HEAT O F FORMATION O F LEAD CARBONATE BY A. L. MARSHALL AND B. B R U ~ S ”
In connection with an investigation on the velocity of decomposition of lead carbonate it was found necessary to have reliable data for the heat of formation of this compound. The data available in the literature are from measurements by Thomsen’ and Berthelot2 and there is a difference of 3000 cals. between the two determinations. The values given are calculated in a roundabout manner from a number of other reactions. From Thornsen’s data we have calculated AH for the reaction PbO+COz=PbC03 as - 2 2 , zoo cals., whereas Berthelot’s data lead to a value of - 19,400 cals. I n the light of these conflicting values it seemed profitable to attempt a more direct determination of the heat of this reaction. A recent publication of M. Centnerszwer, Falk and Auerbuch3 gives evidence for the existence of the compound PbO.PbCOa and we have determined the heat of formation of this compound from PbO and PbC03.
Experimental The method used consisted in dissolving PbO, PbC03, and Pb0.PbC03 in I O percent nitric acid (by volume). The calorimeter used was similar to that described by U. Fischer4. About 800cc. of solution was used in the calorimeter. Care was taken to eliminate all effects due to the heat, of solution of lead nitrate from the calculation by having the final concentration in the solution the same in all cases. Heat of Xolution of PhO. Three experiments were conducted with lead oxide. In the first two the oxide was obtained by the decomposition of a very pure sample of cerussite (crystalline PbC03). In these two experiments IO^. portions were used and between the first and second zog. of PbO were added to the solution in order to have the eame final concentration of Pb(NO3)z as in subsequent experiments, The third experimcnt was conducted with PbO from Kahlbaum. Expt I 2
3
.
temp. rise
amt. PhO added
heat capacity calorimeter
AH1
0.943O 1,137 0.856
9 .or crystals 11.73g. powder 9.98 g. Kahl.
849 cals. 879 1036
- 19,820
average of (I),
- 19,000 - 19,820 (2)
-19,410
*Contribution from the Laboratory of Physical Chemistry, Princeton Cniversity. “Thermochemische Untersuchungen,” 3, 333, 442. 2 “Thermochimie,” 2, 345. Acta. Univ. Latviensis, 11, 289 (1924). 2. anorg. Chem., 78, 57 (1912). 1
HEAT O F FORMATION OF LEAD CARBONATE
I
I85
Heat of Solution of PbO.PbCO3. The oxycarbonate was obtained by heating cerussite at 33oOC. in carbon dioxide at one atmosphere pressure to constant weight. The substance lost exactly j o percent of its carbon dioxide. Three experiments were conducted consecutively in the same solution giving the proper final concentration of Pb.(N03)2. temp. rise
amt. PbO.PbCOs added
IO
0.555
I1
0.532 0.438
16.04 g. crystals >? 16.00 14.00 ”
Expt.
I2
heat capacity calorimeter
A.Hz
1053 cals.
-8930 cals.
I047
-8550
I058
-8130
average
- 8540
It will be observed that between experiments ( I O ) and (12) there is a difference of 800 cals. and jn experiments (I) and (2) a difference of 8 2 0 cals. This corresponds t o a difference in the heat of solution of lead nitrate in a I O percent nitric acid solution and a, solution containing 9 percent nitric acid and 4.3 percent lead nitrate. Heat of Solution of Cerussite (PbCO3). Six experiments were carried out using very pure crystalline cerussite. Before each determination the solution mas saturated with carbon dioxide and a platinum foil cover was placed in the calorimeter to prevent loss by spattering. The reaction goes with an ebssrption of heat but, in the first stage, a small temperature rise was observed. This appeared to be due to the increased solubility of the carbon dioxide evolved during the reaction and a correction had to be introduced to take account of this effect, In the last two experiments a small amount of lead carbonate was added to the solution just prior to the experiment and this practically eliminated the initial rise. Expt .
temp. rise
4
-0.375 -0,325 -0,355 -0.330 - 0.440 -0,390
5 6 7 8 9
amt. PbCOi added
’
5 0 . 0 g. crystals 5 0 . 0 5 g. ”
49.92 g. >’ 5 0 . o g. fine powder 46.8 g, crystals 43.61 g. powder
heat capacity calorimeter
1064 cals.
AHI
1050
2130 cals. 1820
IO54
2000
1057
1870
847 840
2 130 2010
average
1990 cals.
A correcticn has to be introduced in the last experiments for the amount of water evaporated. This amounts to about 0.1g. for 50 g. PbC03 corresponding to about 50 cals. absorbed and gives a corrected value AHz= - 8690 cals. and AH3= 1690 cals. From these data one can calculate AH for a number of reactions. PbO+CO2=PbC03; AH4. AH4=AH1-AHa= - 2 1 , 1 0 0 cals.
1186
A. L. MARSHALL AND B. BRUZS
PbO+% CO2=% PbO, PbC03; AHj AH5= AH1- AHz= - 1 0 , 7 2 0 cals. % PbO. PbCO3+% COz=PbC03; AH6 AK6=AH2-AH3= -10,380 cals. PbO+PbC03=PbO. PbC03; AH, AH7=2AH5-AH4= -340 cals. From the table on lead carbonate it will be seen that the difference in heat content between very finely powdered cerussite and the massive crystals is within the experimental error of the method which amounts to ~ I O Ocals. The value of AH obtained from the reaction PbO+C02 PbC03 is approximately a mean between that of Thomsen and Berthelot. We are indebted to Messrs. R. T. Major and N. F. Myers for assistance in the conduct of some of the measurements. Summary The heats of the following reactions have been determined: PbO+COz = PbC03 AH=ZI,IOO cals. AH = - 3 40 cals. PbO+PbCO, =PbO.PbCOs E
Princeton, 12'ew Jersey.
