T H E HEAT O F ABSORPTIOX O F H Y D R O G E S BY PALLADIUM BLACK AT O’* B Y LOUIS J. GILLESPIE AIVD HENRY A . hMBROSE
The only direct measurements of the heat of absorption of hydrogen by palladium that have the appearance of accuracy are those of Mond, Ramsay and Shields.’ These authors discussed the earlier work of Favre.? From pressure measurements at varied temperature, the heat of absorption (change of heat content) has been calculated by ;110utier3and by Dewar,? using the data of Troost and Hautefeuillej and of Roozeboom,6respectively, and by Gillespie and Hall,’ using their own data. Both Moutier and Dewar found the heat evolved upon absorption to increase with rising temperature; Gillespie and Hall found the opposite behavior. There is also some discrepancy as to the value of the heat calculated for 0’; Dewar finding about 9323 cal/mole Hz, and Gillespie and Hall finding about 6000 for the first hydrogen solution, 8860 for the horizontal isotherms, and about 9740 for the second hydrogen solution, the integrated value up to the composition of Pd2H being 8 ; 8 o . The heat measured at oo by Mond, Ranisay and Shields increased only very slightly with the hydrogen content, was assumed constant, and the average value found to be 9 2 1 0 , if a certain correction was introduced which seemed necessary,’ or 9362, if the correction was omitted. The agreement between the heat measured by %fond,Ramsay and Shields and that calculated by Dewar is probably fortuitous, as Dewar used only three points for the calculation of the three necessary parameters. A difficulty, likely to be present in heat measurements in all systems in which a variety of phases is capable of existence, is that in the heat measurements, necessarily rapid compared with the measurements of equilibrium pressure, phases may be formed other than those that can remain in phase equilibrium, and can then persist during the measurement of the heat. When these other phases have different heats of formation from those of the equilibrium-phases, an error will result in the reported heat measurement. I t is evident that more calorimetric data are desirable. When, in some preliminary experimentation, we found that nearly or quite the whole of the Contrihution from the Research Laboratory of Physical Chemistry, Massachusetts Institute of Technology, No. 270. Mond, Ramsay and Shields: Phil. Trans., 191A, 105 (1898). Favre: Compt. rend., 68, 1306 (1869); 78, 1257 (1874). iMoutier: Compt. rend., 79, 1242 (1874). ‘Dewar: Proc. Chem. Soc., 1897, KO. 183, 197. Troost and Hautefeuille: Compt. rsnd., 78, 686 (1874); Ann. Chim. Phys., 151 2, 273 ( 1 874). “oitsema: Archives nGerlandaises, 30, 44 (1896); 2. physik. Chem., 17, I (18951. H e reported his own, and Roozeboom’s data. Gillespie and Hall: J. Am. Chem. SOC.,48, 1207 (1926).
’
3 106
LOUIS J. GILLESPIE A S D HENRY A. AUBROSE
pressure-composition isotherm of Gillespie and Hall could be traced by merely adding hydrogen to palladium, without resorting to the heat treatment used by them, it seemed that' we were in a favorable position to undertake heat measurement 8. Experimental The palladium black was prepared from palladosammine chloride by the method of Gutbiers as before.' One variation of technique was made in preparing the hlack, which may have been responsible for the greater ease of absorption of hydrogen:-after the bulk of the ammonium chloride (formed in the reduction of the palladosammine chloride by hydrogen) had been removed in a current of carbon dioxide, the palladium weighed and introduced into the apparatus and the further small quantities of ammonium chloride removed by heating in a vacuum to the boiling point of mercury (as in the previous procedure), the palladium was further heated nearly to the softening-point of the pyrex container, the mercury diffusion pump still being connected. Further traces of ammonium chloride were seen to leave the palladium; and there seems to be no doubt that our palladium was freer from ammonium chloride than that of Gillespie and Hall. With this palladium their isotherm could be traced up to a point on the second rising portion of the isotherm, with or without the aid of heat treatment. Sometimes it happened that hydrogen driven off by heat did not all return to the solids on subsequent cooling, but this was the exception, and heat, treatment never produced a pressure appreciably lower than the value on their isotherm. In order to avoid certain difficulties, the heat was measured when palladium and hydrogen were brought together within a container of constant volume; thus we measured a change of energy. The desired changes of energy and of heat content were then computed for the constant-pressure, constanttemperature change of state: H2
+ 4 Pd
= z
PdzH (all at
oo, I
atm.),
(1)
where PdaH gives the average composition of two solid solutions. The essential features of the apparatus are shown in Fig. I . Palladium black, three times treated with hydrogen and then each time exhausted at a high temperature (in the capsule A) is contained in the sealed capsule A, provided with a thin flattened top. After the introduction of the capsule, a measured quantity of hydrogen is introduced into the tube B, which is closed by the steel stopcock I. The volume of that part of the container of hydrogen outside the ice calorimeter has been made very small in comparison with that of the tube B. The calorimeter assembly includes the ice calorimeter C ; a radiation shield D of copper, silvered on the inside; and the ice-bath E, insulated with felt two inches thick. The whole assembly is mounted on a base that can easily be raised into the position shown in the figure, or lowered. \Then in the low position, the ice-bath is filled with ice, the calorimeter given
* Gutbier: J. prakt. Chern:,
(2) 79, 23j
(1909).
HEAT O F ABSORPTIOS O F HYDROGEN BY PALLADIGM BLACK
3107
time to cool to on and then a layer of ice is formed on the inside wall of the calorimeter by immersing a tube cooled in liquid air in a layer of mercury (not shown) in the inner tube of the calorimeter. Then the assembly is raised to the position shown and the calibrated capillary tube F, which is provided with a vernier reading to 0.1mm., is connected at the ground-glass joint. The average mass of mercury per millimeter of capillary is 0 . 0 0 3 0 j 2 g. Khen the creep had become steady, the glass capsule A was shattered by means of the device H. By turning a nut, a long pointed rod, passing through a packing-gland, was forced down against the top of the capsule, breaking
Fig
I
Essentials of the Apparatus
it without a failure. The creep was in the same direction as the motion of mercury in the calibrated capillary that attended the reaction The average creep was determined before and after the reaction, and the creep per minute, multiplied by the time of the observation, was subtracted from the total motion observed. This correction was usually less than one percent. For computing the heat from the motion of mercury, the conventional value of the calorimeter constant was used: 0.01 j 4 6 grams of mercury per calls. This value was checked to about I percent by dropping known weights of water and of copper at known temperatures into the calorimeter. The number of moles of hydrogen absorbed was computed from the quantity originally put in the reaction tube and the final pressure and volume (measured after breaking the capsule, by adding a known quantity of hydrogen and applying the ideal gas law).
3 108
LOUIS J. GILLESPIE A N D HENRY A. AMBROSE
Results Table I gives the mass of palladium black and the moles of hydrogen absorbed, also the loss of energy, the loss of energy per gram of palladium, and the composition of the solids in moles of hydrogen per gram of palladium. The initial pressure never exceeded 2 atm., and the effect of a change of pressure from z to I atm. on the energy or heat content of hydrogen is negligible, as ascertained by the use of an exact equation of state. The effect of this change of pressure on the energy or heat content of the solids can also be neglected. Hence the change of energy or of heat content per unit mass of palladium is determined by the composition.
TABLE I The Experimental Results 104
Run
Moles H z
g. Pd
-2
x
L.
- A I./g.Pd
moles H, p P d
6 99 8 63 IS c ) j 0 89 j 68
I
2.IOi2
0
001474
I2
91
2
z.0900
0
001804
15
87
3
0.8573
14
01
4
2.0724
o 001623 000184
6.127 7 593 16.460
0
I
I8
0.jjl
5
3,2991
0 0012Ij
IO j I
3 .I86
'
For comparison, the data of Mond, Lamsay and Shields mere recalculate on the assumption that hydrogen entered the calorimeter at 23', by subtracting 2 3 X C, and the value of RT wzs also alibtracted to obtain -3r. Table I1 gives the recalculated values.
TARLE I1 Data of Mond, Ramsay and Shields (Recalculated) Run
-A
u
8.78
io'xmoles H1 g.Pd
3
41.22
0.0046;9
2.I.ij
43.97
0.004982
26.40
29.9'
0.
ob1663
- A L./g.Pd
4
2
14.62 28.40
Moles H:
9.98 '9.40 28.09
I
0 . 0 0 3 23 2
17.Cj
Discussion of Results In Fig, 2 , the loss of energy in calories per gram of palladium is plotted against the composition for both sets of data; our observations being indicated by the centers of plain circles, and those of Xond, Ranisay and Shields by the centers of circles with marks. The straight line shown was determined by the method of least squares for all points lying on the horizontal portion of the pressure-composition isotherm (betwen about 0.9 x IO-; and 2 7 . 7 X I O + moles Hs/g. Pd.)Y. JtYthin these limits the slope of the line, which is the loss of energy per mole of hydrogen, appears constant, 3s we should expect. The The equation found is - A 7 . hydrogen per gram Pd.
= 874~-0.01,
where n is the number of moles of
HEAT OF ABSORPTION OF HYDROGEN BY PALLADIUM BLACK
3 Io9
two lower points of Mond, Ramsay and Shields lie in this region and fall on our line. Their two higher points correspond to the second rising portion of the isotherm, and are high, qualitatively in accord with the finding of Gillespie and Hall from pressure data, that the heat is higher in this region. The lowest point of the curve corresponds to the first rising portion of the isotherm and is also low, but the experimental error is so great, owing to the fact that the heat measured was only I cal., that this point cannot be used to verify the conclusion from the pressure data that the heat is lowest in this region. Indeed, our calculations (not here reported in detail) show that our data as a whole (but not including this inaccurate point) indicate that the 301
FIG.2 The loss of energy in calorie per gram of palladium plotted against the composition.
heat in this region is not lower than that in the region of the horizontal isotherms, provided our average precision is better than 0.8 percent. However, the calculations show that this average precision cannot be better than 0.5 percent, and the finding of Gillespie and Hall, that the heat along the first rising isotherm is the lowest per mole, is not definitely proved or disproved by the present work. To decide this, the apparatus would have to be rebuilt, to permit the introduction of a greater mass of palladium. In any case, the integrated value up to the composition of Pd2H does not suffer seriously in precision owing t o this uncertainty, since only about 3.8 percent of the heat is liberated along the first rising isotherm. The values we obtain for the heats attending the change of state ( I ) , which are obviously, from Fig. 2, in agreement with the appropriate data of blond, Ramsay and Shields, are - A U = 8740, and - A H = 9283.
3110
LOUIS J. GILLESPIE AND HENRY A. AMBROSE
The integrated value calculated by Gillespie and Hall for - A H is only 8780, 5.4 percent lower. Some of this can be traced to the empirical deviation
plot used by them. I n drawing this plot, they removed curvature a t the lower temperatures in order to keep close to the datum for 0'. This procedure we have found inconsistent with the rational equation : log p = A - B log T - C / T . (2) This equation assumes ( I ) the ideal gas law for the gas, (2) that the partial molal volume of hydrogen in the palladium is negligible compared with the molal volume of the gas, and (3) that the change of heat capacity attending the evaporation of hydrogen from the palladium is constant. Assumption ( I ) we have verified by the use of an exact equation of state for hydrogen;l0assumption ( 2 ) is valid, according to the density measurements of Wolf;" and assumption (3) is certainly safer than the assumption t'hat the change of heat capacity is not only constant but also zero, which assumption leads to the disappearance of the term in log T and often is satisfactory for chemical equilibria. Application of equation ( 2 ) to the pressure data given for the horizontal isotherms by Gillespie and Hall in their Table I gives the following results. Least squaring with equal weights of the data for all temperatures gives for - A H , 8940 cal., only 3.7 percent lower than our result; and similar treatment of the data without the datum for oo gives 9534 cal., which is 2 . 7 percent high. The values of the pressure so calculated foro' are 3.87 and 3.39 mm., respectively, instead of the experimental value found, 4 mm. Their experimental value at oo is doubtless somewhat too high, and the measured heat now reported must be regarded as better than the value they derived, as it appears to agree with the calculated heats within the uncertainty of the calculation. Summary The loss of energy attending the absorption of hydrogen by palladium black has been directly measured in an ice-calorimeter. Such of the data of Mond, Ramsay, and Shields as correspond to compositions not exceeding that of the second solid phase (Pd2H plus excess hydrogen) are in excellent agreement with our data. The loss of heat content found for the reaction H2 4 Pd = z Pd2H (oo, I atm.), where PdzH gives the average composition of the two solid phases formed, is 9280 calls. A recomputation of the data of Gillespie and Hall indicates that this value is in agreement with their pressure data within the uncertainty of the computation, which is several percent and principally due to the experimental error in measuring the small pressure at 0'.
+
Using the extension of equation (2) to gases not ideal, given, with equations for latent heats, by Gillespie: Proc. Am. Acad. Arts Sci., 66, I 53 (19301. IlWolf: Z. physik. Chem., 87, 575 (1914).
