problems that Muranaka brought to my attention and were discussed by Rorabacher [1]). As Muranaka pointed out, the correct values for the criteria for rejection are found in Rorabacher’s work (1) and are 0.412 and 0.466 for the 90% and 95% confidence levels. Comparing the Qmeasured to the correct values for each confidence level, the conclusions remain the same as described in my original article (2): student 1 should reject 0.708 at the 90% confidence level, but not at the 95% level: students 2 and 3 may keep their questionable data points at both confidence levels. I would like to thank Ken Muranaka for bringing to my attention Rorabacher’s article that published the corrected rejection criteria for use with the Q-test (including the 95% confidence level) and a variety of other different confidence levels for sample sizes up to 30 (1). I highly recommend this paper to anyone teaching the practice of outlier rejection. It does a thorough job of discussing the problems found in analytical chemistry texts dealing with Dixon’s development of the Q test (1). I have noted similar problems in physical chemistry laboratory texts, which usually only publish criteria at the 90% confidence limit level (5, 6 ). Literature Cited 1. Rorabacher, D. B. Anal. Chem. 1991, 63, 139–146. 2. Thomasson, K.; Lofthus-Merschman, S.; Humbert, M.; Kulevsky, N. J. Chem. Educ. 1998, 75, 231–233. 3. Experimental Statistics; National Bureau of Standards Handbook 91; National Bureau of Standards, Department of Commerce: Washington, DC, 1966; p T-27. 4. Dixon, W. J. Biometrics 1953, 9, 74–89. 5. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 6th ed.; McGraw-Hill: St. Louis, 1996; Chapter 2. 6. Sime, R. J. Physical Chemistry, Methods, Techniques and Experiments; Saunders: Chicago, 1990; Chapter 7. Kathryn A. Thomasson University of North Dakota Chemistry Department, Box 9024 Grand Forks, ND 58202-9024
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Properties of Zeolite A Obtained from Powdered Laundry Detergent The article “Properties of Zeolite A Obtained from Powdered Laundry Detergent: An Undergraduate Experiment” (J. Chem. Educ. 1997, 74, 569–70) addresses the need to introduce undergraduate students to a material widely used in industry. In this publication, Smoot and Lindquist present experiments which demonstrate properties of zeolites which are of great importance to industry. However, reference is made to the use of the protonic form of zeolite A (H-A) as a dehy-
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Letters dration catalyst for the preparation of an alkene from an alcohol. The authors ion-exchanged zeolite Na-A to NH4-A, and then heated the solid to 400 °C overnight in air to drive off ammonia and leave “zeolite H-A”. Although this practice is used with many zeolites to generate acidic catalysts, it does not work with highly aluminous zeolites such as type A. We have ion-exchanged samples of Na-A with NH4+, as described by Smoot and Lindquist, and calcined the resultant NH4-A. After just one hour of treatment at 400 °C in air, the solid was found by X-ray powder diffraction to be amorphous. Although other cationic forms of zeolite A are stable up to 800 °C, upon calcination at 400 °C the structure of zeolite NH4A collapses to an amorphous silica-alumina substance. While the silica-alumina itself has catalytic properties, and is able to catalyze the dehydration reaction, it is misleading to refer to it as zeolite H-A. The thermal instability of zeolite NH4-A, and the inability to form zeolite H-A has been described in detail elsewhere in the literature (1, 2). We have also investigated the thermal stability of the ammonium form of another highly aluminous zeolite, type X. Like NH4-A, NH4-X was found to be amorphous after a short period of heating at 400 °C.
nor reflections were still evident. The minor reflections were largely absent in the 300 °C sample although the major reflections were still present, consistent with a significant loss in crystallinity. We agree that a 400 °C treatment would yield solely amorphous material. We further verified the ability of the 300 °C material to effect dehydration of alcohol. Specifically, as confirmed by gas phase FTIR, 2-propanol vapor was converted to propene over the 300 °C sample in a tube furnace maintained at 300 °C. Even better conversions were previously observed using amorphous material at 400 °C. In summary, the partially crystalline material prepared at 300 °C will function as a dehydration catalyst, but higher conversions have been observed using higher temperatures and an amorphous catalyst.
Literature Cited
Photon-Initiated Hydrogen-Chlorine Reaction
1. Kühl, G. H. J. Catal. 1973, 29, 270–277. 2. Dondur, V.; Rakic V. Thermochim. Acta. 1985, 93, 753–756. Pamela J. Davis and Herman van Bekkum Laboratory of Organic Chemistry and Catalysis Faculty of Chemical Technology and Material Science Delft University of Technology Julianalaan 136 NL-2628 BL Delft The Netherlands Eric N. Coker BP Chemicals Ltd. Chertsey Road, Sunbur y-on-Thames Middlesex TW16 7LL United Kingdom
[email protected] The author replies: After our own powder X-ray diffraction of ammonium ion exchanged zeolite-A calcined to the acid form via ammonia evolution, we agree with Pamela Davis, Herman van Bekkum, and Eric Coker that the 400 °C temperature renders the zeolite amorphous. We prepared the ammonium ion exchanged zeolite according to the method described in our paper. Subsequently, a portion of the material was heated to 200 °C and held at temperature for 4 hours and, after cooling, retained for powder X-ray diffraction analysis. A second portion of the material was heated to 300 °C for 12 hours. Powder X-ray diffraction was performed on all three samples. The instrument used was an Enraf-Nonius Generator equipped with a Guinier film camera. The ammonium exchanged zeolite exhibited a pattern with a profuse number of Bragg reflections typical of a cubic structure. The 200 °C material exhibited some loss in crystallinity although the mi470
David Lindquist Department of Chemistry University of Arkansas at Little Rock Little Rock, AR 72204
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In preparation for a recent second semester general chemistry laboratory, we examined the experiment described in “Photon-Initiated Hydrogen-Chlorine Reaction” (1). To our surprise, we saw that the mechanism given for the H2 + Cl2 gas phase reaction has an incorrect termination step. The reaction scheme in the absence of O2 is given as (2) Cl2 → 2 Cl• Cl•
+ H2 → HCl +
(1) H•
(2)
H• + Cl2 → HCl + Cl•
(3)
The usual steady state treatment treats an inhibition step H• + HCl → Cl• + H2
(4)
In addition a termination step must be included. Possible termination steps include H• + Cl• → HCl from the original paper, and H• + H• → H2 or Cl• + Cl• → Cl2 In order to discriminate among these possible termination steps, the ratio of [H•] to [Cl•] needs to be computed. A steady state treatment of steps 2, 3, and 4 reveals that [H•] / [Cl• ] = k 2 [H2] / (k3 [Cl2] + k4 [HCl]) Examination of the experimental rate coefficients (3, 4) suggests that [H•] / [Cl• ] should be about 0.001. Since the rate coefficients for the three possible termination steps vary by less than an order of magnitude (4), the correct termination step for H2 + Cl2 must be the recombination of two chlorine atoms Cl• + Cl• → Cl 2
Journal of Chemical Education • Vol. 76 No. 4 April 1999 • JChemEd.chem.wisc.edu
Chemical Education Today
The author replies:
rather than the given + → HCl Note also that this discussion leaves out the complication of the oxygen molecule, which is clearly present in the reaction mixture. H•
Cl•
Literature Cited 1. Egolf, L. M.; Keiser, J. T. J. Chem. Educ. 1993, 70, A208. 2. Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper and Row: New York, 1987; pp 295–300. 3. Miller, J. C.; Gordon, R. J. J. Chem. Phys. 1981, 75, 5305–5310. 4. National Institute of Standards and Technology at http:// fluid.nist.gov/cgi- bin/CKMechReact (accessed May 1998). Richard Schwenz and Lynn Geiger Department of Chemistry and Biochemistry University of Northern Colorado Greeley, CO 80639
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Precision and Accuracy in Measurements I enjoyed the article by Richard Treptow entitled, “Precision and Accuracy in Measurements: A Tale of Four Graduated Cylinders” (J. Chem. Educ. 1998, 75, 992–995). I found the “concept charts” (Figures 4–5) helpful and intend to use them in the spectrochemistry training seminars I teach for users of our analytical instrumentation. However, there is a problem with the graduated cylinder experiment as it is really about the resolution of a measuring instrument and how this can affect the precision. The poor precision of the cylinders graduated in 1 mL divisions is clearly worse than those with 0.1 mL graduations, as illustrated by the measurements given in Table 1. But this is a matter of the resolution of the respective measuring instruments. It is possible that this distinction was omitted for pedagogical purposes. Although the two are related, there is a very real difference between precision and resolution and this should be noted. I must thank the author for pointing out that poor resolution can also result in poor precision, a relation that escaped me in a recent note on the terminology of measurement (Phys. Teach. 1997, 35, 15–17). Perhaps due to a concentration on instrumental analysis, I noted only the opposite effect: Consider the five measurements, 15.1, 15.2, 15.1, 15.0, and 15.2. If the instrument could not resolve the decimal, then the measured values would all be 15, with zero standard deviation and perfect precision. There are many aspects of the measurement process beyond just accuracy and precision (both short and long term). These include resolution, sensitivity, response time, range, and input impedance. There are interesting and often curious inter-relationships between all of them. Volker Thomsen Spectro Analytical Instruments, Inc. 160 Authority Dr. Fitchburg, MA 01420
I thank Volker Thomsen for his thoughtful response to my article. Had I known of his publication in The Physics Teacher I would surely have cited it. Here are a few comments concerning his letter. My article acknowledges that the two cylinders graduated in 1-mL divisions suffer from poor resolution when it states that they are readable only to the tenths place. I defined error to be anything that causes a measurement to differ from the true value. By this definition, an error results from the combined effects of poor instrument resolution and poor skill of the person using the measuring device. I choose not to separate the effects of resolution and skill since they are intimately linked. By keeping them together, I never find myself in the predicament of viewing a set of scattered measurements, such as 15.1, 15.2, 15.1, 15.0 and 15.2, and having to conclude that they have zero standard deviation and perfect precision. Richard S. Treptow Department of Chemistry and Physics Chicago State University Chicago, IL 60628-1598
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Corrections The review of Statistical Mechanics for Chemists appearing on page 1217 of the October 1998 issue should have listed the reviewer as: John Gunn Department of Chemistry Université du Montréal Montreal, PQ, Canada
❖❖❖ The review of Molecular Mechanics across Chemistry appearing on page 31 of the January 1999 issue should have listed the first author as Anthony K. Rappé. ❖❖❖ The review of The Art of Molecular Dynamics Simulation appearing on page 171 of the February 1999 issue should have listed the reviewer as: Stephen P. Molnar Foundation for Chemistry Upper Arlington, OH 43212-1112
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