r2 c:\files\courses\361-2lab\08labs\htcomb_old_instrument.docx 9/23/2008 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 (for the old calorimeter) 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. Open the calorimeter carefully as follows: (1) Raise the thermometers slowly by sliding the top part of the holder up. It is held in place by a spring loaded latch. Move the holder out of a detent by moving the ball knob away from the vertical rod and sliding the holder upward just a bit. NOW LET GO OF THE BALL KNOB and slide the holder and thermometers upward until it clicks into the upper detent. (If you do not let go of the ball knob, the thermometers may be raised so much that they come out of the holes at the bottom and they will swing about.) (2) Lift the lever located in the back left corner and change its position from facing right to facing left. (3) Swing the top open and then lift the inner can out. Notice the orientation of the electrical contacts on the can with respect to those on the main body of the calorimeter.r2 c:\files\courses\361-2lab\08labs\htcomb_old_instrument.docx 9/23/2008 2 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 distilled 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. 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 loose 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. It is therefore best to wrap the wire around each electrode in such a way that the wire goes through each hole twice. For example, thread the wire through the hole, make one complete turn around the outside and then go through the hole in the opposite direction. Pull it firmly with the needle nose pliers (hold the electrode while doing this to avoid bending the electrode) and wrap the loose end around the post. Repeat with the other side. 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 the OVER light will come on. The meter may still read zero but if the over light is on, the meter is off scale. Nowr2 c:\files\courses\361-2lab\08labs\htcomb_old_instrument.docx 9/23/2008 3 touch one multimeter lead to the top of the bomb so as to make contact with the ball with a slot, which goes to one