Advanced Propellant Chemistry

20 A New Determination of the Heat of Formation of Oxygen Difluoride W A R R E N R. BISBEE, J A N E T V. H A M I L T O N , R O N A L D R U S H W O R T H , T H O M A S J. HOUSER, and J O H N M. G E R H A U S E R 1

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Downloaded by CORNELL UNIV on September 28, 2016 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0054.ch020

Rocketdyne Division, North American Aviation, Inc., Canoga Park, Calif.

The ΔΗ of reaction at 298.16° K. of oxygen difluoride with hydrogen was measured using a Parr fluorine combustion bomb modified to contain a metal ampoule employing a burst diaphragm. This modification permits heat of reaction measurements on systems where reaction occurs sponta neously upon mixing the reactants. The ΔΗ of the reaction OF + 2H --> H O + 2HF (infinite dilution) was found to be —223.26 ± 0.38 kcal./mole. From this and existing thermodynamic data, the standard ΔH of oxygen difluoride was calculated, and theO—Fbond energy was determined. These values are —4.06 and 50.7 kcal./mole, respectively. 0

0

2

2

2

0

f

*"phe currently accepted value of the O — F bond energy (45 kcal./mole) is calculated from the standard heat of formation of O F (7.6 kcal./mole) which was based on an average of three values obtained i n 1930 (7, 8 ) the precision of which was quite poor. T o determine a more reliable heat of formation of O F and thus a better O — F bond energy, the heat of reaction of the following system was measured: 2

2

OF, + 2H

S

H 0 + 2 H F (infinite dilution) 2

Experimental Materials. The O F , obtained from the A l l i e d Chemical Co., was found to be greater than 9 9 % pure. Active fluoride was analyzed b y an iodometric method. B y infrared analysis 0.22% C 0 and 0.02% C F were found; S i F was not detectable; no H F was found. The hydrogen was a prepurified grade of 99.9% minimum purity ob tained from the Mathesoh C o . 2

2

4

1 2

Present address, Beckman Instruments, Albuquerque, Ν. M . Present address, Western Michigan University, Kalamazoo, Mich.

21 5

Holzmann; Advanced Propellant Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

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2 16

ADVANCED PROPELLANT CHEMISTRY

Apparatus and Procedure. The thermochemical measurements were made using a Parr fluorine combustion bomb and a National Bureau of Standards ( N B S ) isothermal calorimeter ( N o . 63090) manufactured by the Precision Scientific C o . The bomb cylinder and a l l internal parts of the bomb were Monel. A M o n e l ampoule was fitted into the top of the bomb to retain the O F sample. The ampoule apparatus reduced the internal volume of the bomb from 380 to 315 cc. T h e ampoule screws


2

Figure 1.

Nickel Parr combustion bomb with ampoule

Figure 2.

Exploded view of ampoule


20.

BiSBEE

ET A L .

Heat of Oxygen Difluoride

2 17


into the bomb head i n the position normally occupied by the outlet tube. This prevents the use of the outlet valve. Additional support is provided by a M o n e l clip which fits around the cylinder body of the ampoule and is attached to the inlet tube. A diagram of the ampoule is given i n Figures 1 and 2.

Figure 3. Effect of wedge breaking mechanism on rupture disk The internal volume of the ampoule was 8.7 cc. The top of the cylinder body and the cylinder head were designed w i t h a 30° angular seat to accommodate a V - i n c h M o n e l burst diaphragm. A mechanism, which fits inside the bomb, was designed to rupture the burst diaphragm i n the ampoule. This consisted of a piston w i t h a knife-like wedge head (Figure 3) and a small spring made from spring-tempered Monel wire. The piston and spring were held i n a compressed position by nickelchromium alloy fuse wire of known calorific value, which was strung between the two internal electrodes i n the bomb. Spring loading of the breaking wedge was accomplished by compressing the piston and spring until the hole i n the piston was aligned w i t h that i n the breaking mechanism (Figure 3 ) . A p i n was then inserted to hold the piston and spring i n a compressed position while the fuse wire was attached to the two electrodes. This wire passed through a small indentation i n the knife-like wedge head of the piston which is positioned between the two electrodes. The copper p i n was then removed, and the piston was held i n place w i t h the fuse wire. A pinpoint breaker was also tried; 2


2


18

however, because it merely punctured a small hole i n the diaphragm, it increased the chance of obtaining incomplete reaction and was unsatis factory. T o solve the problem of loading and weighing a sample i n the ampoule, one side exit was designed to accommodate a small 90° angle valve. The weight of the O F samples was determined by weighing tne empty ampoule, which had been previously evacuated, and then weighing the filled ampoule. The O F sample was condensed from a vacuum line into the ampoule, which was weighed on an analytical balance and then attached to the bomb head. Buoyancy corrections were applied only to the weight of the O F sample. A quantity of 50 m l . of water was placed i n the oomb to absorb the H F formed during reaction and thus reduce corrosion. The reaction bomb was assembled, pressurized w i t h hydrogen (75.0 p.s.ig.), and sealed. T o start the reaction, the sample was released into the hydrogen by electrically fusing the nickel-chromium alloy wire. This released the piston which ruptured the diaphragm and allowed the reactant gases to mix. Reaction occurred rapidly and completely; the temperature rise of the calorimeter was measured b y means of a platinum resistance thermometer, constructed and calibrated by the Leeds and Northrup C o . The thermometer, a four-lead cable type, was used i n conjunction with a Leeds and Northrup G-2 Mueller bridge and a high sensitivity galvanometer. Dickinson's method was used to obtain the corrected resistance change (3). This method involves the use of the equation R = Β+ + 0.63 (ΔΚοδβ) where R* is the initial resistance before the reaction is initi ated, and Ra, is the resistance at t , the time on a resistance-time plot where a vertical line drawn through this point w i l l subtend equal areas to the left and to the right of the curve. T o check the mass balance of the reaction, the reaction products were analyzed after each run by a thorium nitrate method for fluoride and by a sodium hydroxide titration for hydrogen ions. The two methods of analysis agreed within experimental error but were always lower than stoichiometric for each O F 2 - H 2 run. It was be lieved that the approximately 5 % of H F unaccounted for was consumed in the slight corrosion of the stainless steel screw heads i n the ampoule. Qualitative analysis of the screw heads d i d show the corroded film on tne screws was the metal fluorides. N e w screws were used i n each run, and the necessary thermal corrections were made on the data for this side reaction. The deficiency of H F was used to make these corrections which amounted to the formation of roughly 0.0015 mole of iron and chromium fluorides. Since the AH ° of iron and chromium fluorides are nearly the same, the correction was made assuming only the formation of iron fluoride ( Δ Η / = —177.8 kcal./mole). As an additional check, an O F 2 - H 2 run was made w i t h the stainless steel screws replaced by nickelplated steel screws. N o corrosion was found with the new screws, and the H F was found to be i n proper stoichiometric amount within the experi mental error of the analytical technique. The heat of reaction, using the nickel-plated steel screws, agreed within experimental error with the re sults which had been corrected for the small amount of corrosion. Calibration. The energy equivalent of the calorimeter was deter mined by burning N B S sample 39h benzoic acid. The combustions were 2

2


2

s

w

f

0


20.

BiSBEE ET AL.

Heat of Oxygen Difluoride

2 19

carried out with the sample ampoule i n place and pressurized to 450p.s.i.g. with N to prevent collapse of the diaphragm. The N B S benzoic acid was reported to have a heat of combustion of 26,434 abs. j . / g r a m mass (weight in vacuo) under standard conditions at 25° C . with an estimated u n certainty of ± 3 j./gram. This value was converted (9) to the bomb conditions (temperature 28° O , pressure of oxygen 30 atm., mass of sample about 1 gram, volume of bomb 0.315 liters, mass of water about 1 gram ) used during standardization and found to be 26,432.8 abs. j./gram. T o evaluate the energy equivalent of the standard calorimeter, E , the following quantities were evaluated for each standardization experiment. The weight of benzoic acid burned was converted to m , the weight i n a vacuum. The ignition energy, q was calculated from the mass of fuse wire burned times the heat of combustion of the fuse wire, 5.86 j./mff. The energy released by combustion of N to H N 0 (aq.), q , was cal culated using 57.8 kj./mole as the energy of the reaction. The deviation, Ae of the energy equivalent from that of the standard calorimeter system was calculated from the summation of the product of the weights and heat capacities of water, oxygen, nitrogen, benzoic acid, and the Hastelloy cup (weight about 9.41 grams) containing the benzoic acid. The sum mation of these heat capacity corrections multiplied by the reciprocal of the temperature coefficient for the platinum resistance thermometer gave Ae . The energy equivalent, £ , was then calculated according to E q u a tion 1: 2

8

8


i9

2

3

n

lf

t

e

E

a

= [ ( - A f t (28° C.) m, + q + q )/AR ] {

n

c

-Ae

(1)

t

where — AE = 26,432.8 abs. j./gram, and &R is the corrected tempera ture rise i n ohms. The standard calorimeter system for this series of experiments was specified as the N B S calorimeter containing the Parr fluorine combustion bomb plus sample ampoule. The standard bomb was defined as the Parr fluorine combustion bomb plus sample ampoule minus the water, the benzoic acid pellet, the Hastelloy cup containing the pellet, the oxygen, and the nitrogen used to pressurize the ampoule. In a series of five calibration determinations, the mean energy equiv alent for the system was 203,063.7 j./ohm or 48,533.4 ± 6.5 cal./ohm. The uncertainty is the standard deviation of the mean. b

C

Results and Calculations The data are referred to a standard temperature of 25° C . The energy unit used is the calorie which is defined as equal to 4.1840 ab solute joules. The quantity of heat observed during the reaction, Q, was calculated from Equation 2: Q = (E, + Ae*)àR

(2)

e

where E is the energy equivalent of the calorimeter; Ae is a correction for deviations from the standard calorimeter system and was computed from the heat capacities of O F , H , and H 2 O ; àR is the corrected temper ature rise. The heat capacity values used were 10.35 cal./deg. mole ( 5 ) , 2

8

2

2

c


220


Table I· Run No. 1 2 3 4 5 6

η (moles OFi)

Data on O F - H 2

2

Ae% cal./ohm

AR ohm

col.

cal.

505.0 505.0 505.2 505.2 505.0 504.1

0.10417 0.10580 0.11325 0.11174 0.10637 0.06469

5108.3 5188.3 5553.6 5479.6 5216.2 3172.2

-75.7 -84.1 -80.9 -80.4 -76.9 -51.5

0.023042 0.023474 0.025011 0.024748 0.023437 0.014507

Q

e

01

ft is the number of moles of O F j ; Δ#* is the correction for deviations from the standard calorimeter system; AR is the corrected temperature rise in the calorimeter; Q is the quan tity of heat observed during the reaction; qi includes corrections for nonideality of the reactant gases, condensation of water in the vapor phase, and heat of dilution of H r solution to infinite dilution; q% is the energy supplied by corrosion of the screw heads; — ÙJER is the β


C

6.89 cal./deg. mole (6), and 0.999 cal./deg. gram for O F , H , and H 0 , respectively. The energy of reaction per mole i n the thermodynamic standard bomb process, AER°, was calculated at 301° K . for each experiment from Equation 3: 2

-AE ° R

2

2

= (Q —

Advanced Propellant Chemistry

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