Frederick M. Hornyak' University of Tampa Tampa, Florida
A Flashbulb Bomb Calorimeter
This report describes a do-it-yourself experiment in thermochemistry using Sylvania Blue Dot N-2 flashbulbs as calorimeter "bombs." As compensation for the lack of accuracy and precision, t,he student gains an intimate acquaintance with the working elements of a calorimeter; a demonstration experiment with standard equipment may be less satisfying in this respect. In addition, certain chemical complicatious involved in the determinat,ion of a heat of formation are emphasized. The flashbulbs are filled with almost pure aluminum (99.0070 minimum) in an atmosphere of pure oxygen. The flash begirls when a fine tungsten filament, heated to incandescence, ignites primer beads of zirconium, potassium perchlorate, and a binder a t the ends of t,he lead wires. The primer beads then generate a heat stream sufficient t,o ignite the shredded aluminum. sec and peak brilliance The combustion lasts about is rearhed a t 13 millisec. Although a minimum of 3 v is specified for firing, a large ignition dry cell with 1.5 v was used and found satisfact,ory. The ralorimeter is made by bending a layer of pasteboard around a 150 ml beaker just below t,he lip and securing it with tape. A second layer of past,ehoard around the beaker extends to the top of the lip. Then two pasteboard circles are cut out. One is reserved for t,he 1,op of the calorimeter and t,he ot,her is taped t,o the bottom; masking tape is preferred. ~ o m borsembly. A 1engt.h of copper wire (20 gauge) is soldered to the soft metal t,ip of t,he flashbulb base. A second piece of wire is tiehtlv " twisted arouud both the bulb base and a wooden splint as shown in the figure. The second wire piece thus serves not only as a second electrical lead hut also to secure the "bomb" within the calorimeter. The boll) hase is apparently an alumiuuin alloy and ordinary solder cannot he used to fasten the second lead. To insure good electrical contact, t,he metal surfaces should be clean, and it may be necessary to remore t,he thin plastic film partially covering the bulb base. A piwe of Scotch tape applied to the splint and beaker lip a t 1: is sufficient to hold the homh assembly steadily ill place. The mlorimeter is con~pletedby pipetting 100 ml of didilled water into t,he beaker and fitting a stirrer and thermometer (0.1 or 0.2 degree divisions) through
rn -
the pasteboard top which is subsequently taped down; cork borers are used to cut holes. The stirrer, made from 4 or 5 mm solid glass rod, has a single right-angle bend a t the bottom with the tip flattened into a paddle shape. After firing, the maximum temperature rise is noted; the usual extrapolation to zero time is not employed. The student must then dissect the calorimeter, determine a value for the heat capacity, and calculate the energy evolved in combustion. The heat capacity of each item in the calorimeter assembly is determined by m~lt~iplying its weight by its specific heat. The M-2 bulb was broken into its metal and glass components which were weighed, giving an estimated heat capacity of 0.8 cal/deg. A value of 1.0 cal/deg was assumed for the thermometer. As shown in the table the student calorimeter has a total heat capacity of about 113 cal/deg. The heat produced by the flash react,iou can then be determined from the temperat.ure increment which should be about 0.80 degree centigrade. More accurate experiments using a Beckmann thermometer in a Dewar flask of known heat capacity showed that the M-2 flashbulb developed 88.1 cal; an average deviation of 1.8 cal. was observed. Results obtained with t,he student calorimeter were in good agreement with this value. The enthalpy change (-200 kcal) for the reaction
+
Alw
'/a
Olio, -*
'11
AIsOrro
is related to the energy chauge in the bomh (per gramatom of Al) by t,he equation AH = AE -" L,.
+ AnRT
AE= AH+a/,RT
At 25' the last term amounts to only 0.45 kcal. 4 consideration of the precision of the experiment shows that Calculated Heat Couocitv of Colorimeter a t 25'
~
Water Beaker Stirrcr Copper leads Thermometer hf-2 Flashbulb
99.7 56.0
0.998 0.20"
99.5 11.2
The author wishes to thank hfr. 11. E. Armstrang of Sylvania I3lcctrie Prorluct~Inc. for detailils of flashbulb construction. 1 I'rceent atldrpss, Ikpartment of Ch~mistry,Virginin PolyLrchnic Institutp, nlarksburg, Virginia. Volume 38, Number 2, Februory 7 967
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97
the difference between AH and An is not significant so t.hat Ah' is taken to he -200 kcal/mole. A comparison of the heat of t,he flashbulb reaction wit,h AE shows t.hat 11.0 mg of aluminum is consumed. Given this value, the student can measure the heat produced irr his calorimeter and perform t,he above calculatio~isin reverse, oht,aiuing the heat of comhustio~l of aluminum a t const,ant volume and constant pressure. Finally, the lat,ter value can he douhled giving a value for AHo,of AliOac,,. Alnminum oxide, corundum, or alumina as it is variously called, exist,s in several cryst,alline niodificat,ions. There is m n e ambiguity in a t,hermodynamic st,atement whirh gives no sperification. I t is found by x-ray diffract,ion t,hat. when nluminum is burned a-corundum is formed along with a type designated X-AI2O3. The enthalpy of transition for t,he conversion of x-A1203 to a-A1203is about - 10 kr:al/~nole,therefore douhling t,he heat of comhustion of Al docs not give the heat of format,ion of a-corundum; the necessary corrections give a final value2 of about -402 kral/mole. Another low t,emperat,ure modificat,ion, y-Al&, has a less negative heal; of format,ion than a-A1203and is easily convert,ed t,o t,he latt,er by application of heat. For this reason inrcsumublv) it, is not formed in comhustion.
98
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Journal of Chemical Education
Since the M-2 flashlamp rmitains ahout 16 mg of A1 foil, combnst,ion is only about 757, complete owing to a lack of oxygen. Indeed, an esvess of oxygen ~ w u l d lead to prohihitively high pressures a t the high temperature at,t,ained during burning. The excess aluminum melt3 dr~riilgthe flash and is splatt,ered against the interior walls of t.he hulb. The P-25 (press) hulb, which contailis ahout 33 mg of Al, can also be used as a bomb. Its larger volume makes it somewhat less convenient to handle; a 200 ml beaker is recommended. Its heat capacity is ahout 1.5 cal/deg. and it liberat,es ahout 100 cal of heat showing t.hat 26 mg of aluminum is consumed. Recently (1958) zirconium foil has heen introdr~ced as a flashhulh filler. Smaller than a peanut,, the Zrfilled Sylvania AG-I supplies about the same amount of light as the 11-2 whirh is four t,imes it,s size. I n general, zirconium burns mit,h a light out,put 100% great,er t,han aluminum with less pressure at. the flash peak. The enthalpy of formation3 of ZrOzo, is -258 kcal/mole whereas t,he value for '/zmnle of .4I2O3(,,is -200 kcal. Hence, on an equal weight basis zirr~~nium liherat,es murh less heat than aluminum does. a Thcrmodynnrnic data are tskcn from L A T ~ M E Wl'esrm1.1. R~ 31., "The Oxidation States of t h r Elcmmts and Their Potentids in Aqueous Solutions," 2nd ed., Prrnbiee-Hall, Inc., New Yorlc, 1952, pp. 270 and 282.
