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LINDAWOODWARD
chemistry for kids
The University of Southwestern Louisiana Lafayens, LA 70504
How Bright Are You? Energy and Power Hands-on Activities H. Eric Steltberger California State University, Fullerton, CA 92634 Why is air needed to bum a candle? Is air and the oxygen e o m it also needed to bum or oxidize food in our bodies? What are the chemical products that result when candles and food are oxidized? What are calories? What can calories tell us about the energy of a candle or of a cashew or walnut? How are calories determined? What can the wattaee of a light bulb predict about its brightness? How brigh;in wattshould you be compared to a liaht bulh if vuu walked or ran up some stairs? Would you be brighter if>ou ran or waked upstairs? How manicashew or walnut pieces must be burned or oxidized in your body to supply you with the energy to walk or run upstairs? These and other questions were used to introduce oxidation, energy, and power to junior high school students a t a Saturday Science session in chemistry a t California State University. Saturday mornings are assimed each spring semestcrio dcpart&ents to provide hands-on activities i;l chemistry, mathematics, physics, biolom, and earth science to increase interest in science. In addition to students and teachers, parents also are invited. Interestingly, parents become just as excited as their children when doing the science activities. The objectives of the initial questioning and briefdiscussions were to introduce students to the concepts below that are operationally grouped into the three pass:
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Ozidation is the combination of oxygen with an oxidizable substance. The burning of a candle in air and fwd in our bodies are examples of oxidations. Carbon dioxide, water, and heat enerw are released as common ~roducts. The heat energy released in oxidation is measured in units of kilocalories (kcal)and can be determined with a calorime-
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The brightness of a light bulb is measured in watts. The watt is a unit ofpower that is directly related to the brightness of s bulb during the time that it is turned on. The energy that a person uses in the time it takes to Nn upstairs can be calculated in watts and thereforeexpressed in brightness. The procedures used for the Saturday chemistry activities are as follows. Caution: Goggles must be be worn at all times in the laboratory. Part I. Oxidation Lieht a birthdav candle that is held uorieht with clav or in tce hole of a sGall, two-hole rubber stbpier. Place a 550mI, Erlenmever flask over the candle. The flame will extinguish after few seconds. Upend the flask, squirt in onehalf to three-fourths dropperfid of bromthymol blue (BTB)
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indicator solution,' stopper, and swirl. The BTB turns yellow indicating the presence of an acid. Carbonic acid is formed when Con eas., an oxidation oroduct. reacts with the water that is present in the indica'tor sol&ion. Asmall amount of water. in the form of a thin film in the flask. is also produced when the wax is oxidized. s q u i r t a n equal amount of BTB into a second Erlenmeyer flask for wmparison. The solution, when stoppered and swirled, remains blue (or blue-green depending on the pH of the water in the BTB solution). Humans exhale CO. eas after oxidizine foods that contain carbon atoms. ~ o t &for carbon dioGde, strongly exhale into the second Erlenmeyer flask wntaining the BTB solution then swirl. Repeat this procedure until the blue solution in the flask turns yellow indicating the presence of carbonic acid. Use cupped hands to form a tube connectine the mouth to the mouth of the flask. A straw may be used to bubble breath through the indicator solution to accelerate the color change, but this may cause accidents if students suck on the straw. To dramatize the oxidation of food in our bodies, one could (a) eat a cashew or walnut nrior to exhaline into the flask. and then (bjexhale against a mirror or t h i inside of a mktal jar lid to show the builduo of a film of moisture. To further demonstrate that the oxidation of a candle and food such as a cashew or walnut both produce COz, one e r over a can proceed as follows. Place a a ~ r l e n m e ~ flask burning nut held with a probe over the opening of an empty beverage can. After the flame is extinguished, upend the flask, squirt in BTB as before, then quickly stopper and swirl. The resulting yellow solution indicates that carbonic acid was formed from the reaction of carbon dioxide and water.
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Part 11. Calories of
a Cashew or Walnut
After a brief discussion of oxidation based on obsewations of the BTB color changes, the class was introduced to calories and calorie determinations using homemade beveraee can calorimeters. Students were shown how to use the flame from a burning cashew or walnut to heat water in the calorimeter. Operationally, calories were calculated as the product of the grams of water (measured in mL) heated in the calorimeter and the resulting temperature change in degrees Celsius. Kilocalories (kcal) are calories divided by 1000.
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kcal = (mL water) (degree Celsius change)11000 'One gram of bromthymol blue in 500 mL of water and 500 mL of ethanol. Volume 69 Number 4 April 1992
307
To facilitate this activity,' students were given a data sheet to record the grams and kind of nut used, milliliters of water in the calorimeter, initial and final temperatures of the water and the above formula for calculating calories. Procedures for the calorimetry activities are as follows. Preparing a Calorimeter
Cut an aluminum beverage can in half with scissors. Use pliers to crimp approximately 2-3 mm of the sharp edge toward the outside of the can and down. This removes the sharp edge. Use nylon tape to fasten three wooden dowels to the side of the can so that the can sits as a tripod approximately 10 cm above a level surface. Place between 50-100 mL of water into the can (record the actual amount), and then measure and record the initial water temperature with an alcohol thermometer. Using the Calorimeter
Place a weighed piece of nut on the table, then force the sharp point of a probe (from the biology department) a short distance into the nut. Lieht the birthdav candle. then use its flame to light the nucslightly rota& the burning nut burnine while holdine it under the calorime- - ~for - ~ even ter. Gently swirl thtwater with tKe thermometer. Record the highest temperature reached after the nut has stopped burning. Note: Half pieces of cashew and quarter pieces of walnut burn profusely once started. One must take care to dace the nut in the center under the calorimeter and not to burn the wooden dowel legs. Apiece of aluminum foil placed under t h r calorimeter win catch dropped burning pieces. ~
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The students typed the information from their data sheet into a computer. A readout provided them with the calculated kilocalories which they entered, along with their identifying initials, into a class data table on the chalkboard. The class was also reminded that the kcal informationwould be used in the following activity. The table encouraged students to compare and to self-analyze their results. Individuals with inconsistent results often retried the activitv. Parents in oarticular were surprised to find that kilocaiories are t h e same calorie units-listed in diet books. How Bright and Powerful Are You? Upon completion of the calorimetry discussions, students received a second handout in which they were asked to predict: (a) The brightness of a light bulb in watts that they think they would light up when walking and when running up two flights of stairs and (b) the number of nut pieces their bodies will oxidize in doing this. Students entered into the com~utertheir body weight in pounds,3the number of seconds they used to wsik an> run upstairs. and the number of kilocalories of their nut. The cornput& in turn provided a printout of each person's mass in kilograms, expenditure of energy in kilocalories, brightness (or power) in watts, and the equivalent number of nut pieces. Procedures and calculations used in the power activities follow. Determining the Number of Seconds to Move Upstairs
This requires two assistants. One assistant, at the bottom of the stairs, claps a piece of wood against the floor wine the s h a r ~report as a startine sienal for the runner and ?or the other assistant at the topofthe stairs to begm the timine with a stoowatch. The timing is completed when the walker or runner reaches the topof the st&. 308
Journal of Chemical Education
Calculating kcal of Energy to WalWRun Upstairs
The energy or work (J)for a person to move up the stairs equals force times the vertical distance (m) of the stairs. Force is body mass (kg) times acceleration (9.8 m/s2 since the person is moving upstairs against gravity). The body's efficiencyis estimated at 40%. One kilogram equals 2.2 lb. J = Ib x 1/22 ib per kg x 9.8 m/s2 x floors x rn stairdflonrx U0.4 kcal = JI 4180 J per kcal Calculating Brightness or Power in Watts
Watts are units of power and are defined operationally as energy per time to walk and run upstairs, i.e., Jls. Calculating Numbers of Nut Pieces
Based on the number of kilocalories to walk upstairs, the number of nut pieces to walk upstairs is: nut pieces = kcal x 11kcal per one nut piece The students placed the information from the computer printout into the data table on the chalkboard with their previously recorded calorimetry results. This method was excellent for providing a data base in the development and generalization of theioncepts. The table was used also to suggest additional experimentation by hypothesizing about the relationship of some of the variables, e.g., boys versus girls, two floors versus six, nuts versus spaghetti. students particularly seemed to enjoy the py%echnics of the burning nut in the calorimeter experiment. "Really weird!" was one particular exclamation that was picked up by a reporter covering the activities for a newspaper. In eeneral. the students and oarticularlv their oarents did i o t reaiize the small numder of kilochories that one expends when walking upstairs compared to the large number ofkilocalorieswe routinely ingest. And, conversely, the large number of stairs one has to climb (and not use the elevator) in order to lose a small amount of weight. The energy, in general, was the same whether one walked or ran up the stairs since the time to do this is not part of the energy equation. The activities also provided students with experience about Dower. They found, of course, that their brightness or power in wattskaried inversely &th the secoudgit took to walk or run upstairs. Most students intuitively realized that running racher than walking upstairs is more powerful, producing a higher wattage but had no experimental basis for verification.After predicting extremely low wattages for themselves when walking upstairs, participants were surprised that their wattage was relatively high and in direct proportion to their body mass. Summary The junior high school students (and their parents) in this Saturday session learned some chemistry and something about themselves while having fun with the activities. A computer was used for the calculations; therefore, the focus was not on the mathematics and precise defmitions but on general concepts and ideas. Interested readers are invited to send the author a self-addressed, stamped envelope for the calorimetry and light bulb data sheets and for two short computer programs written in Basic for Apple I1 series computers.
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a variation of this activity, see Borgford, C. L.; Summerlin, L. R. ChemicalActivities. Teacher Edition: American Chemical Societv: Washinaton. DC. 1988:D 116 3An i d bathroom s&ie converted to kilogram units could be used for this activity. Students in general did not know their body mass in
kilograms.
