Instructor Side
JCE Classroom Activity: #13
The Effects of Temperature on Lightsticks by the Journal’s Editorial Staff
fold here and tear out
About the Activity Lightsticks contain dilute hydrogen peroxide in a phthalic ester solvent enclosed in a glass vial, which is surrounded by a solution containing a phenyl oxalate ester and a fluorescent dye. When the lightstick is “snapped” and the vial is broken, the two solutions react to produce an intermediate that transfers energy to a dye molecule. Visible light is emitted when the excited dye molecule returns to the ground state. Additional details on chemiluminescence and the reaction can be found in this issue of the Journal (1) and other sources (2). The activity was written for groups of three students because of the number of lightsticks needed and the cost of the lightsticks. Each student or group should have three lightsticks for direct comparison at the three different temperatures. (Working in groups allows all three lightsticks to be activated simultaneously so students may assume that the difference in light intensity is not due to the length of time the lightstick has been reacting.) The activity can also be done by individual students or at home if desired. The activity was tested using 6-inch CYALUME SNAPLIGHTS (3). The activity can also be done with smaller lightsticks. When the activity is finished, the lightsticks can be given to students or kept for longer testing. It is safe to dispose of used lightsticks in a regular trash container. Students should observe the following in the “Try This” section on the reverse side: 1. The temperature of the lightstick does not change when it reacts. Chemiluminescence produces light without heat. 2. The hot-water lightstick will glow more brightly than the room-temperature and ice-water lightsticks. The roomtemperature lightstick will glow more brightly than the ice-water lightstick. CAUTION: students should not use water that is hotter than 70 °C or heat lightsticks directly. The lightstick’s plastic casing will melt at higher temperatures. 3. Carbon dioxide is a product in the reaction (1, 2). Bubbles of CO2 can be observed forming in the lightstick. Bubbles form vigorously inside the hot-water lightstick and more slowly inside the room-temperature lightstick. No noticeable bubbles form in the ice-water lightstick. 4. When the three lightsticks return to room temperature, they should have roughly the same light intensity. The light intensity of the hot-water lightstick may have diminished because more of its reactants were converted to products.
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Answers to Questions 1. The lightsticks are activated at the same time to insure that the differences in light intensity and bubble formation in steps 2 and 3 cannot be attributed to the length of time each lightstick had been activated. 2. The lightstick at the highest temperature is brightest because the rate of reaction is fastest at the highest temperature. 3. The lightstick at the highest temperature produces the highest rate of bubbling because the rate of reaction is fastest at the highest temperature. 4. The bubbles will stop forming when all the reactants have been converted to products. 5. A lightstick stops glowing when all the reactants are converted to products and no longer produce the intermediate that excites the dye molecules. 6. The lightstick that was at the highest temperature would have the lowest light intensity at room temperature because of the smaller concentration of reactant remaining—the reaction rate was fastest at the highest temperature so more of the reactants would have been used up. The lightstick that was at the lowest temperature would have the highest light intensity, because the low temperature slowed the rate of the reaction leaving a greater concentration of reactants.
Integrating the Activity into Your Curriculum This activity is simple to prepare and perform, but requires thought on the part of the student. It can be used early in the year to give students practice in making detailed observations and devising reasonable explanations for those observations. It can also illustrate the use of qualitative versus quantitative observations. It is easy to make qualitative observations with the lightstick system. Quantitative measurements are more difficult to make but can be done (4, 5 ). This activity can also introduce a unit on chemical kinetics.
References, Additional Information, and Related Activities 1. Salter, C.; Range, K.; Salter, G. J. Chem. Educ. 1999, 76, 84–85. 2. Shakhashiri, B. Z.; Williams, L. G.; Dirreen, G. E.; Francis, A. “Cool-Light” Chemiluminescence; J. Chem. Educ. 1981, 58, 70. Schreiner, R.; Testen, M. E.; Shakhashiri, B. Z.; Dirreen, G. E.; Williams, L. G. In Chemical Demonstrations; Shakhashiri, B. Z., Ed.; University of Wisconsin Press: Madison, 1983; Vol. 1, pp 146–152. 3. CYALUME and SNAPLIGHT are registered trademarks of the American Cyanamide Company. Lightsticks are available from some camping supply stores, sporting goods stores, discount stores, and various chemical suppliers. Suppliers offer lightsticks in a variety of sizes and colors; 2-inch lightsticks are sold for use in night fishing. Lightsticks are often available in discount stores around Halloween. 4. Estell, J. K. J. Chem. Educ. 1991, 68, 225–228. 5. Bindel, T. H. J. Chem. Educ. 1996, 73, 356–358.
This Activity Sheet may be reproduced for use in the subscriber’s classroom. JChemEd.chem.wisc.edu • Vol. 76 No. 1 January 1999 • Journal of Chemical Education
40A
JCE Classroom Activity: #13
Student Side
The Effects of Temperature on Lightsticks by the Journal’s Editorial Staff Chemiluminescence is the emission of visible light as the result of a chemical reaction without a noticeable change in temperature. Some familiar items that use chemiluminescence are lightsticks and glow necklaces and bracelets. They are often sold at amusement parks, hardware stores, and in some discount and toy stores, especially around Halloween. Very small lightsticks are used for night fishing and are sold in sporting goods stores. In nature, fireflies use chemiluminescence for signaling. When you “snap” a lightstick to activate it, you are setting a chemical reaction in motion. The lightstick contains a solution of a chemical (phenyl oxalate ester) and a fluorescent dye. There is also a small glass vial inside the lightstick containing a hydrogen peroxide solution. When you break the glass vial, the solution in the lightstick and the hydrogen peroxide from the vial react. This causes a transfer of energy to the fluorescent dye molecules, which excites electrons in the dye molecules to a higher energy level. When these excited electrons return to the ground state, light is given off. In this activity, you will investigate the effect of temperature on lightsticks.
Try This You and two partners will need the following: three lightsticks; two glass containers such as beakers or coffee cups; hot water (no hotter than 70 °C—about the temperature of hot coffee); ice; cold water; a darkened room. __1. In a darkened room, each person should take one lightstick out of its package. Feel the outside of the lightstick to determine its approximate temperature. Each person in the group should then “snap” a lightstick at the same time and shake it. Feel the outside of the lightstick again. Has the temperature changed? __2. Fill one of the glass containers with hot water. (CAUTION: do not use water that is hotter than 70 °C. If the water is hotter, the plastic of the lightstick may melt. The lightsticks should not be placed in water that is being heated. Heat the water, remove it from the heat source, and then add a lightstick.) Fill the other container with a mixture of ice and cold water. At the same time place one lightstick in the hot water and place one lightstick in the ice water. Leave the third lightstick at room temperature. How long does it takes for a change to occur in the hot-water lightstick and in the ice-water lightstick? What happens to the light intensity of each lightstick? __3. Look closely at the hot-water lightstick without removing it from its container. Notice the bubbles rising to the top of the lightstick. Compare the rate of bubble formation between the three lightsticks. If you have difficulty seeing the bubbles forming, you may have to remove the lightsticks from their containers and hold them up to a light source such as a window. What causes the bubbles to form? __4. Remove the lightsticks from the hot water and the ice water. Allow them to come to room temperature. What happens? How long do you observe changes?
Questions __1. In step 1 all three lightsticks were activated at the same time. Why was it necessary to do this? __2. Is the light intensity of the three lightsticks different at different temperatures? Explain. __3. Do bubbles form in the lightsticks at different rates at different temperatures? Explain. __4. When do bubbles stop forming in a lightstick? Why? __5. When does a lightstick stop glowing? Why? __6. If you keep three lightsticks for several hours at different temperatures (hot, cold, room temperature) and then allow them to come to room temperature, how would the light intensities compare? Explain the reasoning behind your answer. (To check your answer, try the experiment. You may need to add more hot water or ice during the test to maintain constant temperature.) After completing the experiments and answering the questions, ask your instructor what to do with the used lightsticks. It is safe to dispose of used lightsticks in a regular trash container.
Information from the World Wide Web 1. Omniglow’s Student Information; http://www.lightstick.com/student.htm (accessed Nov. 1998). 2. Omniglow’s Student Information—Classroom; http://www.lightstick.com/classrm.html (accessed Nov. 1998). 3. Glow in the Dark Book Report; http://www.glowinthedark.com/report.html (accessed Nov. 1998).
This Activity Sheet may be reproduced for use in the subscriber’s classroom. 40B
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
