In the Laboratory
Secondary School Chemistry
Pressure and Stoichiometry
W
Charles E. Roser* North Carolina School of Science and Mathematics, 1219 Broad St., Durham, NC 27715; *
[email protected] Catherine L. McCluskey East Wake High School, 5101 Rolesville Road, Wendell, NC 27591
A common stoichiometry experiment in lab manuals is the reaction of NaHCO3 with HCl (1, 2). The usual procedure involves determining the mass of a sample of NaHCO3 and adding excess aqueous HCl. Water and excess aqueous HCl are removed by evaporation, leaving solid NaCl, whose mass is then determined. From the initial mass of NaHCO3 and the mass of NaCl produced, the mole ratio between the compounds can be determined and compared to the coefficients in the balanced equation. In a typical 50–90-minute lab period, only one sample can be run. Additional methods of analyzing carbonates include titration, determining the mass loss due to CO2, and measuring the volume of CO2 produced (3). Job plots have been used to determine stoichiometry in complex ions (4), and programmable calculators have been used to analyze Job’s plot data (5). In the procedure described here, the reaction is run in a 20-oz plastic soda bottle fitted with a short piece of 3/16-inch brass tubing and a 6-mL plastic Beral pipet (Fig. 1). The threaded cap with its liner minimizes the loss of CO 2 that might occur with a two-hole stopper and an Erlenmeyer flask. The plastic pipet is filled with 6 M HCl to provide a constant volume of HCl. The sample of hydrogen carbonate or carbonate is placed in the bottle. The cap with the filled dropper is attached to the bottle and the brass tubing is attached to a pressure sensor attached to a Calculator Based Laboratory (CBL) 1 interface and a TI-82/83 graphing calculator. The calculator runs the CHEMBIO program from Vernier Software2 to collect pressure-versus-time data. While these specific directions are written for the CBL, they can be easily modified for other interfaces and graphing and data analysis programs. The HCl is added to the carbonate, releasing CO 2 and increasing the pressure in the bottle until either the carbonate or the HCl is consumed. The system is at constant volume and approximately at constant temperature; the change in pressure in the system is directly proportional to the number of moles of CO2 produced. If students start with a small sample, the carbonate is the limiting reactant. As the amount of carbonate is increased, more HCl will react and more CO2 will be produced, causing the change in pressure to increase. Eventually all the HCl will be consumed and the carbonate will be in excess. No additional CO2 can be formed and the change in pressure should stay constant. The change in pressure is plotted versus the mass of carbonate, resulting in a graph similar to Figure 2. The calculator program INTRSECT3 puts linear regression lines through the ascending and horizontal sets of
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points and finds the intersection of the lines. The intersection point gives the mass of carbonate required to react with the HCl added. The numbers of moles of carbonate and HCl are calculated and used to write a balanced equation for the reaction. The advantages of this procedure are that the students can run a large number of samples (since each sample requires only 100 s of data collection time), develop an understanding of the concept of a limiting reactant, and develop an understanding of the relationship between the amount of gas produced by a reaction and the pressure in the reaction container. Experimental Procedure The masses of samples of the carbonate ranging from 0.50 g to 3.75 g in 0.25-g increments are determined. The plastic pipet is filled with 6.0 M HCl. CAUTION: Use care in handling the HCl. The pressure sensor is attached to the CBL using a DIN adapter and to the brass tubing on the dropper assembly using the plastic tubing provided with the pressure sensor. The CBL is attached to the calculator using the calculator link cable. The program CHEMBIO is run using the stored calibration in atmospheres, a minimum pressure of 0.8 atm, a maximum pressure of 2.5 atm, a time graph, and data collection of 10 samples at 10-s intervals. The carbonate is placed in the bottle, the dropper assembly is tightly attached to the bottle, data collection is started, two data points are graphed, and then the HCl is injected. The bottle is swirled during the experiment to ensure good mixing of the reactants. The calculator displays a pressure-versus-time graph. The initial and final pressure in the bottle are determined from the graph and the change in pressure is calculated. After all the samples are run, a graph of the change in pressure versus the mass Figure 1. The apparatus: a 20-oz plastic bottle with a of the carbonate is plotted and the plastic dropping pipet and intersection point is determined. a length of brass tubing The number moles of carbonate re- inser ted through the quired to react with the HCl and threaded cap and secured produce the maximum pressure with epoxy cement.
Journal of Chemical Education • Vol. 76 No. 5 May 1999 • JChemEd.chem.wisc.edu
∆P / atm
In the Laboratory
x
xxxxxxx
x
Figure 2. Typical plot of pressure change vs mass of carbonate.
x x Mass carbonate / g
by eliminating some of the samples with excess carbonate. The following supplementary materials for this experiment are available online:W detailed instructions for setting up and running the experiment and collecting and analyzing the data; student data sheets; and instructor’s notes including sample data sheets. Acknowledgments
change and the mole ratio of HCl:compound are calculated. Discussion and Results Several hydrogen carbonates and carbonates were tried and good results were achieved with most of them. The results are shown in Table 1 using 6.0 mL of 6.0 M HCl. NaHCO3, Na2CO3, and CaCO3 are recommended because of their low cost. Commercial baking soda works well for the NaHCO3. There are several assumptions made in the experiment. The first is that the reaction goes essentially to completion in 100 seconds because the pressure does not increase significantly over a longer time span, even in samples with a large excess of NaHCO 3. The second is that the system is a constantvolume system. The plastic bottle is rigid enough to ensure this. The third is that the system is being run at constant temperature and the heat released or absorbed by the reaction mixture has a minimal effect on the pressure change. This seems to be true especially if the final pressure is measured at the end of the run after the system has had some time to return to room temperature. The experiment may be modified in one of two ways if the reaction enthalpy is a concern: (i) lengthen the data-collection time by collecting 18 samples at 10-s intervals; (ii) immerse the reaction vessel in a constant-temperature bath. Lengthening the data collection time will significantly lengthen the experiment. The fourth assumption is that the dissolving of the CO2 in the reaction mixture has a minimal effect on the pressure change. This seems to be true even at higher pressures, owing to the low volume of solution used. A factor that does cause a noticeable drop in pressure is the reaction of CO2 with carbonate to produce hydrogen carbonate. This effect is seen in samples containing a large excess of carbonate. The relatively short collection time seems to reduce this effect and the carbonates still give good mole ratios. While this procedure is designed as a laboratory experiment, the data could be used as a laboratory assessment item for stoichiometry. The experiment can be performed in the 90-minute periods of a typical block schedule or shortened
We want to thank the Glaxo-Wellcome Foundation for funding the Winners II grant that provided the resources needed to develop this experiment, Myra Halpin for helping to edit this article, and Mari Nishimura for the bottle drawing. Notes W Supplementary materials for this article are available on JCE Online at http://JChemEd.chem.wisc.edu/Journal/Issues/1999/May/ abs638.html. 1. Texas Instruments Customer Support Line, P.O. Box 6118, MS3268, Temple, TX 76503-6118. Phone: 800/842-2737; URL: www.ti.com/calc. 2. Vernier Software, 8565 S.W. Beaverton-Hillsdale Hwy., Portland, OR 97225-2429. Phone: 503/297-5317; URL: www.vernier.com. 3. The INTRSECT program was developed by Eric Gadol, a 1996 graduate of the North Carolina School of Science and Mathematics, as a modification of the program EDITPART from Texas Instruments. Copies of this program are available from the corresponding author.
Literature Cited 1. Toon, E. R.; Ellis, G. L. In Laboratory Experiments for Foundations of Chemistry; Holt, Rinehart, and Winston: New York, 1973; pp 69–70. 2. Tzimopoulos, N. D.; Williams, J. E.; Metcalfe, H. C.; Castka, J. F. In Laboratory Experiments for Modern Chemistry; Holt, Rinehart, and Winston: New York, 1990; pp 75–78. 3. Dudek, E. P. J. Chem. Educ. 1991, 68, 948. 4. Kindahl, N.; Berka, L. J. Chem. Educ. 1993, 70, 671. 5. House, J. E. Jr. J. Chem. Educ. 1982, 59, 132.
Table 1. Results of Trials with Various Carbonates and Hydrogen Carbonates Compound
Needed for Max ∆P
Max ∆P/atm
HCl:Compound a
Mass/g
Moles
Expt
Balanced Eq
NaHCO3
2.96
0.0352
1.22
1.0
1
KHCO3
3.62
0.0362
1.31
0.99
1
Li2CO3
1.36
0.0184
0.70
2.0
2
Na2CO3
1.98
0.0186
0.74
1.9
2
K2CO3
2.45
0.0177
0.68
2.0
2
MgCO3
1.67
0.0198
0.57
1.8
2
CaCO3
1.80
0.0180
0.70
2.0
2
aMole
ratio.
JChemEd.chem.wisc.edu • Vol. 76 No. 5 May 1999 • Journal of Chemical Education
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