In the Laboratory
Stoichiometry of the Reaction of Magnesium with Hydrochloric Acid
W
Venkat Chebolu* and Barbara C. Storandt Laboratory Science Department, Jefferson Community College, Watertown, NY 13601; *
[email protected] Rosen and McCluskey (1) have previously described the use of pressure to study stoichiometry. The reaction of magnesium solid with hydrochloric acid is used in our introductory and general chemistry labs for teaching stoichiometry. Traditionally, the procedure using water displacement described by Neidig and Spencer (2) has been used for this purpose. Our students, however, have felt challenged by this procedure for determining the amount of hydrogen gas produced in the reaction. They found it to be cumbersome, intimidating, and somewhat frustrating. Birk and Walters (3) have described another procedure for measuring hydrogen gas produced in the same reaction. Their emphasis is on the kinetics of the reaction; however, they make a brief mention of the stoichiometry of the reaction. Their procedure involves assembling glassware that does not conveniently lend itself to use in undergraduate laboratories where glass blowing facilities may not be available. In light of the challenges encountered when using these procedures, a more direct and procedurally simple method for determining the amount of hydrogen produced in this reaction has been developed and is described. This procedure enables students to focus on the central issues of stoichiometry as the data collection is automated using the computer. Students can also complete their calculations, plot their data, and analyze their results within the allotted lab time. This modified procedure enables the students to do at least five trials during a three-hour lab period to check reproducibility. A graph of the amount (moles) of hydrogen produced as a function of the amount of magnesium used in the reaction can help students visualize the linear relationship between the reactants and products. The students are able to determine the stoichiometric coefficients of the species in the reaction by comparing the ratios of the moles of magnesium to the moles of hydrogen produced during each trial. The use of spreadsheets further enables efficient manipulation of data.
pressure. This time period allowed for the reaction to reach completion and for the temperature to approach room temperature. The temperature at the end of the twenty-minute time period was also recorded. This procedure was repeated four more times using varying amounts of magnesium (0.02500 g to 0.10250 g) and using the same volumetric flask. The flask was rinsed with water and acetone in sequence and dried between trials. The maximum amount of magnesium used in this reaction was limited by the total pressure in the flask. Our setup could withstand pressures of about 1.50 atmospheres. The amount of HCl used in each trial was kept constant. Excess HCl was used in each case ensuring that magnesium was the limiting reagent. Discussion and Results The difference between the pressure in the flask at the end of twenty minutes of reaction and atmospheric pressure was assumed to be the pressure due to hydrogen gas produced in the reaction. The volume of the Erlenmeyer flask was determined by measuring the mass of water contained in the flask when filled. The volume of the gas was assumed to be the volume of the flask minus 50.0 mL. The temperature in the reaction flask after the twenty-minute reaction period was used as the temperature of the gas. The amount of hydrogen was determined using the values for pressure, volume, and temperature and substituting them into the ideal gas equation. The mass and amount of magnesium reacted, the amount of hydrogen produced, the final pressure recorded, the change in pressure, and the mole ratio for each trial are shown in Table 1. The continuous monitoring of pressure enables the students to notice the qualitative relationship between the amount of magnesium reacted and the pressure of hydrogen
Procedure Magnesium was weighed in an open vial. This vial was carefully placed in a 500-mL Erlenmeyer flask containing 50.0 mL of 1 M HCl ensuring that the magnesium and HCl would not react. A thermometer and a PASCO Model CI6532A Absolute Pressure Sensor were inserted into a twoholed rubber stopper. This stopper, when attached to the Erlenmeyer flask, enabled the monitoring of the temperature and pressure of the gas phase in the Erlenmeyer flask (Figure 1). The pressure sensor was attached to the PASCO Science Workshop 500 computer interface. Pressure measurements were commenced before the flask was sealed to note the atmospheric pressure. The flask was then sealed with the stopper and gently shaken to allow the magnesium and HCl to react for twenty minutes while continuously monitoring the
Figure 1. Experimental setup interfaced with the computer.
JChemEd.chem.wisc.edu • Vol. 80 No. 3 March 2003 • Journal of Chemical Education
305
In the Laboratory 0.005
∆ Pressurec H2:Mgd
Run
Mg (g)
Moles Mg
Moles H2a
Pressureb
1
0.0247
0.00102
0.00099
1.055
0.049
0.971
2
0.0478
0.00197
0.00187
1.099
0.093
0.949
3
0.0769
0.00316
0.00304
1.157
0.151
0.962
4
0.1006
0.00414
0.00412
1.211
0.205
0.995
5
0.1026
0.00422
0.00422
1.216
0.210
1.000
a
Amount of Hydrogen / mol
Table 1. Experimental Data
The moles of hydrogen were determined using the ideal gas law.
b
Total pressure in the flask after 20 min. All pressures were measured in kPa and converted to atm.
0.003
0.002
Difference between total pressure and atmospheric pressure (1.006 atm).
y = 0.9868x + 9 × 10⫺5
0.001
R 2 = .9987 0.000 0.000
c
d
0.004
0.001
0.002
0.003
0.004
0.005
Amount of Magnesium / mol
Mole ratio for hydrogen versus magnesium.
Figure 2. Amount of hydrogen gas produced as a function of the amount of magnesium solid reacted with hydrochloric acid.
gas produced. A graph of the amount of magnesium versus the amount of hydrogen is plotted (Figure 2). It shows the linear relationship between the product and the reactant. The slope gives the stoichiometry of the reaction. The data collection period is an excellent time for the instructor and students to interact with each other and the data. This is an ideal time to ask students to start thinking of the actual quantitative relationship between the quantity of magnesium reacting and the quantity of hydrogen being produced: “Would 1 g of magnesium produce 1 g of hydrogen or 2 g of hydrogen?” “Does 1 g of magnesium produce 1 atm of hydrogen pressure?” “How would you go about establishing the connection at the molecular level in terms of moles of magnesium and hydrogen?” The students could also be asked to think about the various units of measurement of the two species of interest and to explain the need for converting the quantities of magnesium and hydrogen to moles. This discussion could then be extended to include various interpretations for the coefficients of a balanced chemical equation. This lab methodology also lends itself to computer-based data collection and analysis. This procedure, which utilizes technology to facilitate convenient data collection, would likely preserve student interest in focusing on understanding the concepts of stoichiometry. It is recommended that students make use of a spreadsheet program such as Microsoft Excel to organize their data and complete necessary calculations. Vernier’s Graphical Analysis offers an alternative to using a spreadsheet and can also be used for student data analysis. Use of this method precludes the necessity for repeating tedious calculations for each trial while providing results of all trials in a short time period, enabling further discussion of the quality of results.
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Hazards Usual laboratory safety procedures such as wearing safety goggles must be followed at all times. Mg and 1 M HCl are routinely used in undergraduate chemistry laboratories and present no unusual hazards. The main hazard in this experiment comes from the likelihood of build up of excessive pressures that could result if the quantity of Mg recommended in the procedure is exceeded. Excessive pressure could cause the stopper to come off the flask and fly in the laboratory. Splashing of HCl resulting from this is a possibility. Acetone used for drying glassware is volatile and flammable. Acknowledgment We would like to thank David M. Bowhall, graphic designer at Jefferson Community College, for producing Figure 1. W
Supplemental Material
Instructions for the students are available in this issue of JCE Online. Literature Cited 1. Rosen, Charles E.; McCluskey, Catherine L. J. Chem. Educ. 1999, 76, 638–640. 2. Neidig, H. Anthony; Spencer, James N. The Stoichiometry of the Reaction of Magnesium with Hydrochloric Acid; Neidig, H. A., Ed.; Chemical Education Resources, Inc.: Palmyra, PA, 1989; STOI–369. 3. Birk, James P.; Walters, David L. J. Chem. Educ. 1993, 70, 58–589
Journal of Chemical Education • Vol. 80 No. 3 March 2003 • JChemEd.chem.wisc.edu
