Instructor Information
JCE Classroom Activity: #52 Flipping Pennies and Burning Candles: Adventures in Kinetics W Michael J. Sanger Department of Chemistry, Middle Tennessee University, Murfreesboro, TN 37132 In this Activity, students collect data to determine whether two processes, flipping pennies and burning candles, follow zeroth- or first-order rate laws. Students first collect data on the number of pennies remaining “heads-up” after several successive tosses and then measure the mass of a burning candle over time.
This Activity is an adaptation of a 2002 Journal of Chemical Education experiment (1) in which students plotted data in three graphs: amount versus time (linear for a zeroth-order process), ln[amount] versus time (linear for a first-order process), and amount–1 versus time (linear for a second-order process). The most linear graph determined the reaction order. It has been previously established that the penny flipping experiment is consistent with first-order reactions (1, 2) and the candle experiment is consistent with zeroth-order reactions (1, 3–4).
Integrating the Activity into Your Curriculum This Activity and the previously published experiment (1) are inquiry-based and introduce reaction orders, determining reaction orders, and calculating simple kinetics. The processes of flipping pennies and burning candles can be easier to understand than kinetics experiments involving chemical reactions. Instructors may wish to have students perform kinetics experiments after completing this Activity. The graphing experiment (1) requires access to classroom computers or assigning graphing as outside homework; this Activity requires only simple calculations.
About the Activity Students first shake and toss 100 pennies onto a flat surface. They count and keep pennies that land heads-up in successive tosses. This continues until fewer than 3 pennies are heads-up. Data with fewer than 3 pennies should not be used since the uncertainty of ±1 penny represents a large error. The data point of 0 throws and 100 heads-up pennies should be included. Students then weigh a birthday candle, burn it for 30-second increments, and weigh it again after every increment. The candle should burn for the same interval and in the same vertical orientation for every data point. The candle can be mounted in a digital balance pan using modeling clay or a one-hole rubber stopper and the reading tared before beginning. In this way, the candle remains vertical, any dripping wax or candle wick pieces will be taken into account, and there are fewer difficulties with trying to light the candle for each trial. The candle is burned continuously and mass readings are taken every 30 seconds. Students can also light the candle for each trial, hold it as it burns for 30 seconds, extinguish the candle, and then take a mass reading if a digital balance for continuous data collection is unavailable. If no balances are available, students could collect length data instead. In this case, the length lost for each time increment is roughly constant, but it is more difficult to determine because of the uneven wax edge. Students calculate the number of pennies and candle mass that disappear over time (∆n and ∆m) and the proportion of pennies and candle mass that disappear over time (∆n/n and ∆m/m). Then they determine which set of data is more constant for each experiment, (∆X or ∆X/X). If the ∆X values are more constant, it is a zeroth-order reaction because rate = ∆X/∆t = constant and ∆t is always the same; if the ∆X/X values are more constant, it is a first-order reaction because rate = ∆X/∆t = kX, so ∆X/X = k∆t. A sample set of data is on JCE Online.W perforated
Answers to Questions 1. The ∆n and ∆m values are negative because the number of pennies landing heads-up (n) and the mass of the candle (m) are decreasing as the trials proceed. 2. Flipping pennies is a first-order process. Approximately half of the pennies are headsup after each toss. The proportion lost remains constant. Burning a candle is a zerothorder reaction. The mass lost during each time interval is approximately constant. 3. For the candle data, ∆m values should be identical (see final paragraph of About the Activity), but values will probably vary slightly. Reasons for this could be inconsistent time intervals, changing the angle of the candle, or simple balance or student errors. 4. For the penny data, ∆n/n values should be identical (see final paragraph of About the Activity), but values will probably vary slightly. Reasons for this could be inadequate or biased shaking techniques, simple student errors, and limited data values when dealing with small numbers of pennies.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Background
References, Resources, Sources of Related Activities 1. 2. 3. 4.
Sanger, Michael J.; Wiley, Russell A., Jr.; Richter, Erwin W.; Phelps, Amy J. Rate Law Determination of Everyday Processes; J. Chem. Educ. 2002, 79, 989–991. Tawarah, Khalid M.; An Example of a Constant Rate Reaction; J. Chem. Educ. 1987, 64, 534–536. Harsch, Gunther; Kinetics and Mechanism—A Games Approach; J. Chem. Educ. 1984, 61, 1039–1043. Schultz, Emeric; Dice Shaking as an Analogy for Radioactive Decay and First Order Kinetics; J. Chem. Educ. 1997, 74, 505–507. JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 80 No. 3 March 2003 • Journal of Chemical Education
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JCE Classroom Activity: #52
Student Activity
Flipping Pennies and Burning Candles: Adventures in Kinetics When you see a burning candle, do you think of kinetics rate laws? Probably not. How about a coin toss? Interestingly enough, many everyday non-chemical events follow the same rate laws that govern chemical reactions. In this Activity, you will determine whether the simple processes of flipping pennies and burning candles are consistent with zeroth- or first-order chemical rate laws.
Try This You will need: 100 pennies (other coins may be used), flat surface, cup or bag large enough to hold the pennies, small unburned birthday candle, matches, modeling clay or one-hole rubber stopper (hole should be of correct size so that candle will fit into it), 0.01-g digital balance, timer, paper, pen or pencil, and calculator.
Be Safe! Keep combustibles away from flames and never leave the candle unattended.
Flipping Pennies __1. Make a table with four columns: toss number, number of heads (n), ∆n, and ∆n/n. __2. Turn 100 pennies so they are all heads-up on a flat surface. This is your first data point (toss number = 0, number of heads = 100). __3. Pick up the pennies, place them in a cup or bag, shake them, and toss them onto the flat surface. If any pennies land on top of each other, slide them apart without turning them over. Count how many pennies landed heads-up. Enter the number of the toss (1, 2, 3, etc.) and the number of heads-up pennies in your table. Return the heads-up pennies to the container, and set the rest of the pennies aside. __4. Repeat step 3 until you have fewer than three pennies heads-up. Do not enter this last data point in your table. __5. Calculate the change in the number of pennies (∆n) between toss numbers 0 and 1 from the formula ∆n = n1 – n0 (where n1 is the number of heads left after the first toss and n0 is the number of heads left after the zeroth toss). Place this value in your data table in the ∆n column and the toss number = 0 row. Calculate ∆n for the rest of the rows. There will be no data point in the ∆n column for the final row. __6. Calculate the change in the proportion of pennies (∆n/n) for each row by dividing the ∆n value in that row by the n value in that row.
Burning Candles __1. Make a table with four columns: burn time (in seconds), candle mass (m) (in grams), ∆m, and ∆m/m. __2. Weigh a small unburned birthday candle. This is your first data point, with a burn time of 0 sec. __3. Place modeling clay or a one-hole rubber stopper on the pan of a 0.01 g digital balance. Tare the balance so the reading is 0.00 g, then insert the candle so that it stands vertically, wick end up, on the balance pan. __4. Light the candle and start a timer at the same time. After the candle has burned for 30 seconds, record the balance reading in your table. Keep the candle burning and continue to record the mass reading at 30 second intervals until most of the candle has burned away. Extinguish the candle and clean the balance pan. __5. Calculate the change in mass (∆m) and the change in the proportion of mass (∆m/m) for the candle data in the same way you did for the penny data.
Data Analysis For a zeroth-order reaction, the change in amount (∆n for pennies, ∆m for the candle) is constant for the same time increment. If a process is zeroth order (∆n or ∆m), the number should remain the same (or very nearly the same) for each trial. For a first-order reaction, the change in proportion (∆n/n, ∆m/m) is constant for a given interval of time.
Questions __1. Why are the ∆n and ∆m values negative? __2. Examine the penny and candle data. What is the reaction order for each process? Explain your reasoning. __3. Data for the change in amount (∆n, ∆m) should be the same for a zeroth-order reaction. Why? Look at this data for any zeroth-order reactions you have. Are the data identical? Why might they be slightly different? __4. Data for the change in proportion (∆n/n, ∆m/m) should be the same for a first-order reaction. Why? Look at this data for any first-order reactions you have. Are the data identical? Why might they be slightly different?
Information from the World Wide Web (accessed January 2002) 1. 2.
Notes on Kinetics: Reaction Order; http://www.chem.vt.edu/RVGS/ACT/notes/rxn_order.html Chemical Kinetics Simulation; http://www.chem.uci.edu/education/undergrad_pgm/applets/sim/simulation.htm This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 80 No. 3 March 2003 • JChemEd.chem.wisc.edu
