Calorimetry and Heat Capacity of a Metal Lab

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Florida International University *

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1045L

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Chemistry

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Apr 3, 2024

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CHM 1045L Florida International University Calorimetry and Heat Capacity of a Metal Purpose The purpose of this lab is to (1) determine the relationship between mass, change in temperature, and heat using the concepts of calorimetry, and (2) determine the heat capacity of aluminum. Introduction The first law of Thermodynamics states that the energy in the universe is constant. Therefore, when a hot piece of metal is added to a cup of water, the energy in the metal, the energy in the water, and the energy of the surroundings is constant. Since q is the amount of heat energy gained or lost at constant pressure, the exchange of heat energy between the heated metal and the water can be summarized as: q metal + q water + q surroundings = 0 In a laboratory setting, a coffee cup calorimeter can be constructed using two Styrofoam cups, one inside another to insulate the system so that the heat lost to the surroundings is small. If the assumption is made that the heat gained or lost is negligible, then q surroundings = 0 This isn’t an accurate assumption, but makes the system simpler to understand and work with in a laboratory setting. The previous equation would therefore simplify to: q metal + q water = 0 This can further be rearranged to: q water =− q metal A positive q value denotes an increase in heat (heat gained), whereas a negative q value denotes a decrease in heat (heat lost). In order to make the amount of heat lost to the surroundings small, it is important that the cover be kept on the calorimeter as much as possible. When the cover has to be removed to put the metal into the water, do so as quickly as possible. 1

CHM 1045L Florida International University Pre-lab questions: 1. In the calorimeter which component do you think is losing heat (metal or water)? Explain how you made that decision and cite your sources? 2. When heat is lost, what do you expect the sign on heat to be? Why? 3. In the calorimeter which component do you think is gaining heat (metal or water)? Explain how you made that decision and cite your sources? 4. When heat is gained, what do you expect the sign on heat to be? Why? Procedure: 1. Obtain as cylinder of aluminum that is about 2 cm x 1.25 cm diameter. Weigh the cylinder and enter the weight in the Data Table. 2. Place a 250 mL beaker with about 150 mL of water in it on a hot plate and turn the heat up to 10 on the knob. a. Place the aluminum cylinder in the hot water bath and allow it to boil for at least 5 minutes while you prepare the rest of the experiment. b. Ensure that the aluminum cylinder is completely covered while being heated to ensure proper heat transfer! 3. Assemble your coffee cup calorimeter using the figure provided. Weigh the two styrofoam cups and enter the mass of the calorimeter in the Data section in the blank provided. 4. Add 40.000 mL of water to the calorimeter and weigh the calorimeter and the water together. Add or remove water with a plastic pipet until you have as close to 40.000g of water in the calorimeter. Don’t forget to deduct the mass of the two cups! 5. Place the calorimeter on a magnetic stir plate with a small magnetic stir bar in the bottom of the calorimeter. DO NOT TURN THE HEAT ON – ONLY TURN ON THE MAGNETIC STIRRER. Cover the calorimeter with the cover and allow the water to stir while setting up the remainder of the experiment. 6. Prepare a data table in your lab notebook with columns for time (s) and for temperature (˚C). Use the data to graph your results and obtain a heating curve. a. You should have a graph depicting temperature on the y-axis and time on the x-axis. 7. Place the thermometer inside the boiling water bath containing the aluminum cylinder to obtain the temperature of the hot water bath. a. Once the temperature has reached a plateau (close to 100°C since the water should be boiling), record the temperature and let the thermometer cool down using some room temperature water. b. Once cooled, place the thermometer in the top of the calorimeter as shown in Figure 1. Make sure that the thermometer does not poke a hole into the top cup, and that it is not touching the bottom of the cup. 2 Figure 1: Coffee Cup Calorimeter Setup

CHM 1045L Florida International University 8. Begin to write down the temperature in your data table. Collect the initial temperature data (inside the calorimeter) for 15 to 20 seconds. a. The temperature during this time should be steady without too many fluctuations. If the temperature fluctuates too much, record data for longer than 20 seconds until it steadies. 9. Using tongs carefully but quickly transfer the aluminum cylinder to the calorimeter and cover the calorimeter. You must make the transfer quickly so that very little of the heat is lost from the aluminum during the transfer and so that almost all of the heat is transferred to the water. 10. When the temperature of the water has stabilized for about 10 seconds, stop recording data. a. Enter this value in Data Table 1 under the T final column. b. Write down the temperature before introduction of the aluminum cylinder and enter this value in the T initial column of Data Table 1. Calculate the change in temperature, ∆T. 11. Dispose of the water in the calorimeter but DO NOT lose the stir bar. Place the aluminum cylinder back in the boiling water bath to re-heat. 12. Repeat this experiment with two additional (different at least by 5 g) masses of water for a total of three different trials (T1-T3). a. Assume that the weight of the calorimeter as entered in Data Table 1 is constant for all of these experiments. 13. Pick one mass of water and perform two additional trials of the same mass. Place this data (and re-copy the original data point) in Data Table 2 (R1-R3). NOTE: If at any time the boiling water bath heating the aluminum cylinder evaporates to the point that the aluminum bar is exposed, you need to add more water back into the beaker; however, you must wait until the water reaches boiling again before continuing with the experiment!! Data Mass aluminum cylinder: Mass Calorimeter: _______________ Temperature boiling water bath: Data Table 1 Trial # m Cal + water (g) m water (g) T final water (°C) T initial water (°C) ∆T(°C) T 1 T 2 T 3 3

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