Chemistry for Everyone edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Using Balloons for a Dramatic Presentation of the Acid–Bicarbonate Reaction submitted by:
Miroslav Proks ˇa* and Anna Tóthová Comenius University in Bratislava, Faculty of Natural Sciences, Department of Didactics in Sciences, Psychology and Pedagogy, Mlynská dolina, 842 15 Bratislava, Slovak Republic; *
[email protected] checked by:
Emily Brown Department of Chemistry, McDaniel College, Westminster, MD 21157 Stacy DeWees Hovde Department of Chemistry, Michigan State University, East Lansing, MI 48824
Rubber balloons are useful aids for chemistry experiments, especially for chemical demonstrations (1–6). In this article we describe how balloons may be used in novel ways to demonstrate the popular (7, 8) reaction of sodium bicarbonate with acid. Experiment 1
Objective To demonstrate that a gas is produced in the reaction of sodium bicarbonate with acid and show that it produces an acid solution in water. Materials • 200-mL distilling flask (or 250-mL Erlenmeyer filtering flask) • Rubber balloon • 250-mL wash bottle
funnel). Attach the balloon to the neck of flask so that the NaHCO3 stays in the balloon. During the demonstration, manipulate the balloon to add the NaHCO3 to the flask. After the balloon is fully expanded, gradually open the screw clamp to let the gas bubble through the solution in the wash bottle at the desired rate. The mixture in the flask may froth into the balloon, so we recommend holding the balloon up, if necessary, until the bubbles collapse.
Observation We first observe a vigorous reaction in the flask. A gas is produced and the balloon expands. The red color of the universal acid–base indicator turns to yellow. This reaction lasts for about 30 seconds. After opening the screw clamp, the gas bubbles through the solution in the wash bottle. The pink solution in the wash bottle gradually loses color. We control the speed and duration of the demonstration with the screw clamp.
• Piece of rubber tubing • Screw clamp • Ring stand • Utility clamp • Powder funnel • 7 g NaHCO3 • 50 mL of 1 M HCl • 1 mL of 1 M NaOH • 0.1% solution of phenolphthalein indicator • Universal acid–base indicator (thymol blue provides a similar range of colors)
Procedure Pour 50 mL of 1 M HCl into the flask and add 1–2 drops of universal acid–base indicator. Pour 100 mL of water into the wash bottle and add 1 mL of 1 M NaOH and several drops of phenolphthalein. Connect the flask with the wash bottle with rubber tubing with a screw clamp on the rubber tubing (as shown in Figure 1). Close the screw clamp, then add 7 g of NaHCO3 to the balloon (use the powder
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Figure 1. Setup for experiment 1.
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Explanation The experiment is based on the reaction of NaHCO3 with HCl: + NaHCO3(s) + H3O (aq) Na+(aq) + 2H2O(l) + CO2(g)
(1)
The pH in the flask increases because the addition of NaHCO3 neutralizes the H3O+ and this in turn increases the yellow conjugate base form of the indicator, symbolized In−: HIn + H2O
H3O+ + In−
(2)
In the wash bottle the generated CO2 dissolves to form H2CO3, which neutralizes the NaOH solution and causes the pH to decrease. The pink solution of phenolphthalein loses color as its conjugate acid is produced by a reaction that might be symbolized as the reverse of the reaction in eq 2, where HIn now represents the colorless, acid form of phenolphthalein. Experiment 2
Figure 2. Setup for experiment 2.
Objective To observe by experiment some chemical and physical properties of the gas produced in the reaction.
Discussion
Materials The same materials as used for experiment 1, except the wash bottle, phenolphthalein, and NaOH are replaced with a 1-L beaker, three candles of different heights, and a butane lighter. Procedure Securely place three candles of different heights into the beaker, as shown in Figure 2. Connect the flask to the Lshaped glass tubing with rubber tubing with the screw clamp on the rubber tubing, and light the candles. Initiate the reaction as described in experiment 1 and open the screw clamp after the balloon is fully expanded to let the gas gradually fill the beaker. First we observe a vigorous reaction in the flask. The gas expands the balloon and the red color of the universal indicator turns to yellow. The burning candles go out sequentially from lowest to highest after the screw clamp is opened. Explanation Carbon dioxide does not support combustion and therefore the candles go out. Carbon dioxide is denser than air. Carbon dioxide settles at the bottom of the beaker so the shortest candle goes out first. Hazards These experiments are not dangerous. The HCl and NaOH solutions should be handled carefully because they are corrosive. Phenolphthalein is a suspected carcinogen.
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We have suggested procedures that give the results described, but presenters may want to change the scale or the chemicals according to their needs. For example, vinegar may be substituted for the hydrochloric acid, sodium carbonate for the bicarbonate, or household ammonia for the NaOH. The presentation can be adapted to different age levels by emphasizing the surprising changes for elementary students or the pedagogical aspects for high school and college students. We often perform the demonstration to music. Acknowledgment This work was supported by Ministry of Education of Slovak Republic (KEGA 3/2362/04) Literature Cited 1. Arnáiz, Francisco J. J. Chem. Educ. 1993, 70, 1020. 2. Bergquist, Wilbur. J. Chem. Educ. 1993, 70, 586. 3. Fenster, Ariel A.; Harpp, David N.; Schwarcz, Joseph A.; Brice, Luther K. J. Chem. Educ. 1986, 63, 629. 4. Editorial Staff. J. Chem. Educ. 1997, 74, 1328A. 5. Jackson, Barbara Albers; Crouse, David J. J. Chem. Educ. 1998, 75, 997. 6. Mitchell, Edwin T. J. Chem. Educ. 1941, 18, 269. 7. Shakhashiri, B. Z. Chemical Demonstrations, A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison, WI, 1989; Vol. 3, pp 96–102. 8. Carvalho, Ana Paula; Mendonça, Ångela F. S. S.; Piedade, M. Fátima M. J. Chem. Educ. 2002, 79, 1464A.
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