How To Use A Bomb Calorimeter

Transcription

1 University of Lethbridge Chemistry 2740 Laboratory Department of Chemistry & Biochemistry COMBUSTION CALORIMETRY Required materials In this experiment, you will obtain the combustion energy of a food product. Each group must bring to the laboratory one food. Given the equipment currently available in the Department, some foods cannot be studied. The sample must fit into a small sample cup (about 2 cm across) and you need a single piece with a mass of about 1 g. Chocolate is an example of a food that can easily be cut into appropriate pieces. Nuts and coated candies (M&M s, etc.) are often of a roughly appropriate size and can be studied directly. Other foods may be suitable if they can be formed into a pellet with equipment available in the lab. Most breakfast cereals can be pelletized since they have components that make them slightly sticky. Many dry goods (flour, starch, sugar) would almost certainly also work out. On the other hand, pasta and rice are too dry. Foods with an extremely high fluid content (soft drinks, jams, many fruits) must be desiccated before use and so require prior arrangement with the instructor. Although you only need one food item, it is recommended that each group member bring a different food, just in case technical difficulties get in the way of your first choice. Nonfood items which have appropriate chemical and mechanical properties and whose combustion is of interest (wood, coal, etc.) may be studied instead of a foodstuff if desired. However, it may be necessary to study smaller samples than recommended below for some materials so consult the instructor before proceeding. Theory The bomb calorimeter to be used in this experiment is a constant volume device. Accordingly, it measures changes in internal energy. It is an almost adiabatic device. Hence, if a reaction is carried out within it, we have ΔEtotal = 0 = ΔEreaction + q (1) or q = -ΔEreaction where ΔEreaction is the change in internal energy in the reaction and q is the heat released. Page 3-1

2 The heat released is related to the heat capacity of the system by q = CVΔT (2) The calorimeter is operated while containing both the bomb and a quantity of water whose total heat capacity is much larger than that of the products of the reaction. CV can therefore be taken to be a constant for a given calorimeter filled with a given amount of water. Enthalpy and internal energy are related by H = E + PV (3) so that ΔH = ΔE + Δ(PV) (4) Solids and liquids have small volumes and low compressibilities so that their appearance or disappearance during a chemical reaction has little effect on the product PV. Gases on the other hand have large volumes. Most gases behave at least approximately like an ideal gas so that we can use PV nrt (5) Bomb calorimeters are also operated in such a way that the overall temperature change is small, typically 2 5 K. The only thing that can change significantly in the right-hand side of the above equation is therefore the number of moles of gas. Combining this with equation 1, we get ΔH = ΔE + RTΔngas (6) Page 3-2

3 Safety and equipment handling You will be using high pressures of oxygen. High concentrations of oxygen can promote combustion so be on the lookout for sources of sparks in your immediate working area. Also be on the lookout for leaks in the equipment. Highly pressurized gases apply large forces to their containers so be careful to follow the instructions below closely to avoid launching dangerous projectiles across the lab. Use a blast shield to protect yourself during the pressurization of the bomb and when the sample is ignited in the calorimeter. Many parts of the calorimeter are electrically live when the ignition switch is pressed so leave the ignition unit unplugged until needed, then unplug it immediately after ignition to minimize shock hazards. The bomb looks sturdy, but it is a precision-built device that can be ruined by careless use. Be particularly careful not to scratch or dent any surfaces involved in sealing the bomb (screws, seals, etc.). The bomb is designed to be closed by hand. Do not use any tools on any part of the bomb. The bomb should never be cleaned with anything but water. The ignition switch is designed to deliver just the right amount of power to ignite a 10 cm fuse wire. Do not use a longer piece. The thermometer provided is a very expensive precision instrument. Be careful with it. The equipment must be carefully cleaned and dried after the experiment to keep it in proper operating condition. Page 3-3

4 Procedure Figure 1. A view of one of our bombs and of a section of the calorimeter. 1. Figure 1 shows a view of one of our bombs and of a section of the calorimeter. Our second bomb is similar in design but has two valves instead of just one. All the equipment should be clean and dry when you get it, but check the following before starting: a. The inside of the calorimeter is reasonably clean. b. The bucket and bomb are thoroughly dry. c. There are no bits of fuse wire attached to the terminals of the bomb. d. You have at least two reasonably clean and dry sample cups. 2. You will begin by calibrating the calorimeter with benzoic acid. Weigh a benzoic Page 3-4

5 acid pellet into a sample cup. (All weights are to be obtained on a balance that can be read at least to a tenth of a milligram.) Also weigh a 10 cm length of fuse wire. Note the (constant volume) heat of combustion of the fuse wire, which is given on the bobbin. Combustion of the fuse wire is exothermic so make sure to get the sign right. 3. Place the cup in the bomb head and attach the fuse wire. The fuse wire should be wrapped tightly two or three complete turns around each terminal. It must be in good contact with the sample but must not touch the sample cup. This is extremely tricky but take the time to do it right. If you don t, the calorimeter will misfire and you will have to start all over again. The easiest way to do this is to first crimp the wire as shown in Figure 2. Once the sample and fuse are in proper position, handle the bomb head carefully as you do not want to disturb the contact of fuse to sample. Figure 2. Crimping of the fuse wire. 4. Put the bomb head carefully into the bomb body. Screw the cap down tightly. 5. You will now flush the bomb with oxygen before filling it. Take the bomb to the oxygen filling station. Arrange things so that you can stand behind a blast shield while filling the bomb. With the bomb shown in Figure 1, remove the venting nut and attach the oxygen filling fitting in its place. For the bomb with two valves, tightly close the pressure-relief needle valve and attach the oxygen filling fitting to the self-sealing valve. Standing behind the blast shield, open the main cylinder valve. Open the supply valve and let the bomb fill slowly to a pressure of 20 atm. Note that when you stop, the pressure gauge will immediately start to drop. This is not an indication of a leak in the system so don t worry. After closing the supply valve, release the pressure in the line with the lever located just below the valve. Remove the oxygen filling fitting. Vent the bomb either by opening the needle venting valve (two-valve design) or by screwing the venting nut on about halfway, then pressing on it (one-valve design). 6. You will now fill the bomb with 20 atm of oxygen. The steps are identical to those above and must be carried out with the same care and caution. At the end however, Page 3-5

6 you screw the venting nut down tightly if using the one-valve bomb. 7. Verify that the ignition switch is unplugged. Put the bucket in the calorimeter and the bomb in the bucket. Plug the ignition lead into the bomb s external terminal post. Using a volumetric flask, add 2 L of distilled water to the bucket. Be careful to pour all the water into the bucket. 8. This calorimeter allows bubbles to come out of the bomb at a fairly steady rate. Despite this, there is more than enough oxygen in the bomb to ensure that this experiment still works. 9. Close the calorimeter, install the thermometer, and start the stirrer. Record the temperature every 30 s for 5 min. For optimal precision, use a telescope to read the thermometer. 10. Place the blast shield between you and the bomb. Plug in the ignition switch. To ignite the sample, you simply press the ignition switch. If all goes well, a red pilot light will come on for a second or so. As soon as the light goes out, release the switch. Record the time of ignition. If the light does not go off within 5 s, there is a short circuit somewhere in your system. Unplug the ignition switch, remove the bomb, vent it, dump out the bucket, dry everything, check the fuse, and go back to step Unplug the ignition switch. Remove the blast shield and start recording the temperature again at 30 s intervals. If the temperature does not rise significantly during the first minute or so, the bomb misfired. Remove the bomb, vent it, dump out the bucket, dry everything, check the fuse and sample, and start over. 12. Continue recording the temperature until the readings level off or the slope becomes constant for 5 min (generally about 10 min observation time post-ignition). 13. Remove the bomb and vent it. Open it up. Make sure that the sample has completely burned. If it hasn t, start over. (This is unlikely if everything else went well.) There will generally be some fuse wire left on the terminals. Carefully remove and weigh these pieces of fuse. Page 3-6

7 14. Clean the sample cup and terminals. Thoroughly dry everything. 15. Repeat with your food sample (pelletized if necessary). 16. When you are finished, leave the bomb completely disassembled, clean and dry. Analysis The most tedious part of the analysis of calorimeter data is determining ΔT. Both the pre-ignition and post-ignition data should have a linear section. You want to extrapolate these linear sections to the time of ignition. Figure 3 illustrates the procedure. Figure 3. Extrapolation of the linear sections of calorimeter data to the time of ignition. Before leaving the laboratory, please enter names, date, and experimental data into the computer. Page 3-7

8 Calculations and Report Remembering that qtotal for each run is equal to the heat released by what was in the cup plus the heat released by the amount of wire combusted, calculate CV for your experiment and use it to calculate the specific combustion energy of your food sample. Calculate the combustion energy per gram for your sample and compare that value to the literature or label value. Remember that nutritional calories (Cal) are equivalent to kcal. Page 3-8