In the Laboratory
Exploring the Ideal Gas Law through a Quantitative Gasometric Analysis of Nitrogen Produced by the Reaction of Sodium Nitrite with Sulfamic Acid Anne Yu Department of Chemistry, Pomona College, Claremont, California 91711, United States
[email protected] The gasometric analysis of nitrogen produced in a reaction between sodium nitrite, NaNO2, and sulfamic acid, H(NH2)SO3, provides an alternative to more common general chemistry experiments used to study the ideal gas law, such as the experiment in which magnesium is reacted with hydrochloric acid and others in which Boyle's law and Charles' law are investigated (1). This experiment, in which the percent sodium nitrite of a sample is determined with the use of equipment commonly found in a general chemistry laboratory, was first published in 1946 (2), and data culled over the last several years show that modifications to the original procedure have improved results in this classic laboratory exercise while also reducing student anxiety related to difficulties in the original procedure. In addition to the ideal gas law, this experiment provides students with an opportunity to analyze data using principles and concepts that constitute core topics often presented in the first semester of general chemistry, such as stoichiometry, redox equations, Dalton's law of partial pressures, the vapor pressure of water, and barometric pressure. The fundamental reaction in this analysis is NO2 - ðaqÞ þ Hþ ðaqÞ þ NH2 SO3 - ðaqÞ f N2 ðgÞ þ HSO4 - ðaqÞ þ H2 OðlÞ
ð1Þ
A 0.25 M sulfamic acid solution is added in excess to samples of an unknown mixture containing sodium nitrite and sodium chloride. The 55-85% sodium nitrite mixtures need to be dried at 110 °C for 1 h prior to the lab period and stored in desiccators as the samples are hygroscopic. For each run, 20 mL of the sulfamic acid solution is required to completely react approximately 0.15 g of unknown mixture. Once the apparatus is assembled and tested for the leaks, the entire reaction proceeds in 20-30 min so that three to five runs may be executed within a 3- to 4-h lab period. Procedure The reaction takes place in a 50 mL Erlenmeyer flask sealed with a no. 2 one-hole rubber stopper with a 24 in. length of 6 mm glass tubing. A 15 in. length of 3/16 in. rubber tubing is attached to the 6 mm glass tubing on one end and the other end of the rubber tubing is attached to a 3 in. length of 5 mm glass inserted into a no. 00 one-hole rubber stopper placed at the top opening of a buret. An additional 20-30 in. of 1/4 in. rubber tubing, the type commonly used for Bunsen burners, is used to connect the tip of the buret to a leveling bulb. A funnel can also be used as a leveling bulb. An 800 mL beaker filled with water is required
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Figure 1. The assembled apparatus.
to provide a constant-temperature bath for the Erlenmeyer flask being used as a reaction vessel. The apparatus is shown in Figure 1, and additional details are provided in the supporting information. Before the reaction initiates and after it is completed, one must ensure that the water in the buret is at the same level as the water in the leveling bulb. This allows the student to employ the barometric pressure as the total pressure of the system. Because the nitrogen is collected over water, its pressure can be calculated after determining the vapor pressure of water at the experimental temperature as the total pressure is the sum of the two. The apparatus must be assembled and checked to ensure that leakage is minimized. The nitrogen gas produced displaces the water in the buret such that the volume can be easily determined using the final and initial volumes. According to eq 1, approximately 0.20-0.25 g of pure sodium nitrite is needed to react completely with 20 mL of 0.25 M sulfamic acid. Approximately 0.14 g of unknown mixture is used for each run; thus, sulfamic acid is present in excess. An easily acquired and inexpensive replacement for the glass vials used in the original experiment (2), which must be cleaned with water, acetone, and briefly oven-dried in between runs, are gelatin capsules. Between the time the capsule is introduced to the acid solution in the reaction vessel and the beginning of the reaction, the student can adjust the leveling bulb to ensure that the level of the volume of the water in the bulb and the buret have equalized. This adjustment often takes several minutes as does
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 12 December 2010 10.1021/ed100353b Published on Web 10/06/2010
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In the Laboratory Table 1. Comparison of Results Using the Glass Vials and Gelatin Capsules Results with Glass Vial
Results with Gelatin Capsule
Sodium Nitrite in the Unknown Mixture (%)
Number of Students
Average (%)
Standard Deviation
Number of Students
Average (%)
Standard Deviation
85.0
22
79
4.7
51
80.
4.3
75.0
21
70.
7.4
55
71
2.4
70.0
27
66
7.7
53
67
5.5
60.0
21
56
2.2
46
58
5.2
55.0
44
54
4.1
39
55
4.7
the time required for the acid to dissolve through the capsule. Consequently, the delay in the reaction that is inherent in using gelatin capsules allows for the experiment setup to be executed more precisely. Hazards and Disposal Students should wear eye protection and lab coats. Sulfamic acid should be handled carefully to avoid spilling and contact with the skin. Spills can be neutralized with sodium bicarbonate, which should be readily available in the lab. Sodium nitrite is not combustible, but is a strong oxidizer and, similar to sulfamic acid, is harmful if swallowed and can be irritating to the skin, eyes, and respiratory tract. Contact with the skin and eyes should be followed with flushing with water. If fresh air does not improve breathing after inhalation, then seek medical attention. The instructor should caution students that when cleaning up after each run to dispose of the gelatin capsule remainders into the trash, rather than down the sink. Data Analysis The data collected are the barometric pressure, temperature, volume change, and initial mass of the sodium nitrite unknown. The pressure of the gas is experimentally made equivalent to the barometric pressure and can be calculated employing Dalton's law of partial pressures after determining the vapor pressure of water at the experiment temperature. The student is then able to employ the ideal gas law PV ¼ nRT ð2Þ to determine the amount of nitrogen gas evolved. From this the percent composition of sodium nitrite in the initial sample can be determined from the stoichiometric relationship given in eq 1. Results Three years of student lab results are shown in Table 1. Each student reported the average of at least three trials. The table lists the averages and standard deviations of all student results for each of the unknown mixtures. The original article claims that an accuracy of 0.1 to 0.2% may be expected (2), but the student data show otherwise. There is an expected error ranging from 1-5% as a result of several inherent experimental errors. For example, a 1 °C rise in room and bath temperature would result in a 0.5% error. The solubility of nitrogen gas may cause another 0.15% error for a sample that generated a volume 1370
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of 40 mL. The reaction of empty gelatin capsules with the acid solution showed no contribution to the volume of gas produced. The error increases as the percent composition of sodium nitrite increases in the unknown sample. This is likely due to the increase in reaction time. As more time is required to allow complete evolution of gas, there is also an increase likelihood of leakage. This would decrease the volume change and thus the amount of sodium nitrite calculated. Additional results similar to those reported in Table 1 from previous years of using glass vials can be provided upon request. The results indicate that there is an overall improvement in the average percent composition using gelatin capsules with an overall smaller range of standard deviations. For optimal precision and accuracy, unknown samples that contain approximately 55-70% sodium nitrite work best. The data in Table 1 are helpful in making peer comparisons should an instructor distribute a range of samples among the students. In the original experiment, an open glass vial was used, and upon contact with the sulfamic acid, the reaction proceeded immediately. It was common for the reaction to start inadvertently before students had time to equalize the water levels due to accidental tipping over of the open sample vial in the beaker containing sulfamic acid. This would have led to misleading volumes and greater deviations in the averages if runs had initiated too early and on an inconsistent basis before water levels had been properly adjusted. The inadvertent tipping over of the open glass vials was an enormous source of frustration, and replacement of the glass vials with gelatin capsule has removed the anxiety associated with the experiment, increased efficiency during the lab period, and reduced the waste of chemicals. Summary The gasometric analysis of nitrogen produced in the reaction of sodium nitrite with sulfamic acid is rich in chemistry and can be executed largely with items commonly found in a general chemistry laboratory. The experiment allows students to employ a variety of concepts presented in the lecture portion of the curriculum as they analyze data. The quantitative nature of the experiment is increasingly more precise and accurate with replacement of the glass vials with gel capsules as it provides more time to allow equalization of pressure. Though there are inherent and unavoidable errors, the 50-70% sodium nitrite samples have resulted in better accuracy, requiring less time to completely react than the higher sodium nitrite content. Once the apparatus is set up, students are able to perform multiple runs in a 3- or 4-h lab
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In the Laboratory
period, which should yield ample data for a quantitative analysis of the results. Acknowledgment The author thanks the members of the Pomona College Chemistry Department who were initially involved in implementing this experiment, especially Chuck Taylor for suggestions and discussion, Elena Branford (class of 2010), who tested the modification, and the general chemistry students whose results are included here.
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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Literature Cited 1. Wilbraham, A.; Staley, D.; Simpson, C.; Matta, M. Chemistry Laboratory Manual, 2nd ed.; Addison-Wesley: New York, 1990; pp 135-142. 2. Brasted, R. J. Chem. Educ. 1946, 23, 320–321.
Supporting Information Available Instructions for the students; notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
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