Heat of Combustion of Zirconium a General Chemistry Experiment When oxygen reacts with another material, energy in the farm of heat is released. The heat released is referred to as the heat of combustion. Methods used for the determination of heat of combustion in physical chemistry courses are usually too complicated and expensive for use in general chemistry laboratories. A simplified calorimeter would thus be very desirable to illustrate this experimental technique. The experimental system consists of a flash bulb (zirconium in an oxygen atmosphere inside the flash bulb'), surrounded hy water, fired electrically using a battery. The increase in water temperature depends on: 1) the heat released from the burning zirconium, 2) quantity of water, 3) heat capacity of water, 4) heat capacity of the glass and metal of the flash bulb and the calorimeter.
Experimental The calorimeter is assembled using a seven ounce polystyrene cup covered by a square of cardboard. Fifty ml of water is placed in the cup, the flash bulb (AG1 is recommended) is fully immersed in the water so that it does not touch the calorimeter. The flash bulh is held by the connecting wires which are passed through a slit in the cardboard cover. The wires should not touch each other. A thermometer is inserted through a hole in the cardboard tap so that maximum immersion is obtained. Note that the thermometer should not touch the flash bulb. The system is allowed t o equilibrate with the surroundings until a constant temperature is obtained. This temperature is recorded and the flash bulb is ignited using a 1.5-V dry cell. The water is stirred or swirled after firing and the maximum temperature obtained is recorded. The weight of zirconium in the flash bulh (AG1 = 0.0223 g) is obtained by weighing the zirconium wire from several bulbs and averaging the weights.
Calculations The following equations are used to calculate the heat of combustion of zirconium. 1 cal h e a t = - X grams of water A 1 deg lg r
AT t measured increase in water temperature weight of 50 ml of water heat capacity of water heat released from burning zirconium
I
X
I
heat A H = -in calories per gram g of Zr
AH =
heat
-in males Zr
calories per mole
AH (theoretical)' = 262.29 kcalimole
Discussion
In this experiment we have assumed that the heat capacities of the calorimeter, the glass flash bulb, the metal of the flash bulb, wires, etc. are negligible. We have also ignored heat leaks such as those around the thermometer, wires, etc. and have assumed that the zirconium wire is pure when in reality it is covered with an explosive paste. No safety problems are associated with breakage of the flash bulb since the few bulbs that do break are contained within the cup and cardboard comprising the calorimeter. The results obtained by the students usually fall within 5% to 10% of the theoretical value.
Boise State University Boise, Idaho 83726
Richard C. Banks Loren Carter Ellis R. Peterson
Volume 5 2 . Number 4 . April 1975
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