A SIMPLE CALORIMETER fm STUDENT USE ROBERT LIVINGSTON* AND WILLIAM HORWITZ Institute of Technology, University of Minnesota, Minneapolis, Minnesota
T
HE calorimeter which is described here was designed to enable the undergraduate student to make reasonablv precise measurements of heats of
mixing, quickly and with the need of a minimum of technical skill. The construction of the apparatus is relatively simple, and its general principles should be
certain corrections for differences in temperature which are necessary when an external container is used for the second liquid, the calorimeter is provided with an internal container which keeps the two liquids in thermal contact. This internal container is in the form of a dump bucket which permits rapid mixing and thereby minimizes the correction for heat lost during mixing. For the sake of mechanical simplicity and to keep the apparatus readily portable, the external ther-
of the large Dewar flask. It can be easily removed for cleaning and drying. The dump bucket is in the form of a truncated hemisphere and is open on top. It is supported by two wire hinges hooked over small metal pegs which are fastened to the wall of the calorimeter. It is dumped by means of a thread, H, which is fastened to the outer edge of the bucket, a t K, and passes through a small hole in the cover. The cover, F, of the Dewar is made of paraffined wood and has three holes drilled in it. The shaft and drive pulley, G, of the stirrer pass through one of these, a Beckmann thermometer is supported in the second by means of a rubber collar, L, and the thread, H, slips through the third. For the sake of mechanical simplicity the stirrer shaft is supported by an oiled bearing which is fastened to the cover, C, of the can. The stirrer is driven by a small motor, equipped with a reducing gear. Both the stirring motor and the Dewar are fastened to a metal plate. The Beckmann thermometer slips into PIGLTE 2 the can through a close fitting sleeve, M, which is fastened to the cover, C. The Beckmann thermometer, motor, and bed plate are not shown in the figure. The use of the calorimeter can be easily understood from the following outline of procedure for the determination of the molar heat of neutralization. A known volume (about 400 ml.) of dilute acid is introduced into the can, A, and one fourth of this volume of a sodium hydroxide solution slightly more than four times as concentrated is pipetted into the bucket, D. The excess of base is used to minimize the effect of any carbonate present. Before being used both solutions should have stood in the laboratory long enough to have come (practically) to thermal equilibrium. The Beckmann thermometer is adjusted to indicate room temperature on the lower part of its scale. The apmostat has been omitted. The calorimeter proper is paratus is then assembled as is shown in Figure 1. supported in a large Dewar flask provided with an The motor, which has a speed of about 300 R.P.M., is insulating cover. then started. The rate of stirring should be slow The detailed construction of the apparatus is in- enough so that the resulting heating of the calorimeter dicated by Figure 1. The calorimeter proper consists and its contents is not greater than 0.002'C. per minute. of a metal can, A , equipped with a rotary stirrer, B, The thermometer is read a t regular (two-minute) a tightly fitting cover, C, and an internal container or intervals until the rate of cooling or heating becomes dump bucket, D. This bucket was designed to hold constant. This requires a t least fifteen minutes. The 100 ml. and the can, exclusive of the bucket, 400 ml. temperature is then read a t one-minute intervals for The calorimeter used in the experiments which are five minutes. At the end of the fifth minute the bucket reported here was made of silver plated copper. It is dumped to mix the solutions by pulling the thread, would be preferable, however, to use one made of an H. Since the apparent heat capacity is influenced inert metal, since silver plate does not have a very long slightly by the method of mixing, it is necessary to life when subject to attack by acids and bases. To follow the same procedure in all experiments. It has avoid electrolytic corrosion, i t is, of course, necessary been determined empirically that satisfactory results to have all parts which come in contact with the solu- may be obtained by raising and lowering the bucket tions made of the same metal. The calorimeter can ten times a t five-second intervals. In this way the rests on small corks which are cemented to the inner wall concentrations and temperatures of the solutions inside
and outside of the bucket are rendered practically identical in less than one minute. For a t least five minutes more, the temperatures are recorded a t oneminute intervals. At some time, the reading of the Beckmann thermometer should be compared to an accurate tenth degree thermometer. To compute the molar heat of the reaction it is necessary to know the heat capacity of the system and the number of equivalents of the substance present in the least amount. The heat capacities of the solutious can be computed from their volumes, densities, and
is introduced by this simplifying assumption is largely compensated for by the use of a value of the "water equivalent" which is based upon the same assumption. The corrected rise in temperature can be obtained from a plot of the readmgs of temperature and time. The points which precede the steep rise should lie on a straight line, and the initial temperature can be obtained by extrapolating this line to the time when the bucket was first dumped. The corrected final temperature can be obtained by drawing a straight line through the final series of points and extrapolating back to the
TABLE 1
A
COYPAR~SON OR
Concrrlrolion of NoOH
STAW*BD VALWS OF TBB HBATSor NBVTRALIEATION TO VALUBSBASBDUPON MB&SVFSSIBNIO ~
T TAB H
SIMPLB CALORIXB~EB
Pn u n l .
- A H rals.
Conccnlralion of acid 0.2503N HCI 0.2503N HCI 0.2503N HCI 0.2503N HCI 0:2503N HCI 0.04985N HCI 0.04985N HCI 0.1674N CHGOOH 0.2087N CH&2OOH 0.04174N CHCOOH 0.04174N CHaCOOH 0.04174N CHCOOH 0.2505N CnHr.COOH 0.2505N GHs.COOH 0.2434N CHdX.COOH 0.2399N N NaHSOd 0.2399N N NaHSO, 0.2399N N NaHSO,
IT.
U0C.
(abrrracd)d
- A H calories (rmndnrd aoluar)
diffwmce
between the observed and the standard values of A n is n since these values were used to determine the heat capacity of the calorimeter, the without significance. The v a l u a i n the percentage differenceealvmn are departures from the mean and as sveh are a measure of the precision of the measurements. b ~omputingthe standard valves it has been assumed that the tunpernteoe5dent for this reaction is the same as for the corresponding reaction with acetic acid. a he "standard values" are from Thamsm's mesrurements which appear to have been made at about 18'C. d The pipet used in these experiment. delivered 99.3 ml. in*ead of 100.0 ml.
specific heats. m i l e the heat capacity, or "water e&ivalent," of the calorimeter can be computed from the and ~soecific heats . . .- weights .. - -o ~ ~ ~of the materials of which it is made, it is more satisfactory to determine the "water equivalent" by calibrating the, apparatus with a reaction for which the molar heat is accurately known. The neutralization of hydrochloric acid with sodium hydroxide is excellent for this purpose. This method of calibration has the advantage that it (partly) compensates for systematic errors due to faulty calibration of the thermometer and volumetric glassware used and also largely eliminates the uncertainty in the "water equivalent" which would otherwise be introduced by the effect of the rate of mixing.' Since the time of mixing is very short in these experiments, i t is sufficiently accurate to correct the rise in temperature for heat loss by a graphical method, assuming that the heat is liberated instantaneously the first time the bucket is dum~ed. Any error which
.
-
1 I t should be realized that while this type of calorimeter is c a ~ a h l eof vieldina results which are as reproducible as the tdrnmmerei ,cadi& (about 0.1 per cent. when the temperature ri,e ii 2%) it is not propcrly designed to give results of high absolute accuracy. (1:J. Whitc, "The modern calorimeter." The Chemical Cataloa C o . Inc.. S e w York City, 1928.) If the e5ect of the rate of mixing on the apparent "water equivalent" is not taken into account, the results while highly reproducible may he in error by as much or one, or even two per cent.
time of mixing. Since thermal equilibrium is not established immediately and since there is a lag in the response of the Beckmann thermometer the h t point or two, which correspond to measurements made directly after mixing, may not lie on the straight line and should be disregarded. Usually the points corresponding to the measurements preceding mixing lie on a straight line so accurately that if the bucket is dumped a t the instant the last reading is made, this reading may be taken as the initial temperature of the reaction. If this is done, only the final temperatures need be plotted and a larger scale may be used conveniently. Table 1 gives the results of a number of experiments which were performed to test the calorimeter. The initial concentrations of base and of acid are given in the first two columns. The centigrade temperature a t which the heat was determined is listed in the third, and the corrected rise in temperature in the fourth column. The observed molar heats of reaction (column five) were calculated by means of the usual formula,
where N, and Va are the normality and volume of the substance present in smallest amount (the acid in these experiments), At is the corrected rise in temperature, C ,, C ,, and C , are the heat capacities of acid, base, and salt solutions, respectively; Cois the heat capacity of the calorimeter, and the subscripts t and t' indicate the initial and final temperatures, respectively. A value of 31.0 cals. was used for CC.= This value was based upon five experiments performed with 0.25 N hydrochloric acid. In the performance of these experiments the usual analytical precautions were observed; calibrated glassware was used throughout, the base was "carbonate free," and all dilutions' were made with freshly boiled conductance water. The standard values for heats of reaction were taken This value is 5 cals. less than that calculated from the weights and specific heats of the components of the calorimeter (making allowance for the thermometer). For a discussion of this discrepancy see the preceding footnote.
from the literature (1-8), making allowance for the heats of dilution of the reactants and products. In the case of chloracetic acid, the observed value of - 14,340 cals. a t 25°C. is probably as accurate as any available. SUMMARY
A relatively simple portable calorimeter, which is suitable for student determinations of heats of mixing, is described. This instrument can be operated quickly, requires relatively little manipulative skill, and yields results which under favorable conditions are reproducible within 0.05 per cent. It is not designed to give results of high absolute accuracy, and should be used as a comparative instrument by calibrating with a reaction for which the molar heat evolution is known. A table is included of the results of a number of determinations of beats of neutralization, which were performed to test the calorimeter. m e results are compared to standard values taken from the literature.
REFERENCES
(1) THOMSEN, "Thermochemistry," Longmans, Green and Co., New York City, 1908. (2) "International Critical Tables." McGraw-Hill Book Co.. Inc.. New Yark City, 1933. AND ROWE. 1. Am. Chem. Soc.. 43, 779 (1921). (3) RICHARDS (4) RICHARDSAND GUCKER, ibid., 51, 723 (1929).
..
5) [6) (7) (8)
RICHARDS, Mom, AND H ~ B ibid., , 51, 729 (1929). LAMBEET AND GILLESPIB, ibid., 53, 2638 (1931).
BURYAND DAVIES.J. Chen. Soc.. 1932. 2415. Brcnows~~ AND ~ossrN1. "The thermo&emistry of chemical substances," Reinhold Publishing Corp., New York City, 1936.
