830
THAIRL. HIGGINS AND EDGAR F. WESTRUM, JR.
are directly c ~ m p a r a b l e . ~Earlier ~ comparisons have shown good agreement for the critical coagulation concentrations obtained by the two method^.^^,^^ The present work constitutes a (48) See, e.g., (1556).
S. A. Troelstra, Disc. remark, Kollozd-Z., 146, 65
1-01. 6;
direct test of the comparability of aged AgI sols and sols in statu nascendi. In all cases identical results were obtained for the critical coagulation concentration as well as for the stability limit. (49) K. Schuls and B. Teiak, Arhzu kem., 26, 187 (1534). (50) E. MatijeviC, Kolloid-Z., 146, 74 (1556).
THERMOCHEMICAL STUDY OF THE SODIUM AND AMiVOSIUM HYDROGEN FLUORIDES 13- AXHYDROUS HYDROGEN FLVORIDE' BY THAIRL. HIGGISSAND EDGAR F. WESTRUM, JR. Departmenz of Chemistry, University of Michigan, Ann Arbor, Michigan Recezzed ,Voeember 88,1960
Enthalpies of solution of various hydrogen fluorides have been determined in an electrically calibrated isothermal calorimeter which is stirred by an oscillatory motion. In addition to the enthalpy of solution data reported, the enthalpy of formation of liquid hydrogen fluoride a t 298.15"K. is calculated to be -71.8 kcal./mole. Enthalpies of forniation of the following fluorides are calculated to be: " 4 F = -111.0 kcal./mole, NHIHFz = -191.4 kcal./mole, P\'H4H3F4= -337.4 kcal./mole, NaHFz = -218.0 kcal./mole, and NaH2F3= -292.5 kcal./mole. These data are used to calculate enthalpies of decomposition of the compounds, and wherever possible these values are compared with enthalpy increments determined from the temperature dependence of pressure measurements made by other investigators. B partial confirmation of the assignment of t r o units of residual entropv to ammonium monohydrogen difluoride by Benjamins, Burney and Westrum is obtained.
Introduction Like water, anhydrous hydrogen fluoride is an associated liquid composed of a mixture of single molecules and of more complex aggregates. The abnormally high melting and boiling points, the high dielectric constant, and the high enthalpies of vaporization and fusion may all be cited as evidence of the association through hydrogen bonding. Thr resultant high dielectric constant of hydrogen fluoride favors the separation of ions and hence the solution of ionic cryst'als. It is a fluid medium of relatively low viscosity ; ionic mobilities are correspoiidingly greater and its usefulness as an electrolytic solvent is thereby enhanced. AIoreover, its abilit'y to form hydrogen bonds makes it an excellent solvent for many organic materials. The corrosiveness, together with the problems and hazard of handling the anhydrous liquid hydrogen fluoride, have deterred investigators from studying its solutions to the same extent that other non-aqueous solvents, ammonia for example, have been studied. Pioneering investigations of solubilit'ies in anhydrous hydrogen fluoride were carried 011 by Fredenhagen and his co-workem2 Further studies on fluorides were made by Bond and Stowe3 and of organic materials by Klatt.4 Wt'h t'he advent' of plastics such as Teflon (polyt,etrafluoroethplene), Kel-F (polytrifluoroethylene), 'and Polyethylene, the number and scope of physical chemical studies on hydrogen fluoride solutions have increased in more recent years. Yi'ikolnev and Tananaev5 det,ermined act'ivities ( I ) From a dissertation submitted b y T. L. Higgins in partial fulfillment of t h e requirements for t h e Doctor of Philosophy degree in the School of Graduate Studies of the University of Michigan. (2) K. Fredenhagen a n d G. Cadenbach, Z . physik. Chem., A146, 245 (1930); K. Fredenhagen, 2. Elektrochen., 37, 684 (1531); K. I'redenhagen, G. Cadenbach and W. Klatt, Z . p h y s i k . Chem., 8164, 176 (1933). (3) P. Bond a n d V. Stowe, J . .4m. Chem. Soc.. 63,30 (1531). (4) TT. K l a t t , Z . anorg. allgem. Chem., 234, 185 (1937).
of lithium, sodium, potassium and ammonium fluorides and of several alkyl and aromatic amines in liquid hydrogen fluoride from boiling point elevation studies of these solutions. Although no thermochemical measurements hare been reported iiivolving anhydrous hydrogen fluoride solutions, studies of the enthalpy of solution of various silicates in aqueous hydrogen fluoride have been made. For a detailed study of anhydrous hydrogen fluoride solutions, desiderata are enthalpy of solutioii measurements as a function of solute concentration to enable a Debye-Huckel treatment of these solutions. Enthalpy of solution data, along with molal entropies and activity data, permit the calculation of ionic entropies and ionic free energies of formation, etc. I n some instances the determination of enthalpies of reactions involving fluorides can be achieved nith a smaller number of calorimetric reactions if anhydrous hydrogen fluoride is used in preference to the aqueous solution. The purpose of this study was the des-elopmc>ntof techniques for \Torking with anhydrous hydrogen fluoride in precise calorimetric investigations and the use of these techniques in determining the enthalpy of solution of various hydrofluorinated ammonium and sodium fluorides. A further, immediate stimuluq to the determination of the enthalpies of reactions involving ammonium fluorides and ammonium hydrofluorides was the desirability of testing the data of Beiijamins, Burnry and TV-estrum.6 Confirmation of the enthalpy increments of these reactions iyould reinforce the assignment of residual entropy at 0°K. to the conipound ammonium monohydrogen difluoritlr. In the present research, therefore, the enthalpies of solution in liquid anhydrous hydrogen fluoride ( 5 ) N S Nikolaei and I \- Tananaev h e s t S e k f o r n F i z - K h z m Anal.. Akad. N a u k S S S.R 20, 184 (1950). (6) E. Benlamins G. -1.Burney a n d E. F Kestriirn, .Jr (unpublished d a t a ) .
May, 1961
SODIUM ASD AI\IMONIUM HYDROGEK I:LUORIDEH ISAKHYDROUS HI-DROGES FLUORIDE 83 1
have been determined for these several compounds : ammonia, ammonium fluoride, ammonium monohydrogen difluoride, ammonium trihydrogen tetrafluoride, sodium fluoride, sodium monohydrogen difluoride, sodium dihydrogen trifluoride, potassium monohydrogen difluoride, sodium acetate and hexamet'hylbensene. The ammonium fluoride and sodium fluoride systems were chosen since temperature-composition studies for t,he systems NH4F-HF7 and lJaF-HFs are available, and the thermochemical quantities relating to these compounds are unknown. From the enthalpy of solution data, enthalpies of formation of most of these compounds have been obtained and by making use of decomposition pressure measurements the corresponding free energies of formation have been evaluated.
Experimental Calorimeter and Submarine.-The use of anhydrous hydrogen fluoride a? solvent in these studies necessitated that the calorimeter be a completely closed system to prevent loss of the solvent and absorption of water vapor. Consequently, the number of apertures into the caloriineter were minimized, the sample holder was immersed in the hydrogen fluoride, and a means devised for breaking it open. Moving shaft seals which might tend to leak hydrogen fluoride and create heat through friction were eliminated by developing a method for stirring the calorimetric fluid which avoided the use of a propeller-type stirrer. The design finally adopted was that of suspending the calorimeter proper inside a submarine, immersed within a stirred thermostat. The kmperature sensing element was a copper resistance thermometer bifilarly wound on the outside of the calorimeter. To minimize heat exchange between the calorimeter and surroundings, the space between the calorimeter and the wall of the submarine could have been evacuated. Although this \vas not done in the measurements of the present paper, subsequent modifications of this apparatus provided such an insulating vacuum. The calorimeter and submarine are depicted in schematic cross-sect,ion in Fig. 1. The calorimeter ( I ) was machined from a rod of fine silver to eliminate any silver-alloy brazed joints in contact with the solut'ion. It had a volume of about 65 ml., an over-all length of 9 em., an outside diameter of 4.1 cm., and an average wall thickness of 1.6 mm. The calorimeter vefisel was gold-plated on all surfaces to protect the silver from corrosion by sulfur compounds and to present a reflective surface to reduce radiational heat exchange. The calorimetcr was suspended within a polished chromium-plat'ed brass submarine (2) by a thin wall Monel tube ( 3 ) ,5 . 3 nim. outside diameter, terminated by a Monel nipple (4)a t the lower end. The submarine provided a dead air gap of one cm. width w-hich, according to, \yhite,g reduces convection and conduction of heat to n minimum. Xear the upper end of the Monel support tube, there was a Monel fitting ( 5 ) , which screwed into a copper plug ( 6 ) . This was soft-soldered onto the end of t,he submarine support tube ( 7 ) . The copper plug ensured that the calorimeter support tube was thermally anchored to the bath temperature a t its upper extremity. The hIone1 nipple (4), brazed with silver-alloy to the >.Ionel support tube, screwed into the bop of the calorimeter compressing a Teflon gasket, which prevented leakage of the calorimeter contents t h o u g h the t,hreads. The nipple contained a Teflon sleeve and a threaded internal bushing to adjust the gasketiny action of the Teflon sleeve on the sample breaker rod (8). The bushing had a. screwdriver slot on the bottom to facilitat,e the adjustment. The calorimeter was stirred by a reciprocating rotation of the entire calorimeter-submarine assembly. This was achieved by gears and links such that the nsscmbly (7) R. D. Euler and E. F. Westrum, Jr., J. Phys. Chem. 65, in press ( 19151).
(8) R. D. Euler and E. 12. Westrim, Jr., (unpublished data). (9) M-.P. '~Vhitr."The .\IodPrn Calorimeter," Reinhold Pilhl. Gorp, N,.i\-York. h-.B.. 1928.
Fig. 1.-Calorimeter and submarine: (1) calorimeter; ( 2 ) submarine, (3) calorimeter support tube; (4) monc.1 nipple; ( 5 ) nionel fitting; ( 6 ) copper plug; ( 7 ) submarine support tube; (8) sample breaker rod; (9) sample holder: (IO) baffle vane; (11) resistance thermometer: (12) heater; (13) Bakelite binding post. was rotated 360" in alternate directions at a, rate of 22 reversals/min. A baffle vane (10) inside the calorimeter served to provide a considerable amount of vertical mixing of the calorimetric fluid \Then the calorimeter and submarine rotate and reverse direction. The stirrring thus provided is very efficient in terms of the small production of heat. That it, was quite adequate is evidenced by the fact t.hat 97% of the t,ot,al temperature rise for an?- given reaction was observed on the thermometer within one minutcl after the initiation of the reaction. The calorimeter vessel was sealed with a "Ball-V" seal (i.~.. by n bead on thv cover which fit tightly aqainst a chamfered edge on thtb lower portion of the calorimeter). The submarine was sealed with a Keoprene O-ring gasket. The 1-ml. capacity sample holder (9) \vas cy$-lindricdin shape and may be likened in its general appemtnce t o i~ snare drum with platinum foil drum lit, mm. platinum foil discs were held ~ n u g Teflon foil gaskets by a silver masher and a threaded clnmping nut. The bottom clamping nut had a "Y" suppoit machined onto it, the axial leg of which was tupped and scre-n-ed onto a threaded stud project~ingup from the bottom of the calorimeter. The contents of the sample holder were exposed to the calorimetric fluid by pushin. the s:implts breaker rod through t,he platiniim discs. This rod \vas made of thin wall Lloiiel tuhinp 2.8 inm. onicr diametcr; the lon-er end T ~ closed S by a solid Mom1 rod \vhicii was silver-alloy-brazed into place. The iipper end of the rod extended out through the top of the thermostat. The sample holder \ms so designed that it could be loaded in the dry box and the weight of the sample dctermined precisely on a balance within the dry box. It then iva calorimeter, the anhydrous €IF eolwnt a measurement begun. In order to detcrrnine the enthalpy of reaction of gaseous ammonia with liquid hydrogen fluoride, a small lLlonel val replaced t'he sample breaker rod. Gaseous ammonia nadded from an external reservoir connected to the ammon valve by a short, piece of 0.56 mm. inside diameter 3lnnc.l tubing xith 0.07 mni. will thicliness :ind 5 0 cm. of polyr%liylencc:ipill:iry tubiiig of siniihr t1i:imc~tt'~.
832
THAIR L. HIGGINS AND EDGAR F. WESTRUM, JR.
The thermostat employed was a 24 liter hemispherically bottomed cylinder insulated by glass wool batting maintained to within =t0.004' of the desired temperature. Electrical Circuits.-An external girdle of 42 feet of No. 40 B. and S. gauge enamelled Fiberglas insulated copper m7ire and ten feet of No. 36 B. and S. gauge Fiberglas insulated Advance wire served as a resistance thermometer (11) and heater (12), respectively. The wires were bifilarly wound, enamelled with Formvar, and baked. The leads on the thermometer were No. 32 Advance wire and on the heater were KO.29 copper wire. Copper washers were soldered to the ends of these leads. These washers are brought into contact with identical copper washers on Bakelite binding posts (13) attached to the inside top of the submarine. Beyond the binding posts the leads were of No. 24 Advance wire on the thermometer and No. 29 enamelled copper wire on the heater. These leads passed out through the axis of the submarine support tube and around a stiff wire helix (to protect the leads from wear due to the oscillation of the calorimetric assembly) to a suitable terminal strip. A simple thermometric bridge circuit, using a decade box as the variable resistance component and a high sensitivity galvanometer as the null detector, made possible a temperature sensitivity of 0.0002° with a current of 5 ma. flowing in the circuit. Although an absolute calibration of the resistance thermometer was not needed to determine the temperature rise, since the decade box settings could be translated directly into electrical energy, a calibration to ascertain the temperature a t which measurements were made was achieved by comparison of this thermometer (placed directly into the bath surrounded only bv a thin rubber membrane) with thermometers calibrated by the National Bureau of Standards. The electrical energy added to the calorimeter for calibration purposes was measured with an autocalibrated White potentiometer, resistors and standard cells calibrated by the National Bureau of Standards. Method of Operation.-The sample holder was first rreighed empty and then loaded with the compound under investigation and reweighed. This operation was done in the ambient atmosphere unless the compound was hygroscopic and required loading in a dry box. After obtaining the weight of the sample by difference, the platinum discs were lightly coated with paraffin to ensure liquid tightness and the sample container put into place within the calorimeter. The entire calorimeter was assembled, weighed empty, flushed r i t h anhydrous nitrogen, and w a ~ then ready for loading with anhydrous HF, which was condensed from the storage cylinder into a thin wall copper container. From this container the desired quantity of H F was transferred as liquid to the calorimeter. The resistance thermometer and heater were protected with rubber sheeting, and the calorimeter cooled slightly with solid carbon dioxide. Upon opening the valve on the copper container the hydrogen fluoride transferred to the calorimeter. Between 50 and 55 g. of hydrogen fluoride usually was employed. After placing the calorimeter within the submarine and making the electrical connections, the entire assembly was put into the thermostat and the stirring mechanism started. After attainment of a uniform drift, two caliliration energy inputs usually were made and the chemical reaction observed upon breaking the platinum discs. Two additional calibration electrical inputs were typically made on the products of the reaction. Separate determinations indicated that breaking the platinum foil on the sample container was a process which produced a detectable thermal effect of 0.42 f 0.03 cal. The heat effect is due primarily to the frictional heat developed a t the seal a t the top of the calorimeter. This seal was necessarily tight since the contents of the calorimeter were a t a pressure of 950 mm. and any leakage of the contents would lead to disastrous consequences, as well as a negative temperature drift due t o vaporization. As a further check on the heat developed, the breaker was made so that it could be pulled back into its original position after an enthalpy of solution measurement and a blank determination were made for each run. The actual heat of breaking of platinum foils has been described as less than 0.02 cal.lo From the apparent average energy equivalent of the calorimetric system, the observed temperature rise, the (10) J. E. Wertz, Anal. Chem., 22, 1227 (1950).
Vol. 65
correction for the heat of breaking, and an appropriate factor, the apparent molal enthalpy of solution a t the midpoint of the temperature rise was calculated. This value was corrected to 298.15'K. by use of the molal heat capacity increment for the dissolution reaction. C Performance and Calibration of Calorimeter.-The thermal interaction of the calorimeter with respect to its surroundings can be expressed in terms of thermal leakiness modulus, which is the drift of the calorimeter in (deg./min.) er deg. of thermal head. The average value of the thermal Peakiness modulus obtained from these experiments was 1 .O X 10-2 min.-l with dry air in the submarine. The heat of stirring was found to be 1 .8 X 10-80 min.-l; that due to the thermometer current was calculated to be 2 X lO-'O min.-l. The calorimetric procedure and the analytical treatment of the observations were tested by determining the heat of solution of potassium chloride in water and comparing these results, as presented in Table I, with literature data. The , Table I, represents the computed value symbol - L ~ ( s )in for the enthalpy of solution a t infinite dilution, and is related to the enthalpy of solution a t any finite concentration by
+
-LI(s) = AHs @L in which 41,represents the relative apparent molal heat
content.
TABLE I HEATS OF SOLUTION OF POTASSIUM CHLORIDE IN WATER AT 25' (Molallty)l/n
0.343 .320 .324
Weight
KCI, g.
At, OC.
Heat absorbed (cal.)
AH, cal./ mole
0.5779 0.2327 32.393 4211 .5032 .2018 28.338 4213 .5174 .20GO 29.171 4219
@L,
cal./ mole
-Lz(s), cal./ mole
80
4131 4133 80 4139 Av. 4134 f 3 80
Spedding and Miller11have surveyed the existing data on the heat of solution of potassium chloride in water, correcting it to 25' by means of the molal heat capacity increment for the dissolution reaction and to infinite dilution. The average value for -Lt(s) computed from data prior to that of Spedding and Miller was 4134 f 5 cal./mole. This value was obtained by the use of both isothermal and adiabatic calorimeters and furnishes a very favorable com arison with the value of 4134 f 5 cal./mole obtained by 8pedding and Miller in their work and the identical value obtained in this study. Further details concerning the calibration and performance of this instrument, the calculation of the enthalpy of reaction, and the experimental arrangements may be obtained from the dissertation.'2 Preparation and Purity of Reagents.-The high purity, anhydrous NHs used for the enthalpy of soliltion studies was from the same cylinder used and analyzed by Benjamins6 in his phase studies on the NH,F-NH,HFt system. Part of the NH4F used was taken from the calorimetric sample prepared by Benjamin@ and part was obtained by the same method of preparation used by Eenjamins. The anall-pis of the original sample was 51.46% fluoride and 48.78% ammonia (theoretical: 51.29 and 48.719/, respectively). The sample prepared for this work was analyzed for fluoride using the volumetric method of Willard and Winter13 and for nitrogen by the Kjeldahl method with the following results: fluoride 51.29% and ammonia 48.36y0. The KH4HFt was taken from a calorimetric sample prepared tiy Burney6 and its analysis was 100.0% of theoretical for hydrogen and fluoride ions. The KH4HaF~ was synthesized as follows: NH4HF2,prepared by Euler,' was converted with 99 .Syo pure anhydrous H F in a hIonel reactor to the desired composition by weight. The composition of the sample WLP tested by titration of the available H F with standard KaOH to the phenolphthalein end-point in an ice-bath. The ratio (11) F. H. Spedding and C. F. Miller, J . Am. Chem. S o c . , 74, 31% (1952). (12) T. L. Higgins, "A Thermochemical Study of the NHa.nIIF-IIF and NaF.nHF-HF Systems," Dissertation No. 21184. University Microfilms, Ann Arbor, Michigan. (13) H. H. Willard and 0.B. Winter, Ind. Enp. Cheni., Anal. Ed., 6, 7 (1933).
A K D ANMOKIUM HYDROGEN FLUORIDES IN ANHYDROUS HYDROGEN FLUORIDE May, 1961 SODIUM 833
of H F to NHIF found was 3.01 i: 0.01 (theoretical, 3.00). Recrystallized reagent grade N a F was used after drying and storage in a polyethylene bottle. The NaHFz was a portion of the calorimetric ~ a m p 1 e . l ~The reported analysis was 100.03yo of theoretical for hydrogen ion and 100.5% of theoretical for fluoride ion. S a H z F awas prepared by reaction of NaF with the high purity anhydrous HF in a Fluorothene beaker within a closed copper reactor. The H F was introduced by vacuum distillation from a copper system. The sample was analyzed by titration of the available H F with standard XaOH to the phenolphthalein endpoint in an ice-bath. The ratio of H F to SaF found was 2.02 i: 0.03 (theoretical 2.00). il calorimetric sample of K H F P reported to contain 100.370 of the theoretical amount for hydrogen ion and 99.8% for fluoride ion, was used in these experiments. Eastman Kodak Company hexamethylbenzene wag used in the preliminarv studies. Reagent grade anhydrous sodium acetate was rpcrystallized from distilled water and dried in an oven at 120' for 24 hours. Special p u r i h hydrogen fluoride (99.8%) obtained from the Pennsylvania Salt Manufacturing Company was used as solvent in these studies. Titration with XaOH indicated t h a t the hydrogen fluoride was 99.75y0 HF on the assumption that the only anion present was fluoride.
Experimental Results Enthalpy of Formation of Hydrogen Fluoride.Since the value for the heat of formation of hydrogen fluoride enters into many of the subsequent calculations, it was considered advisable to check all the original data and the corrections which might have been applied to it. The value reported in Circular 50016 of -64.2 kcal./mole is based upon the data of von Wartenberg and Fitzner,17 of Ruff and lIcnzel.ls and of von Wartenberg and S c h u t ~ a . ' ~An identical value of -64.2 I 0.2 kcal./mole was obtained by Westrum and Pitzer'5 from the enthalpy of decomposition of KHFz together with values for the enthalpy of formation of KF and KHF2. In the thermochemical equations in this paper the temperature of 25' (298.15' IIolalsolute. 1T. evolved. ity)' r'P ". 3C. Cd.
0.216
--
. .,*>J
,213 ,210
-AH kcal. lllolr
42 45 4'2 47 1 2 .5(j 4 2 . 4(\
dimiioniuni fluoride ( S H , F ) 28.24:< 0.07:320 0.3!J4(j (X)751 37. 58!) 4520 33.6i0 08750 :
1 4 . 3I 14.30 11.31 14.31
,
Ammoniiiiii ~nonoliydrogeiitliflnoride (KH,HF,) 0,272 0.2270 0 .28TT 22.975 5. i : 3 0.182 0.104
