In the Laboratory An Inquiry-Based Chemistry Laboratory Promoting Student Discovery of Gas Laws W A. M. R. P. Bopegedera Department of Chemistry, Lab I, The Evergreen State College, Olympia, WA 98505; bopegedd@evergreen.edu Gas laws are covered in most undergraduate general ware provided by Vernier could be used also, if desired.) Stu- chemistry courses and even in some high school chemistry dents analyzed their data individually, outside the lab period. They were directed to plot graphs W and make inferences on courses. Once the concept of pressure and its units are intro- duced, chemistry texts launch into a discussion of gas laws the relationships between properties of gases based on these (1). Experiments to enforce the understanding of gas laws graphs, prior to attending a discussion session to go over their are often done in the laboratory after these concepts are cov- lab work. Each student submitted his or her lab report a few ered in lecture. From a description of Boyle’s original work days after this discussion. Students conducted experiments (2) to simple demonstrations and laboratory experiments (3– and explored the relationships between the following prop- 27), many articles on the topic of gas laws have been pub- erties of gases. lished in this Journal. An interesting article on the assessment 1. Pressure and volume (temperature and amount of gas of students’ and teachers’ understanding of gas laws was pub- held constant) lished recently (28). A laboratory experiment book for high 2. Pressure and temperature (volume and amount of gas school and college general chemistry, published by Vernier, held constant) covers some of the gas laws (29). For the past few years, I have taken a different approach 3. Volume and temperature (pressure and amount of gas to teaching gas laws by letting students “discover” them in held constant) the laboratory using their own lab data. The subsequent lec- 4. Pressure and the number of moles (volume and tem- ture time is used for problem solving using gas laws. This perature held constant) to determine the universal gas pedagogy was effectively used with chemistry majors and constant nonmajors. I conduct these experiments with readily avail- able, reasonably priced, Vernier software and laboratory Equipment and Chemicals equipment. 1 This equipment is user friendly and training time is minimal. The hardware and memory capabilities of a typical The materials needed for these laboratory experiments desktop computer are sufficient to host the software. 1 Prior are listed below. to conducting the lab experiments on gas laws, students are exposed to the following concepts. • An inexpensive 30-mL plastic syringe, available at any drugstore, was used as the sample cell for Experiment 1. • Chemical foundations, atoms, molecules, ions • For Experiments 2 and 4, a 250-mL Erlenmeyer flask • Introduction to the periodic table served as the sample cell. • Nomenclature of compounds • For Experiment 3, a 50-mL glass syringe was used • The mole concept, molar mass, writing balanced (Becton-Dickinson and Co., product # 512135). chemical equations • Three-way valves were purchased from Cole-Parmer • Stoichiometric calculations, limiting reagents (plastic three-way stopcocks with Luer connection, • Types of chemical reactions product # C-30600-02). • Acid–base reactions • Atmospheric air was used as the “gas sample” in Ex- At The Evergreen State College, students taking general periments 1 and 3. chemistry register for lecture and laboratory classes simulta- • In addition to atmospheric air, He, CO 2 , and N 2 gases neously. Therefore it is possible to use either the lecture class (Matheson Gas, 99.9% purity) were used in Experi- or the laboratory to introduce any concept. (For descriptions ment 2. of the academic environment at The Evergreen State Col- • Students used CO 2 gas in Experiment 4. lege please see refs 30–33 ). Four experiments are used to in- • The instructor repeated this experiment with SF 6 and vestigate the relationships between properties of gases and Ne gases (Matheson Gas, 99.9% purity) and provided only one set of lab equipment was used for each experiment. the results to students for inclusion in their data analysis. Since data acquisition time is short with the Vernier equip- ment, 25 students were able to complete all four experiments Conducting the Laboratory Work in a lab period of three hours. Laboratory time was dedicated to collecting data, which Experiment 1 was then exported to Microsoft Excel spreadsheets for graph- ing and analysis. (Excel is used for data analysis in all the The relationship between pressure and volume of a gas labs throughout the academic year; using it for these labs as was explored while keeping the temperature and the num- well supports students’ skill development. Logger Pro soft- ber of moles of gas as constants. The values for the volume www.JCE.DivCHED.org • Vol. 84 No. 3 March 2007 • Journal of Chemical Education 465
In the Laboratory of the gas were read from the 30-mL plastic syringe and the pressure was recorded with Vernier sensors and software. Stu- dents plotted graphs of pressure values versus volume and pres- sure values versus reciprocal volume and easily inferred that the pressure of a gas is inversely proportional to its volume at constant temperature (Figure 1), and is directly proportional to the inverse of its volume (Figure 2), thus “deriving” Boyle’s law. Experiment 2 A fixed amount of gas was held in a constant volume sample cell (250-mL Erlenmeyer flask) and the relationship between pressure and temperature was explored. The experi- ment was done with four different gases (atmospheric air, He, Figure 1. Graph of pressure data values vs volume of a gas sample CO 2 , and N 2 ). The sample cell was immersed in a water bath at constant temperature. Students analyzed these data and could at approximately 0 ° C and the temperature of the bath was deduce that the pressure of a gas is inversely proportional to its slowly increased by heating. The temperature and correspond- volume at constant temperature. ing pressure of the gas sample were recorded continuously until the final temperature reached 90 ° C. By plotting graphs of pressure versus temperature for the four gases, students concluded that the pressure of a gas sample held at constant volume is proportional to its temperature (Figure 3). Experiment 3 The relationship between volume and temperature of a gas was explored while keeping the pressure and the number of moles constant. A volume of atmospheric air was drawn into the 50-mL glass syringe, which served as the sample cell. The cell was sealed, immersed in a constant temperature wa- ter bath at 0 ° C and the inside pressure was allowed to equili- brate to atmospheric pressure. The value for the volume of the gas was read from the syringe and the temperature was recorded. This was repeated at several different temperatures of the water bath. Students plotted a graph of volume versus Figure 2. Graph of pressure data values vs reciprocal volume of a temperature and deduced that the volume of a gas at con- gas sample at constant temperature. After analyzing these data stant pressure is proportional to its temperature (Charles’s students could deduce that the pressure of a gas is directly propor- law). tional to the inverse of its volume at constant temperature. Experiment 4 The relationship between the amount of gas and its pres- sure was explored while keeping the volume and the tem- perature of the gas constant. A 250-mL Erlenmeyer flask (which served as the sample cell) was evacuated and weighed with an analytical balance. A small amount ( ∼ 150 torr) of CO 2 gas was introduced into the cell, its pressure was re- corded and the sample cell was weighed to determine the mass of CO 2 . This step was repeated several times, each time with a different amount of CO 2 in the cell. The volume of the cell was determined by filling the sample cell with water, de- termining the mass of the water and using the density of wa- ter to arrive at the volume of water (which is equal to the volume of the cell). Students input their data into a “class spreadsheet” and these combined data were used in the analysis. The instructor repeated this experiment with Ne and Figure 3. Graph of the pressure data values of the CO 2 gas sample SF 6 gases and provided these data to students to be included versus its temperature at constant volume. With these data students in their data analysis. W Students were directed to plot graphs could deduce that the pressure of a gas sample held at constant volume is proportional to its temperature. of pressure (P) versus the number of moles (n) (which showed 466 Journal of Chemical Education • Vol. 84 No. 3 March 2007 • www.JCE.DivCHED.org
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