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Introduction: Most chemical reactions are accompanied by a measurable absorption or evolution of heat energy.Using Hess's Law, we can combine several combustion reactions to calculate standard enthalpies of formation.In a bomb calorimeter, we measure the energy of combustion of a sample.

Introduction: Most chemical reactions are accompanied by a measurable absorption or evolution of heat energy. In a bomb calorimeter, we measure the energy of combustion of a sample. Using Hess's Law, we can combine several combustion reactions to calculate standard enthalpies of formation. The "bomb" is a pressure reaction vessel made from a special stainless steel alloy. On the lid of the bomb are two valves, one for filling the bomb with oxygen and the other for releasing the gases after the reaction is complete, and two terminals for connecting the ignition wires. Two electrodes, one straight and one with a loop, are connected to the bottom of the lid. A fuse wire, used to start the combustion, is connected, between the two electrodes. The sample, usually in the form of a compressed pellet, is kept in a stainless steel cup which fits in the loop electrode. When we pressurized the bomb with oxygen, the outward pressure on the lid forces a compression ring against a washer, and seals the bomb The bomb is immersed in an oval shaped bucket which contains 2000 g of distilled water. The bucket is placed in a plastic shell and covered with a lid, to minimize the transfer of heat energy between the bucket and room. A small stirrer agitates the water in the bucket, to keep it at a uniform temperature, and aid the exchange of heat energy between the bomb and the surrounding water. Using a special calorimeter thermometer, we measure the temperature change produced in the water from the combustion reaction The Fgute on the next page depicts the set up, Not shown is the ignition unit, used to pass an electric current through the fuse and thereby start the combustion, When we close the bomb, it contains the sample, some air, and Iml of water (to dissalve any oxides of Nitrogen and Sulfur that are produeed in the reaction) We then add oxygen to a pressure of about 25 atm Side reactions occur in the bomb that would not occur if the sample were burned in air, because the reaction inside the bomb is at higher temperatures than that in air. Any nitrogen present (there is always some from the air in the bomb) is converted to HNO1(ag) Any sulfur Compounds produce HSO(ag), These reactions release additional energy and we must correct for them Taus, afier the temperalure versus time measurements are completod we open the bomb and wash and collect the residue left inside into a beaker We then titrate to determine the amount of acid present