HEATSOF FORMATION OF CHFs AND CH2P2
Sept., 1958
1043
+
with respect t o partial pressures of H2 CO, as these partial pressures decrease less rapidly with increasing z than the quantity (1 - z). Equally good or better representation of the data is obtained by a half-order equation. For nitrided catalysts the apparent over-all activation energies computed from the variations of TO increase with carbon monoxide content of the feed gas up t o 0.7H2 1C0, and the magnitude of the increase seems t o be greater than experimental uncertainties. This trend is also shown by activation energies computed by comparing values of XS a t constant x in Table 111. The fact that relatively constant activation energies are obtained by this method, as well as by the empirical equation suggests that the rate equation can, to a t least a first approximation, be separated into the form r = f(T)g(z). Activation energies for reduced iron catalysts reported by British workers4 varied in the same way with gas composition. Their values were 14.5, 18, 22.3 and 27.5 kcal./mole for feed gases with Hz/CO ratios of 3, 2, 1.12 and 0.67, respectively. If the extrapolation to zero values of S - I is correct, values of ramay be properly related to the feed gas composition. A rate equation
+
ro = apHpl-npcon
200
-
-
(2)
which will be considered in detail in a subsequent paper, was rearranged to ro = aPtfl-" (1 - f)'&
3
= aPt[(I
(3)
-f)/fl"
where a is a constant, p ~ and % pcb are the partial
THE HEATS OF FORMATION OF CHF, AND CHzFzl BY CONSTANTINE A. NEUGEBAUER AND JOHN L. MARGRAVE Contribution from the Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received February 8, 1968
The standard heats of formation at 298.15"K. of gaseous CHFs and CH2F2, have been determined in a rotating bomb calorimeter as -162.60 & 0.65 and -105.50 i 0.22 kcal./mole, respectively.
Introduction Investigations on the properties of gaseous fluorocarbons have been carried out extensively in recent years, primarily because of the use which these gases find in the refrigerating and polymer fields and because of their great chemical stability. Nevertheless, not many quant,itative data are available on their thermal stabilities and heats of formation, probably because of the difficulties involved in making the particular fluorocarbon undergo reaction. No measurements of the heats of formation of the fluoromethanes have been carried out hitherto, with the exception of CFI, the most stable of all fluorocarbons. Six independent determinations of its heat of formation were reported prior to 1956, ranging from -161 t o -231 kcal./ (1) Abstracted in part from the thesis presented by Constantine A. Neugebauer in partial fulfillment of the requirements for the degree of Doctor of Philosophy a t the University of Wisconsin in February, 1957.
mole. This spread illustrates the difficulties encountered in making precise calorimetric investigations on these compounds. The four main problems which have to be solved in fluorocarbon calorimetry are: (1) many fluorocarbons are inert toward the common chemical reagents used in calorimetry, such as oxygen and hydrogen. (2) The thermodynamic state of the final products in the reaction whose heat is meassured may be inadequately defined, such as gaseous HF, amorphous carbon, COz dissolved in aqueous HF, etc. (3) The data needed to make the nonideality corrections are generally not available. (4) Since aqueous H F is a common product in combustion reactions involving fluorocarbons, corrosion problems arise. An isothermal bomb calorimeter was used exclusively in this investigation since most of the reactions would not take place unless the reactants
CONSTANTINE A. NEUGEBAUER AND JOHN L. MARGRAVE
1Q44
were at pressures higher than atmospheric. This made a flow calorimeter, which is generally used for gas reactions, unsuitable. In order to achieve the maximum accuracy and precision attainable with the bomb calorimeter if a solution phase is formed in the reaction, it proved advantageous to construct a device which would allow the bomb to rotate end over end. Thus it was possible to agitate the contents of the bomb after combustion had occurred. The need for this is fourfold: (1) the solution phase will rapidly become uniform in concentration throughout and will thus be in a well defined standard state. (2) The gas phase will reach equilibrium quickly with the dissolved species in solution. (3) Corrosion of the bomb is reduced by preventing the formation of very concentrated HF solutions. (4) The heat exchange between the pool of solution inside the bomb and the calorimeter water is greatly speeded up. This prevents interference in the after period in the measurement of the temperature rise. Experimental Design and Operation of the Rotating Bomb Calorimeter. -A stationary bomb calorimeter, which was used in the determination of the heats of formation of CF2CF2, CH&F2 and CF4, and of conventional design has been described previously.2 This calorimeter has been modified to operate with a rotating bomb. Sunner,a studied the combustion calorimetry of organic sulfur compounds in the f i s t moving-bomb calorimetric system. A similar method was used at the Bureau of Mines's to measure the heats of combustion of various nongaseous sulfur and fluorine compounds. A double valve Parr illium bomb of conventional cylindrical design and a capacity of 360 ml. was used in all experiments. The illium metal is quite resistant to attack by aqueous HF u to about 3 molar, provided the bomb is metal ions were detectable when 100 ml. stationary. of a 3 M solution of HF was formed in the hydrogenation of tetrafluoroethylene, even though the solution was in contact with the metal for about one hour. If the bomb is rotated, however, some corrosion will take place, the extent of which depends on the concentration of the acid produced and the length of the time of rotation. About 1% of the acid reacted during a run when the bomb was rotated for 15 min. and the acid strength was 2 M. The conventional loop electrode for ignition of the bomb charge was employed only in the calibration experiments with benzoic acid. Two short straight electrodes were used to ignite a gaseous mixture by electrically heating either a short length of platinum wire or a thin carbon rod to incandescence. The calorimeter can was constructed from copper sheet and plated with a heavy layer of bright nickel to minimize radiation loss to the jacket. The bomb was held in a 1 inch wide stainless steel ring and tightened into place with six brass screws. Rotation of the bomb was effected by turning a vertical brass drive shaft with an electric motor. Torque was transmitted through two bevel gears so that the bomb itself made 16 r.p.m. Details of the construction are given in reference 1 . During a calorimetric measurement the jacket temperature was kept constant a t 24.800 f 0.003' with a conventional mercury regulator in conjunction with a thyratron relay. A slow flow of water was maintained through a cooling coil to balance the heat input by the heater. I n preparation for a run, one placed the charged bomb into the bomb carriage either in an upright position (for
d
(2) C. A. Neugebauer and J.
L.Margrave, THIS JOURNAL, 60,
1318
Vol. 62
calibration experiments) or upside down (for the fluorocarbon experiments). When the bomb was upside down, the water initially in the bomb covered the stainless steel valves and protected them from the initial blast of the combustion. The can containing the bomb was positioned in the well by visual observation and 6.8 1. of distilled water was added carefully. The water had been weighed to lt0.05 g., and had a temperature somewhat lower than that of the desired initial temperature in the experiment. Three drops of light oil were placed on the surface of the water to prevent excessive evaporation during the run. The water level was always sufficiently high to cover the bomb completely in any osition. A catrimetric type platinum resistance thermometer, Leeds and Northrup serial no. 1132445, was used in all experiments to measure the temperature rise produced by the reaction. This thermometer was calibrated by the National Bureau of Standards in 1955. The resistance of the thermometer was measured with a Mueller type Wheatstone bridge, Leeds. and Northrup designation 9;2, which was The sensing thermostated In an oil-bath a t 30.0 f 0.1 element of the bridge was a high sensitivity (0.05 p v . / ~ . ) galvanometer, Leeds and Northrup Cat. No. 2284-b, with a 12.8 ohm critical damping resistance. I n actual use the galvanometer was somewhat underdamped which gave a more rapid response. The galvanometer was 3.5 m. distant from the scale and the reflection of the filament of the galvanometer lamp from the galvanometer mirror onto the Lucite scale was used as the deflection indicator. A change in resistance in the thermometer of 0.0001 ohm moved the light spot 3 mm. on the scale. Calibration of the Rotating Bomb Calorimeter.-Benzoic acid, NBS standard sample 39g having an isothermal heat of combustion a t 25' of 26.4338 kj ./g. mass under certificate conditions6 was used for determining the energy equivalent of the calorimeter. I n the combustion of both CH2Fz and CHFa, concentrated HF solutions containing dissolved Con, 0 2 and excess fluorocarbons were found. Since the solubility and heat of solution of GO2 and 0 2 in aqueous HF are not known, and a ueous HF causes some corrosion of the bomb walls, a caibration under certificate conditions does not reproduce actual bomb conditions in many respects. This difficulty was overcome by designing comparison experiments in a manner such that the final state of the calibration bomb process was as nearly equal to that obtained in the experiments with fluorocarbons as possible. Thus, the initial pressure of 0 2 was chosen to be the same as the initial pressure of O2 in the combustion experiments; the quantity of benzoic acid burned was taken to be such that the final partial pressure of GO2 would be nearly identical with that obtained in the combustion experiments. I n addition, 50.0 ml. of 2 to 3 N HF was added initially to the bomb, so that the final conditions after the combustion of benzoic acid very closely resembled those met in the actual combustion experiments. Comparison experiments of this type were fist used by Waddington,' et al., in 1956. I n all, three series of calibration experiments were performed. I n the first series the calorimeter was calibrated by burning benzoic acid under certificate conditions in order to find the correct energy equivalent of the system. Two grams of acid was burned in each experiment, rather than the one gram recommended on the NBS certificate, to achieve a greater temperature rise. The appropriate certificate corrections were applied. The bomb was rotated for only 1 min. (16 revolutions). I n the second series a procedure exactly like the one outlined above for the comparison experiments was followed, with the exception that 50.0 ml. of pure water, rather than HF solution, was added initially to the bomb. The bomb was rotated for 230 sec. or longer. Reductions to standard states were made, and, if properly applied, should give the same value for the energy equivalent as the first series of calibration experiments under certificate conditions. The third series of calibration experiments consisted of the comparison experiments with HF present. When calibration experiments were carried out under certificate conditions, reductions to standard states did not have to be applied, since the value for the heat of combus-
.
(1956). (3) (a) 8. Sunner, Suensk. Kern. T a d , 68, 71 (194G); (b) Thesis, Univ. of Lund, Sweden, 1950. (4) W. N. Hubbard, C. Kats and 0.Waddington. THISJOURNAL. SS, 142 (1954). (0) NBS certificate accompanying standard sample 39g. ( 5 ) W. D. Good, D. W. Scott and G . Waddington, ibad., 60, 1080 (7) W. D. Good, D. W. Scott and G. Waddington, T t 60, 1080 (1956). (195G).
~ JOURNAL, s
HEATSOF FORMATION OF CHF8 AND CHzFz
Sept., 1958
TABLE IA CALIBRATION OF THE ROTATING BOMBCALORIMETER, NO H F PRESENT R212 R213 R216 R195 R196 R197 2.12095 2.5559 -2.0033 2.2552 2.02201 0.89109
(1) Lab. no. (2) m, benz. acid, g.
(3) (4) (5) (6)
v, soln., ml.
1.00 0.0 AT, "c. 2.2097 (~/M)AEB (or -16,150.6 AEoo)benz. acid, cal. (7) AE(fuse -102.2 HNO, ign.), cal. (8) AE( rotation), -6 . 6 cal. (9) AE, cor. to stand. states, cal. (10) Coont AT, cal. +9.8 (11) Cor. heat - 16,249.6 evolved, cal. (12) C(ca1or. 7,353.7 water), cal./deg. (13) C(water, cal./ 6,780.4 deg. 1 (14) C(empty 573.3 calor.), cal./deg.
N (HF)
+ +
+
(1) Lab. no. (2) m, benz. acid, g. (3) o, soln., ml. (4) N W " (5) AT, "c. (6) (m/M)AE2 benz. acid, cal. (7) AE(fuse 4-ign.), cal. (8) AE(rotation), cal. (9) AE, cor. to stand. states, cal. (10) Coont AT, cal. (11) Cor. heat evolved, cal. (12) C.,,(calor. water), cal./deg. (13) C(water), cal./deg. (14) Capp(empty calor.), cal./deg.
+
R220 0.88936
1.oo 0.0 1.7285 - 12,657.8
1.00 0.0 1.9390 - 14,249.9
1.00 0.0 1.8351 -13,401.4
1.00 0.0 1.7389 -12,776.1
-52.9
-27.3
-106.8
-28.4
-1.2
-4.8
-0.6
-0.6
+7.6 - 12,704.3
+8.7 - 14,273.3
$7.9 -13,500.8
+7.4 -12,797.7
$40.3 -5,638.7
+40.3 -5,632.8
7,349.9
7,361.7
7,357.0
7,359. I
7,360.2
7,353.4
6,780.0
6,239.3
6 ,782.8
6,785.4
6,783.5
6,783.1
569.9
571.8
574.2
574.3
576.7
570.3
CALIBRATION OF
50.0 0.0 0.7661 (-5,625.4) -29.7
Continuous rotation -23.9
TABLE IB ROTATING BOMBCALORIMETER, HF PRESENT R218 R219 R222 R235 0.88870 0.89298 0.88835 0.89220 50.0 50.0 50.0 50.0 1.98 1.98 1.98 3.17 0.7647 0.7715 0.7631 0.7666 -5,610.3 -5,637.3 -5,608.1 -5,632.4 -30.8 -34.2 -28.2 -18.3 -15.5 -32.9 -10.7 -20.0 -24.2 -24.2 -24.2 -24.2 +39.4 +39.8 $39.3 $39.5 -5,641.4 -5,688.8 -5,631.9 -5,655.4 7,377.2 7,373.7 7,380.1 7,377.1 6,792.1 6,787.7 6,792.3 6,790.8 585.1 586.0 587.8 586.3
50.0 0.0 0.7660 (-5,614.5) -28.2 -6.5 -23.9
THE
tion per gram of benzoic acid given on the certificate has also been derived under bomb, not standard conditions. When the calibration was carried out with 50.0 ml. of HF present in the bomb, however, deviations from certificate conditions were so large that it was necessary to calculate all the reductions to the standard bomb process a t unit fugacity and 25", and, by also using the heat of combustion of benzoic acid under standard conditions, -6312.91 f 0.62 cal./g., in these calculations, the apparent heat capacity of the calorimeter, Cap,, was obtained. A convenient step by step method for making these corrections can be found in the recent literature.8 One starts with the reactants at unit fugacity a t 25", compresses and confines them to bomb conditions, lets the reaction occur at 25", and then one decompresses the products to unit fugacity a t 25". The results for the three series of calibration experiments are given below. In the Tables Ia and Ib the first four items give the initial conditions of the particular experiment; item (4) lists the concentration of the HF solution. Then fife energy factors follow, the sum of which should add up to the corrected heat evolved, item (11). Here AEB stands for the uncorrected heat of combustion of benzoic acid under bomb conditions, AEO, is the corrected heat under standard t,, denotes the heat capacity of the bomb conditions, C contents, the items (7) through (10) are energy corrections. The first five experiments (R195-213) in Table Ia (first (8) W. N. Hubbard, D. W. Scott and G . Waddington, ibid., 88, 152 (1954).
1045
R236 1.78880 50.0 3.17 0.5333 - 11,292.5 -21.8 -26.8 -51.6 +79.4 -11,313.3 7,378.4 6,798.1 580.3
series of calibration experiments) were carried out under certificate conditions, and the average for the heat capacity of the empty calorimeter was 572.7 f 1.5 cal./deg. The last two experiments (R216, 220) in Table Ia (second series of calibration experiments), where 50.0 ml. of water was present during the reaction, give an average of 573.5 =k 3.2 cal./deg. Within experimental error these two values are identical, as they should be if the non-ideality corrections have been applied correctly. The greater average deviation from the mean in the second series must be expected since the sample size of benzoic acid was smaller. It should be noted a t this point that the procedure followed in these twc, experiments was identical to that followed in all other experiments from here on. The conditions for the first three experiments (R218-222) in Table I b (third series of calibration experiments) were practically identical. The average for the apparent heat capacity of the empty calorimeter from these runs is 586.3 f 1.0 cal./deg. This indicates that the temperature rise measured for this set of comparison experiments was somewhat lower than expected, or (586.3 - 572.7) (0.76) = 10.6 cal. less than expected was evolved. This difference could be explained if the product of the solubility times the heat of solution per mole of COa in HF at that pressure of COz is less than that in pure water. It may also be partially due to an over correction for the heat of corrosion, which is necessarily somewhat ill defined. Regardless of the source of this difference in apparent heat capacities, this factor is eliminated in the actual fluorocarbon combustion experiments. The conditions of the bomb processes are very nearly the Same and it wm
CONSTANTINE A. NEUGEBAUER AND JOHN L. MARGRAVE
1046
VoI. 62
possible to apply B blanket correction of 10.6 cal. to each was produced. The reaction went essentially to) run. Correction for the Heat of Rotation.-During the reac- completion, small amounts of CHzFzin the product, tion period the bomb was rotated until the maximum tem- gas being due to appreciable solubility in water. perature was reached (about 230 sec.) to make sure that the Purity of Methylene Fluoride.-The sample of CH2F2. heat exchange between the pool of solution in the bomb and used in these experiments, coded FCD-1592, was obtained’ the rest of the calorimetric system was complete. To de- through the courtesy of the Jackson Laboratory of E. I. du termine the magnitude of the heat of rotation correction, Pont de Nemours and Go. Mass spectrometric analysis the bomb was again rotated for about 400 seconds at, the showed 0.2 mole % fluoroform (CHFI) and 0.1 mole %, end of the after period. The rate of cooling was slower chlorodifluoromethane. The impurity due to CHF, is since the loss of heat to the jacket was partially offset by the probably not serious, since, as will be seen later, it is not heat generated by the rotating bomb. The difference be- very reactive toward oxygen. A small COz peak was obtween the normal rate of cooling and the rate observed when served in the mass spectrogram, but it was present in quanthe bomb was rotating gave the rate a t which the rotating tities too small to influence the final COz analysis. No bomb generated heat while in operation. When 50.0 ml. of other impurities were detectable. water was initially present, enough heat was produced on Procedure.-First, 50.0 ml. of pure water was added to rotation to change the temperature at the rate of about the bomb which was then flushed 5 t o 8 times with the same 0.00000028°/sec., which corresponds to 0.022 cal./sec. oxygen used for the calibration experiments. The bomb When there was 50.0 ml. of 2 N H F solution present in the was then loosely connected to the methylene fluoride tank bomb while it rotated, the heat of rotation observed was and any air which may have been in the connection waa somewhat greater, around 0.05 cal./sec. The difference swept out. The connection was tightened and methylene between the two heats of rotation is the heat of corrosion. fluoride was admitted to a pressure of 5 atm. Finally, the! Weighmgs and Definitions.-The water needed as the oxygen tank was connected again, the filling connection calorimetric fluid vas weighed on a solution balance having swept clear of air as before, and then oxygen was added to a. a capacity of 5 kg. per pan and a sensitivity of 7 mg. per total pressure of 25 atm. In the calorimeter this charge was ignited by electrically division with a 2 kg. load. Weighings for solid combustible samples (benzoic acid, the ignition wires, etc.), whose exact heating (15 v., 5 amp.) a thin platinum wire, 1 cm. long and! weight had to be known for calibration and analytical pur- fastened to the two electrodes. An ignition energy of about poses, were done on a semi-micro Ainsworth balarxe to 30 cal. generally sufficed to initiate the reaction. Rotation of the bomb was started as soon as practical after the igniwithin 0.01 mg. All weights used were calibrated and all weighings were tion, to prevent the formation of very concentrated hydrocorrected to ‘‘in vacuo.:’ For this purpose the density of fluoric acid on the bomb walls and continued for 200 to 2501 sec. The procedure of measuring and correcting the temthe brass weights was taken as 8.4 g ; / d . The atomic weights were those reported in the J. Am. perature rise was identical to that described for the calibraChem. Sac. in 1956. The energy unit is the calorie, defined tion experiments. Analysis of the Bomb Products.-To fix the amount of as e ual to 4.1840 abs. joules. CHZFZwhich undergoes reaction, one must analyze the gas A I reactions were referred to 298.15”K. Uncertainties and Precision.-The uncertainty quoted phase for GOz and titrate the aqueous phase with standard for the final value of a particular heat of formation is com- NaOH. The bomb gases were first passed through an acid scrubber to take out any H F which had vaporized and posed of three parts. The first is the deviation from the mean of the heats ob- were then led through a tube filled with Mg(C104)z for drying served for the individual combustion experiments. The before the COZ present was absorbed on ascarite. Before resulting uncertainty in the mean value of the heat of re- the bomb pressure reached atmospheric pressure, the bomb action is called the precision error, according to R o ~ s i n i , ~was connected to a tank of high purity helium, and a helium sweep was continued for 1.5 hours to make sure that all the and is taken as twice the standard deviation COz in the aqueous phase was swept out. COS-free air was 21/(EA)z/n(n- 1) drawn by an aspirator through the ascarite bulb for another 30 min. to displace the light helium gas and make the bulb where A denotes the deviation from the mean and n the ready for reweighing. The weight gain was equal to the number of trials. of COZproduced in the reaction. The second is called the calibration error and reflects the weight For the analysis of the H F produced in the reaction, the uncertainty produced in the heat of formation by the aver- bomb was rinsed carefully to ensure quantitative recovery age deviation from the mean of the heat capacity measure- of the solution; the acid was diluted to 1 liter; and 100-ml. ments in the calibration experiments of the calorimeter. were titrated to the phenolphthalein end-point with The third is the accuracy error and is an estimate of the aliquots standard 0.1 N NaOH, which was standardized frequently effect, in calories, of various other factors on the final heat against Bureau of Standards benzoic acid sample 39g. of formation: the purity of the reactants used, the quality Shortly before the end-point the solution turned a faint of the estimates which were necessary in making the non- green due to the precipitation as hydroxides of the metals ideality corrections, the definiteness of the thermodynamic dissolved in the corrosion of the bomb. Since the metal states, etc. ions took the place of an equivalent number of hydrogen The final uncertainty quoted for a standard heat of forma- ions, the stoichiometry of the neutralization was preserved. tion is the square root of the sum of the squares of the pre- The green color did not interfere with the end-point because cision error, the calibration error, and the accuracy error. of the intensity of the color change. From the stoichiometry of the combustion of methylene The Heat of Formation of Methylene Fluoride fluoride, 1 mole of COZand 2 moles of HF are produced from The combustion reaction of gaseous CHzFs 1 mole of fluorocarbon. The results of the analyses are with oxygen in the presence of water was used to given in the third and fourth row of Table 11. The COZand HF analyses generally agreed within 0.25%, and within determine its heat of formation 0.44y0 at worst. CH2F2(g) Odg) = COz(g) 2HF(aq., 2.4 N ) (4) Corrections to Standard States.-The solubility of CHZFZ in 50 ml. of water a t 5 atm. of gas pressure was taken as By measuring the heat of reaction under bomb 0.0044 mole. This value was arrived a t by interpolating conditions, correcting it to the reactants and prod- values for the solubility of CFI, CHFs and CHd, and using ucts in their respective standard states, and from the the solubilities of the series CF,, CClFs, CClzF2, CCL, apd the series CC14, CHCl8, CHZC12, CHaCl and CH4,11s12as inknown values of the standard heats of formation of terpolation aids. The heat of solution was estimated in a COzand H F (aq.),*Othe value of the standard heat similar manner, using the difference in the solubility at of formation was obtained. Mass spectrometric different temperatures to calculate the AHsoin, according analysis of the products of the reaction showed that to the relation
+
+
no other product was formed; in particular no CF, (9) F. D. Roasini, Chsm. Reus., 18, 223 (lQ36).
(10) National Bureau of Standards Circular 500,
(11) H. M. Parmelee, Refvrberation Eng.. Dec., 1341 (1953). (12) Solvay Process Divisign, Product Data @beets gr) Ghlorvrnethanes,
1047
HEATSOF FORMATION OF CHFa AND CH2F:!
1958
TABLE I1 THEHEATOF FORMATION OF METHYLENE FLUORIDE Lab. No. AT, "C. n, moles COZ n, '/2 moles H F water), cal./ C(ca1or. deg. C(calor. water), AT, cal. AE(ign.), cal. AE(rotation), cal. AE(cor. to stand. states), cal. Coont (-AT), Cal. nAE2. cal. (12) AEoO, kcal./mole (13) AHfO, kcal./mole a Values were discarded.
+ +
R223 1,1323 0,06024" 0.05999 7,363.8
R224 0.9904 0.05298 0.05257 7,370.1
R225 1,0006 0.05298 0,05322" 7,360.7
R226 1.0398 0.05533 0,05513 7,369,5
R227 0.9907 0.05261 0,05265 7,365.9
R228 1,0093 0.05343 0.05350 7,357.5
-8,338.0
-7,299.3
-7,365.1
-7,662.8
-7,297.4
-7,425.9
$45.0 $7.1 -6.5
f35.9 f4.6 -7.5
+34.1 4-8.6 -7.5
+23.3 +7.2 -7.1
f26.9 f6.7 -7.5
+26.9 f8.4 -7.3
-58.8 -8,351.2 -139.21
-51.3 -7,317.6 -139.09
-51.4 -7,322.7 -139.14
-52.2 -7,450.1 -139.36
-54.0 -51.9 -7,693.4 -7,381.8 -139.30 -139.33 - 105.50 i 0 . 2 2 kcal./mole
kcal./mole, giving a final value of It was estimated as -4.30 kcal./mole. Interpolation is somewhat difficult since both the solubility and heat of solution are a t a maximum for CH2FZ, CH2C1?and CC12F2, the middle members of all the three methane series mentioned above. From calorimetric measurements Rossini and Frandsenta ~ concentration found that the dependence of (b E , f b P ) on for the two component system 02-COz may be represented empirically by a simple equation. I n the absence of experimental data, for the systems 02-CH2F2 and OZCOZCH2Fz,the dependence of ( b E / b P ) r on concentration was assumed to be similar. The steps involved in making the corrections to standard states differ somewhat from those used in the calibration experiments or other experiments where solids or liquids instead of gases are burned; the detailed procedure and solubility data for 02, COZ,etc., are found in references 1 and 8.
Results.-Six out of twelve reaction trials were successful (in the other six an insufficient rotation time was allowed) and essential data for each run are given in Table 11. Row (2) gives the corrected temperature rise observed for the reaction, while the results for the analysis are given in rows (3) and (4). The value for the HF analysis is divided by two for comparison purposes: it should agree with the number of moles of COzproduced and also gives directly the number of moles of CHzFzwhich took part in the reaction. The value for the sum of the heat capacity of the calorimeter and the calorimeter water multiplied by AT gives the number of calories evolved under bomb conditions. This value is then corrected for the ignition and rotation energy, the non-ideality of the system and the heat capacity of the bomb contents (the solution and gas phase). The sum of items (6) to (10) is then the standard internal energy change a t 25" for the number of moles of CHzF2 that took part in the reaction (row l l ) , and in row (12) the same quantity is given per mole. From the stoichiometry of reaction 4 the standard heat of formation at 25" of CHzFz can be calculated (row 13).. The average value of the heat of formation is - 105.50 kcal./mole. The precision error was 0.09 kcal./mole, the calibration error 0.03 kcal./ mole and the accuracy error was estimated as 0.20 (13) F. D. Rossini and M. Frandaen, J . Research Natl. Bur. Standards, 9, 733 (1932).
AH? [CH2F2(g)]a t 298.15"K. = -105.50 i 0.22 kcal./mole
The Heat of Formation of Trifluoromethane The reaction used to determine the heat of formation is similar in principle to the one given for CHzFz CHFa(g)
+ '/tOz(g) + HzO(1) = COz(g) + 3HF (as., 2.5 N ) (5)
However, from mass spectrometric analysis of the product gases it was found that a small amount of CF, also was produced. CF, could be formed by either of two side reactions
+ '/zOdg) = l/zCOz(g) + '/&F4(g) + HF(aq.1 CHFa(g) + '/LUg) '/dXMg) + '/rCFd(g) + '/zH20(1) CHFdg)
(6)
(7)
If the quantity of CHFa consume'd in either of these side reactions is known, the standard heat of formation can be calculated from equation 5 by a method of successive approximations. It can be shown that the two side reactions 6 and 7 are actually identical, since from a combination of either equations 5 and 6 or equations 5 and 7, the same over-all equation can be derived. Purity of the Sample of Trifluoromethane.-The sample of CHF3 used in this investigation was obtained from the General Chemical Division of the Allied Chemical and Dye Corporation. Mass spectrometric analysis indicated that the sample was quite pure. In particular CHzF2was completely absent; COS was present only in traces; and no CF, was detectable. The only impurities were some oxygen and a small quantity of nitrogen. Procedure and Analysis.-The procedure followed in charging the bomb was essentially that described in the previous section, except 7 atm. of CHFI and 23 atm. of O2 were used, bringing the total pressure to 30 atm. The rather large quantity of CHFI was necessary in order to force the reaction to proceed 50% toward completion. The charge was ignited by electrically heating (10 v., 9 amp.) a thin, 1.5 om. long pencil lead carbon rod, since the temperature reached with a hot platinum wire was insufficient to ignite the charge. The heat of combustion of the rod in the presence of H F was determined in separate experiments in which only carbon was burned. It was 5023 cal./g. with an evolution of 0.0528 mole of COZper gram of rod. About 2% of the final number of moles of COZproduced in the combustion were due to this carbon fuse,
CONSTANTINE A. NEUGEBAUER AND JOHN L. MARGRAVE
1048
TABLE I11 THEHEATOF FORMATION OF TRIFLUOROMETHANE (1) Lab. no. R230 R231 R232 R229 0.5240 (2) AT, "c. 0.5280 0.5468 0.3813 0,04163 0.04109 0.04295 0.02967 (3) Moles COZ 0,04142 0.04258 0.04058 (4) moles HF 0.02949 0.00021 0.00037 0.00051 (5) Difference, moles 0.00018 7365.3 (6) C(ca1or. water), cal./deg. 7364.3 7353.9 7355.7 -3859.4 -4021.1 -2804.7 (7) C(ca1or. water) AT, cal. -3888.4 (8) AE(ign.), cal. +30.2 +17.4 $21.5 +32.1 (9) AE(fuse), cal. $96.0 +73.8 +93.0 +84.0 +6.8 +6.9 +5.8 f6.8 (10) AE(rotation), cal. +6.5 (11) AE, cor. t o std. states, cal. +6.9 -2.4 $8.7 -27.5 -27.3 -28.5 -19.7 (12) Coont (-AT), C d . -3747.2 -3910.7 -2719.6 -3807.1 (13) nAE2, cai. -161.92 -163.17 -161.88 (14) AHr, kcal./mole -162.80
+ +
Vol. 62
R233 0.5503 0.04313 0.04257 0.00056 7365.3 -4053.3 +24.4 +107.0 +6.9 f6.9 -28.7 -3937.4 -163.15
ference in the C02 and H F analyses. Since here the difference between two relatively large numbers is involved, its reliability is no more than 40%, which is magnified by a factor of 4. This is the reason why the results for the standard heat of formation consistently increase as the difference between the two analyses increases. If it were asResults and Calculations.-The results for five sumed that the average of the differences between runs are given in Table 111. The third and fourth the C 0 2 and 1/3 H F observed in all five runs is more rows give the result of the analyses, the difference accurate than that obtained in any one run sepais given in row ( 5 ) . The various energy factors, rately, the precision error of the average heat of rows (7) through (12), are listed similarly to those formation would be substantially lower, although in Table 11. For the purpose of the [ ~ E / Q P ] Tthe value for the average heat would stay the same. correction, CHF3 was treated in the same manner The values for the total heat observed under as CH2F2. The solubility of CHF3 at 7 atm. was standard conditions (row 13), divided by the HF or taken as 0.00457 mole in 50.0 ml. of water, and COz analyses alone, without making the correction its heat of solution in water as -3.80 kcal./mole. for the production of CF4, are consistent to within The corrections to standard states are given in i 0.25 kcal./mole, thus showing that the quantity row (11). The sum of items (7) through (12) gives of CF4 produced did not vary as much from run to the internal energy change had the reaction pro- run as indicated by the analyses. ceeded under standard conditions (row 13). However, from the analyses as obtained, the To calculate the heat of formation of CHFs, it precision error was 0.58 kcal./mole and the caliwas assumed that the reactions which were involved were no. ( 5 ) and (7), and the number of moles of bration error 0.02 kcal./mole, giving a h a 1 value CHFs which reacted in both equations was cal- for culated. Then, from an estimated heat of forma- AHfo[CHFa(g)]at 298.15"K. = tion of CHF3 the contribution to the total heat - 162.60 f 0.65 kcal./mole produced per run made by reaction 7 was calcuAcknowledgments.-The authors wish to aclated. The difference in the heats was due solely to reaction 5 and thus a more accurate determination knowledge the financial support of the Wisconsin of the heat of formation of CHFa was possible. Alumni Research Foundation, the Minnesota The calculation was repeated until a consistent Mining and Manufacturing Corporation, the E. I. value for the heat of formation was obtained. du Pont de Nemours Coinpany, and the Office of Since the contribution of reaction 7 to the total Ordnance Research. The sample of methylene heat evolved is small (about 4%), a consistent value fluoride was obtained through the courtesy of the was obtained after onlv 2 or 3 trials. The values Jackson Laboratory of the du Pont Company and the sample of trifluoromethane through the courtesy thus derived are given h row (14). The magnitude of the correction for the reaction of the General Chemical Division of the Allied producing CF4is essentially determined by the dif- Chemical and Dye Corporation. The ashes from the rod did not affect the acidity of the solution. The electrical energy needed to cause ignition was about 20 cal.; the heat of combustion of the fuse per run was of the order of 90 cal. The analyses for COz and HF were performed as previously described. The difference between the COZanalysis and '/a the HF analysis is a measure of the quantity of CF, produced according to reactions 6 or 7.
