exp21_rev9-07 - Experiment 21 Chemistry 541/2 Physical...

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1 Experiment 21 Physical Chemistry Laboratory The Bomb Calorimeter: Heat of Combustion (Revised 6/07) Introduction This experiment uses a constant-volume process to determine the heat of combustion of naphthalene. Before you start. .. The textbook “Experiments in Physical Chemistry” by David P. Shoemaker, Carl W. Garland, and Joseph W. Nibler has a useful chapter on thermochemistry. You should read this chapter before performing this experiment. Chemistry 541/2 Chemistry 541/2
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2 Apparatus The oxygen bomb calorimeter is a device used to measure the heat of combustion of solids and liquids. The sample is burned inside a container filled with oxygen. The container is called a “bomb” because of the high initial oxygen pressure required for combustion and the still higher pressures attained in the reaction. The bomb is immersed in a bucket of water of known temperature. The heat generated in burning the sample is determined by measuring the change in temperature of the water surrounding the bomb. To calculate the heat of combustion for a sample burned in a calorimeter it is necessary to know the heat capacity of the calorimeter. In this experiment, the heat capacity of the bomb apparatus will be estimated by combusting a compound (benzoic acid) with a known heat of combustion. An insulated jacket surrounds the calorimeter assembly so that allowance may be made, if necessary, for heat leak to or from the calorimeter. In the adiabatic method of calorimetry the temperature of the jacket is controlled so that it is always virtually equal to the temperature of the water in the calorimeter bucket and thus no heat leak is involved and no allowance for it is necessary. The thermometer is read only twice, once before ignition and once after final temperature equilibrium. However, the jacket temperature will not be controlled in this experiment. Instead, the calorimeter used in this experiment has a jacket, which remains at constant temperature; results are just as accurate as in the case of the adiabatic method, but more data are required. The calorimeter is normally a little cooler than the jacket at the beginning of the run; therefore, heat leaks to the calorimeter from the surroundings. After combustion, the calorimeter is a little warmer than the jacket, causing the heat to leak from the calorimeter to the surroundings. If the rates of temperature change before and after ignition are known, it is possible to extrapolate forward and backward to particular times between the foredrift and afterdrift periods in order to calculate the rise in temperature for the hypothetical case in which both temperature measurements are made simultaneously. Fig. 3 Calorimeter jacket, Passport temp. controller
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3 The equation you will need to calculate the temperature correction due to drift rate is as follows: t = t c - t a - r 1 (b - a) +/- r 2 (c – b) (1) Where t = net corrected temperature rise t c = temp. after time temperature rise at which the temperature has become constant
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exp21_rev9-07 - Experiment 21 Chemistry 541/2 Physical...

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