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HEAT CAPACITY OE'LEAD BROMIDG, ETC.
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The use of the bimolecular equation, velocity = k[sugar][water] is shown to be unsound, and it is shown that the inversion velocity is best expressed by the equation, dx/dt = k (sucrose molecules per molecule of water) (Hf activity), for any particular inversion. The decrease in water content and the increase in hydrogen-ion activity during inversion, are sufficient to explain the steady increase in the coefficients. SOUTH AUSTRALIA [CONTRIBUTION FROM THE CHEMICAL LABORATORY OF THE UNIVERSITY OF CALIFORNIA]
THE HEAT CAPACITY AND ENTROPY OF LEAD BROMIDE AND BROMINE BYWENDELL M. LATIMER AND HOWARD D. HOENSHEL RECEIVED JUNE 5, 1926
PUBLISHBD JANUARY 8, 1926
The purpose of this investigation was to obtain a more accurate experimental value for the entropy of bromine. At the time it was undertaken the only existing data were determinations of the average specific heat' of solid bromine from the boiling point of liquid hydrogen to that of liquid air, and from the latter point to 190'K. Since that time Eucken's coworkers2 have investigated the specific heat of bromine; a discussion of their results will be included. The entropy of bromine was determined by two independent methods. 1. The specific heat of solid bromine from 14'K. to its melting point was measured and the entropy calculated from the relation,
This expression was integrated by plotting C, against In T and taking the area under the curve. Existing values for the entropy of fusion and the specific heat of liquid bromine were used to obtain the entropy a t 29SOK. 2 . In a similar manner the entropy of lead bromide was determined, and the entropy of bromine calculated using this value, the entropy of lead and the A S for the reaction, Pb Br2 = PbBr2, since we have, from the third law of thermodynamics, Spb SBrl= SPbBrz AS. A S was determined from the experimental values for the change in free energy A F and the change in heat content, AH, of the reaction employing the relation, T A S = AH - AF. Since both the substances investigated retain a large portion of their
+
+
+
(a) Dewar, Proc. Roy. SOC.(London), 76, 325 (1905). (b) Barschall, Z. Elektrochem., 17, 341 (1911). ( c ) Koref, Ann. Physik, 36, 49 (1911). (d) Estreicher and Staniewski, Krakauer Anzeiger, 1912, p . 834. * Suhrmann and Lude, Z. Physik, 29, 71 (1924).
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WENDELL M. LATIMER AND H. D. HOENSWL
Vol. 48
heat content down to very low temperatures, it is desirable to carry the measurements to as low temperatures as possible. A temperature of 14'K., the triple point of hydrogen, was attained by boiling liquid hydrogen under reduced pressure. The liquefaction apparatus was that described by Latimer, Buffington and HoensheL3 The liquefier was built in connection with the calorimetric apparatus in such manner that the cooling effect of the expanding gas above the liquefyi ig point was utilized, and the trouble and loss arising in a transfer of the liquid hydrogen was avoided. Experimental Part Method.-The vacuum calorimeter method was used, with such adaptations as appeared advisable in this particular problem. The material was placed in a small metal calorimeter vessel, isolated from its surroundings by a high vacuum ; a measured amount of electrical energy was supplied, and the rise in temperature noted. In the case of lead bromide, a thin-walled copper calorimeter can, containing some 237 g. OF the material, was used. This material was prepared by precipitation from c. P . lead nitrate and potassium bromide. I t was thoroughly washed, and dried for several days a t 120'. The resulting fine powder was tightly packed into the calorimeter. In order to provide rapid thermal conduction through the mass of the material, several radial vanes had been soldered into the can. As a further aid, it was filled with hydrogen just before it was soldered shut. A silver can was employed For the bromine. Hard solder was used throughout, except for the final seal, which was above the level of the bromine. Otherwise, the construction was that described above. As a further precaution against corrosion, the bromine was frozen immediately, and was kept in that condition during the entire course of the experiments. After the measurements on solid bromine had been obtained, an attempt was made to investigate the liquid, but the vapor attacked the soft solder joint on the cover after one determination. The capacity of the calorimeter was approximately 216 g. C. P. bromine, distilled from phosphorus pentoxide, was used.
Temperature measurements were made in terms of a resistance thermometer. In making the choice between a resistance thermometer and thermocouple, it was believed that the former gives a better average value of the temperature of the calorimeter while it is more sensitive and much less affected by heat leaks through the connecting wires. The resistance thermometer was constructed of fine lead wire. This choice is due to the fact that the approximate linear relation between the resistance of lead and the temperature holds to a lower temperature than for any other metal. Wire of 0.017 mm. diameter was made by forcing the heated metal through a small hole under pressure. For simplicity, this same resistance was also used to supply heat to the calorimeter. The wire was wound on the outside of the can, covering the entire length. For insulation a layer of very porous rice paper saturated with thick Bakelite lacquer was first baked onto the can. Over this the lead 3
Latimer, Buffington and Hoenshel, THISJOURNAL, 47, 1571 (1925).
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wire was wound, the turns being separated from one another by an alternate winding of silk thread. It was then covered with another layer of Bakelite and paper, which served as protection against oxidation by the air, as well as afforded better heat conduction away from the wire. The whole was then covered with a layer of very thin copper foil, to lessen exchange of radiation with the surroundings. This sheath was soldered to the bottom of the can, to provide thermal contact. External connection with the thermometer was made through two d.s.c. B. and S. No. 30 copper wires, which were attached to the lead below the Bakelite covering. The arrangement of the calorimetric apparatus is shown to scale in Fig. 1. The can a was suspended by means of silk threads from the bottom of the large lead block b. To provide environment of an even temperature, the can was surrounded by the copper radiation shield c. During a run this shield was maintained a t a practically constant temperature approximately equal to that of the calorimeter. This constancy was accomplished by thermally isolating it, and providing a large heat capacity, in the heavy block b. Four boxwood pegs, inserted in the bottom of the shield, prevented it from touching the outside can. Thermal contact was provided with the block by fastening the shield, by means of small screws,' to the tightly fitting copper plate k , which in turn was soldered to the lead block. Lead was used for this block because of the high value of its specific heat a t low temperatures. This block was suspended by silk threads to the auxiliary block e, which in turn was fastened by copper studs to the cover of the outside can. The outside can d was fastened to the cover by means of a vacuum-tight soldered joint, and the entire apparatus evacuated through the tubef. The can and cover were of brass and the tube of steel. The steel tube was connected through a de Khotinsky seal t o the glass line leading to the \ mercury diffusion pump. A pressure of less than 5 X 10-6 mm. was maintained during the determinations. The end of the hydrogen liquefying interchanger I and the expansion valve ZJ are shown just above the outside can. Fig. 1. The whole apparatus is immersed in the Dewar tube p in a cold bath as-nearly as possible a t the temperature of
