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AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition And Now for Some Fractions Student Instruction Sheet Challenge Determine the percent of H2O2 in store-bought hydrogen peroxide solution. Equipment and Materials • • • • • • • • • computer with USB port (2) PASPORT USB interface USB 4-Port Hub (optional) PASPORT Absolute Pressure Sensor PASPORT Temperature Sensor DataStudio software rubber stopper, two-hole Erlenmeyer flask, 125-mL graduated cylinder, 100-mL • Balance • manganese dioxide (MnO2), 0.2 g • hydrogen peroxide (H2O2) solution from a grocery store, 100.0 mL • protective gear • glycerin, 1 drop • Student Instruction Sheet • Student Response Sheet If your computer does not have two USB ports, you will need a powered USB 4-port Hub. Note: Safety Precautions Remember, follow the directions for using the equipment. Wear safety glasses and follow standard laboratory safety procedures. PS-2808 Stud ent Instruct ion Sheet 311 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition Background In order to better study and understand the incredible variety of possible chemical reactions, chemists classify reactions in a number of different ways. One common classification scheme recognizes four major types of chemical reactions: 1. combination or synthesis reactions 2. decomposition reactions 3. substitution or single replacement reactions 4. double displacement or double replacement reactions Starting from this classification scheme, reactions can be further classified according to the type of chemistry that occurs – acid-base neutralization, or oxidation-reduction reactions, for example. In this lab activity, you will investigate one example of a decomposition reaction. This type of reaction occurs when a substance chemically breaks down to form new products. The general formula for a decomposition reaction is: AB → A + B The reaction you will investigate in this experiment is the decomposition of hydrogen peroxide. You may be familiar with this compound; hydrogen peroxide solution can be commonly purchased in any pharmacy and it is found in most home medicine cabinets. In fact, the reaction you will be studying takes place in the bottle: hydrogen peroxide decomposes over time to form two products, liquid water and gaseous oxygen. Normally this process occurs slowly, but hydrogen peroxide solution is typically packaged in a dark-brown bottle to further limit the rate of the decomposition reaction. In the exploration that follows, you will add a catalyst, manganese dioxide, to the hydrogen peroxide in order to speed up the decomposition reaction rate. You will be able to monitor the reaction’s progress by using a pressure sensor to measure the oxygen gas that is produced as a result of the reaction. By using your knowledge of chemical reactions, stoichiometry, and the Ideal Gas Law, you will be able to calculate the percent concentration of hydrogen peroxide in the store-bought solution. You will treat the oxygen produced in this reaction as an ideal gas, defined as one in which all collisions between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces. Try to visualize an ideal gas as a collection of perfectly hard spheres that collide but that otherwise do not interact with each other. The Ideal Gas Law describes a predictable relationship 312 S tudent Instruct io n Sheet PS-2808 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition between the pressure, volume, and temperature of such a gas: PV = nRT In the Ideal Gas Law, P is absolute pressure (measured in kPa), V is volume (in L), n is the number of moles, R is the universal gas constant (R = 8.3145 L kPa / mol K), and T is absolute temperature (Kelvin). Using the Ideal Gas Law will let you calculate the number of moles of gas inside your flask before and after the reaction takes place. You will then be able to use the stoichiometry of the reaction to predict the percent concentration of hydrogen peroxide in the store-bought solution you will be testing. Predict Before beginning the eLab, complete the prediction and vocabulary portions of the Student Response Sheet. Explore Computer Setup 1. Plug the USB interfaces into the computer’s USB ports or USB Hub. 2. Plug the Temperature Sensor into one of the USB interfaces. This will automatically launch the PASPORTAL window. To computer 3. Plug the Absolute Pressure Sensor into the other USB interface. To computer 4. Choose the appropriate DataStudio configuration file entitled 12 Chemical Decomp CF.ds. and proceed with the following instructions. Configuration files automatically launch the appropriate display(s), sampling rate(s), etc. Note: PS-2808 Stud ent Instruct ion Sheet 313 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition Equipment Setup 1. Put a drop of glycerin on the barb end of a quick-release coupler that comes with the Absolute Pressure Sensor. Insert the end of the coupler into one end of a short piece (about 15.0 cm) of plastic tubing that also comes with the sensor. 2. Use a second drop of glycerin on the barb end of the other type of plastic connector that comes with the sensor, and insert this connector into the other end of the plastic tubing. Fit this side of the connector into one of the holes in the two-hole rubber stopper. 3. Align the quick-release connector on the other end of the plastic tubing with the pressure port of the Absolute Pressure Sensor. Push the connector onto the port, and then turn the connector clockwise until it clicks (about one-eighth turn). 4. Insert the Temperature Sensor into the second hole of the two-hole stopper. If the sensor fits loosely in the hole, place some rubber tubing around the probe. Ensure that the tip of the probe will not contact the liquid or foam—it must measure the temperature of the gas inside the flask. 5. Use the balance and a piece of weighing paper to measure 0.20 grams of maganese dioxide (MnO2). 6. Use the balance to find the mass of the empty Erlenmeyer flask. Record the mass on your Student Response Sheet. 7. Pour 10.0 mL of hydrogen peroxide solution into the 125-mL Erlenmeyer flask. Determine the mass of the flask containing the solution, and record this value on the Student Response Sheet. Note: Do not insert the Temperature Sensor too far into the Erlenmeyer flask. The stainless-steel probe should not get wet during the reaction. You want to measure the temperature of the gas inside the flask, not the liquid. 314 S tudent Instruct io n Sheet PS-2808 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition 8. Seat the rubber stopper holding the 2 sensors firmly in place into the neck of the Erlenmeyer flask. Record Data 1. Click the Start ( ) button to begin recording the pressure and temperature conditions inside the flask. 2. In order to start the reaction you will need to add the 0.20 g of manganese dioxide (MnO2) to the flask as quickly and efficiently as possible. To do so, fold or curve the weighing paper to form a funnel, remove the stopper from the flask, and pour the MnO2 powder in quickly. Immediately replace the stopper in the flask. 3. Hold down the stopper until the pressure stabilizes. 4. Click the Stop ( ) button to end data collection. Analyze Record calculations in your Data Table on the Student Response Sheet as you complete your analysis. 1. From the graph, find the initial pressure, final pressure, initial temperature, and final temperature. Record them on the Student Response Sheet. 2. Save your DataStudio (on the File menu, click Save Activity As...) to the location specified by your teacher. 3. Answer all the questions on the Student Response Sheet. 4. Follow your teacher’s instructions regarding cleaning up your work space. PS-2808 Stud ent Instruct ion Sheet 315 AND NOW 12. Chemical Decomposition 316 S tudent Instruct io n Sheet PS-2808 FOR SOME FRACTIONS AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition Student Response Sheet Name: ________________________________ Date: ________________________________ And Now for Some Fractions Vocabulary Use available resources to find the definitions of the following terms: catalyst: _____________________________________________________ ___________________________________________________________ decomposition reaction: _________________________________________ ___________________________________________________________ hydrogen peroxide (H2O2): _______________________________________ ___________________________________________________________ ideal gas: ____________________________________________________ ___________________________________________________________ PS-2808 Student Respon se Sheet 321 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition Predict 1. As described in the Background section of the Student Instruction Sheet, hydrogen peroxide decomposes over time to form two products, liquid water and gaseous oxygen. Write and balance the correct chemical equation for this process in the space below. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 2. Based on the equation you wrote above, how do you expect the temperature and pressure conditions inside the flask to change as the decomposition reaction proceeds? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 3. If a sealed air-tight container of an unknown gas contained n number of moles of gas, and you doubled the number of moles while keeping the temperature and volume constant, what would happen to the pressure? Explain your answer. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 322 St uden t Response Sheet PS-2808 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition Data a) Mass of empty flask b) Mass of flask with H202 solution c) Mass of H2O2 solution [(b) – (a)] d) Initial temperature e) Final temperature f) Initial pressure g) Final pressure h) Calculated initial moles of gas i) Calculated final moles of gas j) Moles of O2 produced [(i) – (h)] Analyze 1. Considering the volume of liquid used in this experiment, determine the volume that is occupied by gas inside the flask. Hint: This volume will be needed for additional calculations below. PS-2808 Student Respon se Sheet 323 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition 2. For the experiment’s starting conditions, use the Ideal Gas Law to calculate the initial number of moles of gas in the flask. Hint: Remember to use correct units for all quantities. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 3. Use the Ideal Gas Law and the final temperature and pressure values to calculate the number of moles of gas in the flask at the conclusion of the experiment. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 4. Determine the number of moles of oxygen gas produced by the reaction, and enter this value into the data table. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 5. Based on your analysis of the amount of oxygen gas produced, use the balanced chemical equation for the decomposition of hydrogen peroxide to determine the number of moles of hydrogen peroxide consumed. Hint: Refer to question #1 from the Predict section above. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 324 St uden t Response Sheet PS-2808 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition 6. Determine the percent concentration of hydrogen peroxide in your original sample solution: a. Convert the amount of hydrogen peroxide (number of moles) to mass (grams). ___________________________________________________________________________ ___________________________________________________________ ___________________________________________________________ b. Divide the calculated mass of hydrogen peroxide present by the mass of the original H202 solution. Multiply this ratio by 100 to obtain a calculated value for the percentage of hydrogen peroxide in the solution. ___________________________________________________________________________ ___________________________________________________________ ___________________________________________________________ Synthesize 1. According to the bottle’s label, what is the percent concentration of the store-bought hydrogen peroxide solution you used? ___________________________________________________________________________ ___________________________________________________________ ___________________________________________________________ How does this value compare to your calculated percent concentration? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 2. If your calculated value is different from the percentage listed on the bottle’s label, what sources of error might account for the difference? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ PS-2808 Student Respon se Sheet 325 AND NOW FOR SOME FRACTIONS 12. Chemical Decomposition 3. What was the purpose of the manganese dioxide (MnO2) used in the experiment? Did it react with another substance? If so, what were the new products formed? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 326 St uden t Response Sheet PS-2808