Heat of Formation of Oleums from Sulfur Trioxide and Water The heats of formation and infinite dilution of oleums have been recalculated from all the data available on the subject. CARL V. HERRMA" Grasselli Chemicals Department, E. I. du Pont de Nemours & Company, Inc., Wilmington, Del.
data for the heats of formation of HzSz07(solid) and H2S207 (liquid) should be increased by 2.6 kg.-cal. per mole, t o give 304.8 and 302.25 kg.-cal. per mole, respectively. SUMMARYOF BICHOWSKYAND ROSSINI. These two workers (1) reviewed the available data and gave "best" values for the heats of formation of sulfuric acid, sulfur trioxide, and water from which the heat of infinite dilution can be calculated as shown in Table 111. DATAOF HOWARD TEST. A simple method for the determination of the composition of oleum was devised by Howard (d). It is based on the temperature rise on mixing 150 ml. of 66" BB. (93.19 per cent) sulfuric acid with 50 ml. of oleum in a Dewar flask. A number of unpublished data on the test carried out under exact laboratory conditions were available. The d e terminations were made a t 30" C., and the temperature rise was noted in each case. Table IV gives the calculations of the heat of infinite dilution of the oleums. The weight of
HE only aiTailable values for the heat of formation of oleums are those of Porter (4) which were derived from the experimental data of Knietsch (3). These are inconsistent with the most generally accepted values for the heat of dilution of sulfuric acid solutions and sulfur trioxide. Accordingly, all available data, including those of Knietsch, have been reviewed and a new set of consistent values has been calculated. In order to reduce all the data t o a common basis, the heat of infinite dilution of the oleum involved was calculated from the experimental data and the heat of formation of sulfuric acid a t various dilutions as summarized by Bichowsky and Rossini ( 1 ) . To simplify subsequent calculations, the heat of dilution was expressed as heat per gram mole of sulfur trioxide in the original oleum.
T
TABLEI. DATAO F KNIETSCH
Data from the Literature DATAOF KNIETSCH.The heat of dilution a t about 18" C. of various concentrations of sulfuric acid and of liquid and solid oleums was determined by Knietsch by two methods. I n the first set of experiments 15-40 grams of oleum were added to 5-6 kg. of water in a calorimeter. I n the second set, 0.5 kg. of oleum was added to 400 kg. of water in a large wooden barrel. The actual experimental results were not given, and the values reported were smoothed averages. One illustrative calculation for the first set indicated that 40 grams of 91.05 per cent total sulfur trioxide oleum were added to 5 kg. of water. If this is taken as representative of the first set of experiments, the final dilution can be calculated as 750 moles of water per mole of sulfuric acid. In the second set the final dilution was about 4000 moles of water per mole of sulfuric acid. From Bichowsky and Rossini's data the heat of infinite dilution of the final solution in the first set is 3.59 kg.-cal. per gram mole of sulfuric acid, and in the second set it is 2.24 kg.-cal. Knietsch stated that the two results agreed well, and since it is impossible to distinguish which dilution applied, a value of 2.9 kg.-cal. per gram mole of sulfuric acid was added to all the reported values to reduce them to infinite dilution. Table I gives the results of the calculations. The single determination in which the dilution was known is given separately. DATAOF THOMSEN. This investigator (7) determined the heat of solution at 18" C. of liquid oleum and liquid sulfur trioxide. The experimental data were given and are corrected to infinite dilution in Table 11. Owing to a slight ambiguity in Thomsen's text, Bichowsky and Rossini ( 1 ) assumed that he worked with the solid. This is incorrect and therefore their
7% Boa in Oleum 81.62 83.49 85.26 87.31 89.08 91.05 92.67 94.72 96.62 98.48 99.64
Reptd. Heat of Diln., G.-cal./G. Oleum 194. OB 221.4 245.27 277.6 299.06 327.9 361.4 393.6 483.5 470.6 491.1
Calcd. Heat of Diln. G.-cal./G. kole Total SO8 19,035 21,230 23,030 25,460 26,880 28,830 31,220 33,270 35,920 38,260 39,460
Estd. Heat of Infinite Diln., Kg.-cal./G. Mole Total S0a 21.936 24.13 25.93 28.36 29.78 31.73 34.12 36.17 38.82 41.16 42.36
Experimental Result for Which Dilution Was Given 91.07
327.90
28,826
31.966
TABLE11. DATAOF THOMSEN % SOa in Oleum
Moles Water/ Mole H?S04 after Diln.
Exptl. Heat of Dilution, G.-cal./G. Mole SOa
Heat of Infinite Diln Kg.-cal./G. Mgle SO8
30.23 30.02 30.53 30.44 42.23
TABLE111. SUMMARY O F BICHOWSKY AND Formula His04 Has04 SO8
Ha0
898
State Liquid Infinite diln. Liquid Liquid
&r ~g.-~al./b o .ole 193.75 215.8 104.2 68.37
ROSSINI
DATA
Heat of Infinite Diln. Kg.-cal./G. MAle
so3
22.050 43':23
...
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1941
the oleum and 66" BB. acid was calculated from specific gravity tables given in Sullivan's handbook (6) and the density of water a t 60' F. The heat capacity of the Dewar flask was determined by mixing 9-18 grams of 95 per cent sulfuric acid with about 100 grams of water and comparing the heat evolution estimated from the temperature rise with that calculated from the known heats of dilution and specific heats of the acids involved. Three tests gave 12.4, 11.1, and 12.6 calories per " C. as the heat capacity of the flask and thermometer] and a mean of 12 calories per O C. was used in the calculations. To obtain the heat of dilution at 18" C., the heat content of the oleum, 66" BB. acid, and final mixture were calculated by the specific heat data of Socolik (6) for the acids and those of Knietsch (8) for the oleums. The heat of mixing a t 18" C. was then calculated by adding the heat content of the final acid to the heat absorbed by the calorimeter and subtracting from this the sum of the heat contents of the original 66" BB. acid and oleum. The heat of infinite dilution of the oleum was then calculated by combining this value with the heat of infinite dilution a t 18" C. of the original 66" BB. acid and the final acid mixture (estimated from the data of Bichowsky and Rossini, 1).
899
5% 5
3
PERCENT TOTAL 503 I N OLEUM FIQURE~ 1. HEATOF INFINITP~ DILUTION OF OLEUMS
Results Obtained
The data are plotted in Figure 1, and a mean curve is drawn through the points. Consideration wm given to the relative accuracy of the data in drawing the mean curve. Table V gives the values of the heat of infinite dilution of oleums as taken from this curve. Since the composition of oleums of commercial interest are usuTABLE IV. DATAOF HOWARD TEST ally expressed as per cent Heat of free sulfur trioxide, this Heat Content Content Q of Final method is used in Table V. of Oleum Hs$04 Av. Sp. Mist. Heat AbNet Heat Hqat of,InOleum0 Compn. Referred Heat of Referred sorbed by Evolvedb finite Diln in I n the usual engineering to 180 c., Calorimeter, at 180 c., %-cal./d: % Temp. Risea t o 180 c., Final Mist calculations involving the O.-cal. ole 80s G.-cal. 0.-cal. Hn%&r SO8 0.-cal. Mist. O.-cai: thermochemical properties of 22.1 100.32 81.89 4.8 2.7 94.98 0.357 1,910 32 379 2,640 101 23.6 382 95.44 0.354 102.04 83.30 15.1 8.4 oleum, the heat of formation 34.6 19.2 231 4,060 26.0 96.13 0.363 387 104.58 55.37 27.2 96.54 0.352 288 106.05 86.57 43.2 24.0 4 680 from water and sulfur tri393 97.14 0.352 29.3 410 399 108.18 88.31 59.9 33.3 &el0 oxide is of more value than 6 940 30.7 97.65 0.351 425 494 109.99 89.79 74.1 41.2 97.91 0.351 433 7:590 653 110.92 90.54 83.0 46.1 31.9 the heat of infinite dilution. 32.8 98.54 0.351 499 8,700 644 112.59 91.91 96.6 53.7 9.380 706 33.6 Table VI gives the value of 98.74 0.351 515 113.29 92.48 105.9 58.8 34.8 98.85 0.351 10,000 523 769 114.10 93.14 115.3 64.1 the heat of formation from a Average of three determinations at each point. liquid water and either liquid b The heat content of the 66" BB. aoid was constant at 1210 g.-cd. in all determinations. or gaseous sulfur trioxide. The values for liquid water and liquid sulfu; trioxide TABLEV. HEATOF INFINITE DILUTION OF OLEUMSAT 18' c. were obtained by subtracting the heat of infinite dilution Heat of Heat of of the oleum from the heat of infinite dilution of sulfur Infinite Diln., Infinite Diln % SO1 in Oleum K ~ . - ~ ~ ~ ~~l~ ./G. % 808 in Oleum Kg.-cal. a. M& trioxide. The values for liquid water and gaseous sul'Free Total so: Free Total do: fur trioxide were obtained by adding the heat of vapori0 81.63 22.05 60 92.65 34.05 zation of sulfur trioxide as estimated by Bichowsky and 10 83.47 23.95 70 94.49 36.20 20 85.30 25.90 80 96.33 35.40 Rossini (10,300 calories per gram mole of sulfur tri30 87.14 27.90 90 98.16 40.70 oxide). 40 88.98 29.90 100 100.00 43.23 50
90.82
31.95
Literature Cited (1) Bichowsky, F. R., and Rossini, F. D., "Thermochemistry of Chemical Sub-
TABLE VI. HEATOF FORMATION OF OLEUMSBY THE RBACTION HzO (liquid) zSOs (liquid or gas) =HzSOc (2-1) SOa (liquid)
+
ToFree
608 in x of Oleum Equation
Col. 1 Liquid" Kg -cal /
g.mble
0 10 20
1.000 1.136 1.306 1.525 1.817 2.226 2.537 3.859 5.907 12.005
30 40 50 60 70 80 90 m 100 a State of 90s.
SO,
21.18 19.30 17.35 15.35 13.35 11.30 9.20 7.05 4.86 2.55 0
Col. 2 Liquid Kg +a1 /
g.mble H20
21.18 21.90 22.65 23.40 24.25 25.15 26.10 27.20 28.65 30.60
Col.3 Col.4 Liquid Liquid B t u/ B t u
Id. S O 8 476 434 390 345 300 254 207 158 109 57
0
1;.
Hd
2120 2190 2260 2340 2420 2510 2610 2720 2860 3060
...
Col. 6 Gas
Col. 6 Col. 7 Gas Gas Kg.-Gal./ Kg.-cal./ B.t. u./ o.mole SO8 g.mole Ha0 lb. SO: 31.48 31.48 29.60 33.60 36.10 27.65 39.10 25.65 23.65 43.00 21.60 48.10 55.30 19.50 67.00 17 35 15.15 89.50 12.85 154.30 10.30 I
...
Col. 8 Gas
B.t. u./
lb. Ha0 3,150 3 360 3:610 3,910 4,300 4,810 5.530 6,700 8,950 15,430
..
stances", 1st ed., New York, Reinhold Pub. Corp., 1936. (2) Howard, H., J.SOC.Chem. Ind., 29. 3-4
(1910). (3) Knietsch, R., Ber., (1901).
34, 4069-4115
(4) Porter, A. W., Trans. Faraday SOC.,13,
373-400 (1917). (5) Socolik, A. .S., 2. physik. Chem., 158, 305-12 (1932). (6) Sullivan, T. J., Sulfuric Acid Handbook, 1st ed., New York, MoGrawHill Book Co., 1918. (7) Thomsen, H. P. J. J., "Thermochemische Untersuchungen", Vol. 111, Leipzig, Barth, 1883.
