r2 e:\files\courses\361-2lab\08labs\htcomb08.docx 9/28/2010 1 P Chem 361 Bomb Calorimeter - Heat of Combustion Introduction In order to measure the heat of combustion of reactions it is more convenient to conduct them at constant volume than at constant pressure. The heat evolved is absorbed by the contents of an insulated container called a calorimeter and is equal to the change in internal energy (∆U) rather than the change in enthalpy (∆H). If the compound being burned is known, the change in enthalpy (heat of combustion) may be calculated from the change in internal energy (∆U) and the change in the number of moles of gas. If the composition of the material is unknown, the difference between ∆U and ∆H is often unknown and therefore the results should be reported as ∆U. Our object in this lab is to calibrate a bomb calorimeter using benzoic acid in order to determine ∆U for sucrose and a dry cereal. The areas of explosives and nutrition are two examples of areas that need good data of this type. Procedure Calibration (week 1): The heat capacity (cal/K) of the calorimeter must be determined. Although the calorimeter is chiefly water, the entire contents of the system in the calorimeter contribute to the heat capacity. Although the heat capacity can be separated into the contribution of the water and the contribution of everything else, the heat capacity of the entire system is more convenient to use provided the amount of water is the same each time the calorimeter is used. To determine the heat capacity of the system a compound is burned that has a known heat of combustion. Using crucible tongs and a hot flame (light blue cone) heat a combustion cup until it is red-hot to burn or volatilize any residue that might be affected by the combustion step later. Place the hot cup on a wire gauze to cool. 1) Fill the calorimeter bucket and set in the calorimeter. If the calorimeter contains water, empty it out. Weigh the can before adding the water to the nearest 0.1 g on the large triple beam balance. Add about 2 liters of tap water and weigh again. You can use any large capacity measuring device. Check the bottom of the calorimeter housing to insure that it is dry. Lower the can into this enclosure being certain that the electrical contact is in the correct position. 2) Prepare the sample and charge the oxygen bomb Using a top loading balance weigh about 1 g of benzoic acid into a weighing boat. Weigh the combustion cup on an analytical balance. Lift the lever on the pellet press. Using a paper towel wipe clean the attached piston, the looser2 e:\files\courses\361-2lab\08labs\htcomb08.docx 9/28/2010 2 cylinder, and the bore of the large cylinder. Place the cup right side up inside the lower end of the large cylinder and slip it into place on the press. Put the loose cylinder into the barrel. It will slip to the bottom coming to rest on the inside of the cup. Now using a spatula place the benzoic acid into the barrel. If it will not all fit at one time, carefully position the upper attached cylinder over the hole and, after making certain that it is started straight, apply pressure to compress the contents. After adding the remaining benzoic acid if any, compress the pellet very firmly. If the lever goes all the way down, raise the pellet cylinder by turning it a bit and then compress the pellet. Remove the pellet by removing the cup along with the loose cylinder and then replacing the cup. Place the loose cylinder on top of the compressed pellet which is stuck in the barrel. Lower the lever gently until the pellet drops into the cup. Weigh the cup and contents. If some compound is loose, it will almost surely burn even if not part of the pellet. Although loose powder will burn, if there is too much of it, the combustion may proceed so rapidly that it might scatter the contents out of the cup and not burn. Place the top of the bomb on its stand which will enable wiring to proceed more easily. Cut 10 cm of fuse wire from the reel. There is a scale on the reel. The wire when properly attached to the solid wire leads will resemble a 'U' shape with each end bent toward an electrode and attached through the hole and the bottom of the 'U' resting on the top of the pellet. In order to avoid loss of compound attach the fuse wire before placing the cup in the electrode's cup holder. The fuse wire must make good electrical contact with the electrodes. Bend the wire into the desired shape and slip the cup into position with the wire fuse touching the pellet but not the metal cup. The resistance of the wire should not be more than 10 ohms. Use the multimeter to check this resistance. Plug it in, turn it to an appropriate ohms scale. Plug the leads into the correct set of contacts common and ohms). Touch the leads together. The meter should be close to zero. Now separate the leads and it should read infinite resistance. Now touch both leads to the plug holes on the top of the bomb. A modest amount of pressure on the sharp tips of the leads will usually ensure good contact. The multimeter should be relatively stable and should read less than 10 ohms. Tapping the top while measuring ohms should not change the reading. Place one ml of water into the bomb. (Use a pipet.) Lower the top section into the bomb and screw the lid on. It need not be very tight since its function is not to seal but to merely keep the top from being pushed out during the combustion. Sealing is performed by a rubber ring around the upper part of the top section. Close the exhaust valve which is operated by a small knob on the top of the bomb. Snug will do. (Turn it clockwise to close.) Carefully place the bomb in the holder on the bench.r2 e:\files\courses\361-2lab\08labs\htcomb08.docx 9/28/2010 3 THE INSTRUCTOR WILL DEMONSTRATE THIS PROCEDURE - HOW TO FILL THE BOMB TO 25 ATM. Open the oxygen tank value. (Open is CCW, counterclockwise.) The tank pressure, which is registered on the small gauge, must be no lower than 25 atm. Attach the fill line to the top inlet of the bomb. A pressurized bomb can be dangerous. While handling a pressurized bomb, keep it pointed up and do not get any part of your body over the top of the bomb. Fill the bomb, shut off the oxygen, and remove the fill line. 3) Place the bomb in the calorimeter and ready the assembly. Place the bomb next to the calorimeter. Notice that in the